WEBVTT 00:00:00.000 --> 00:00:02.040 I want you to do something really simple for 00:00:02.040 --> 00:00:05.799 me right now. Just snap your fingers. Or, you 00:00:05.799 --> 00:00:07.400 know, if you're holding something, just think 00:00:07.400 --> 00:00:09.339 a single, deliberate thought. Right, exactly. 00:00:09.560 --> 00:00:12.380 Ready? Go. It feels completely instantaneous, 00:00:12.539 --> 00:00:14.779 right? Like effortless. Yeah, totally fluid. 00:00:14.980 --> 00:00:18.260 But in that exact fraction of a second. to make 00:00:18.260 --> 00:00:21.480 that tiny muscle movement, or even that single 00:00:21.480 --> 00:00:25.199 cognitive spark happen, your body just orchestrated 00:00:25.199 --> 00:00:29.019 millions of microscopic, violently energetic 00:00:29.019 --> 00:00:32.299 cellular collisions. It's honestly mind -blowing 00:00:32.299 --> 00:00:34.579 when you really look at it. And every single 00:00:34.579 --> 00:00:37.759 one of those events... was coordinated by a highly 00:00:37.759 --> 00:00:40.899 specific piece of molecular machinery that we 00:00:40.899 --> 00:00:42.899 are doing a deep dive into today. Yeah, it's 00:00:42.899 --> 00:00:45.240 one of those biological realities that genuinely 00:00:45.240 --> 00:00:47.579 alters your perspective once you actually understand 00:00:47.579 --> 00:00:50.240 it. Completely. Like, we navigate the world taking 00:00:50.240 --> 00:00:52.280 our movements and our thoughts as these continuous 00:00:52.280 --> 00:00:54.740 actions, but down at the cellular level, it is 00:00:54.740 --> 00:00:58.240 a bustling, highly regulated, and very mechanical 00:00:58.240 --> 00:01:02.259 universe. And our mission today is to, well... 00:01:02.270 --> 00:01:05.170 Pull back the curtain on that universe. We're 00:01:05.170 --> 00:01:07.670 doing a deep dive into an incredibly comprehensive 00:01:07.670 --> 00:01:10.510 set of source materials all about the biology 00:01:10.510 --> 00:01:14.510 the structure and the the physics of snare proteins 00:01:14.510 --> 00:01:16.909 right snare proteins We're gonna break down how 00:01:16.909 --> 00:01:19.189 these proteins act as the ultimate molecular 00:01:19.189 --> 00:01:22.030 engine driving almost everything you do. I mean 00:01:22.219 --> 00:01:24.500 from releasing your transmitters in your brain 00:01:24.500 --> 00:01:27.180 all the way down to how your cells manage their 00:01:27.180 --> 00:01:29.799 own hazardous waste. Yeah, they do a lot. So 00:01:29.799 --> 00:01:31.640 setting the foundation here, let's talk about 00:01:31.640 --> 00:01:35.120 what a snare protein actually functionally is 00:01:35.120 --> 00:01:37.780 within the cell. OK, so fundamentally, SNARES, 00:01:38.019 --> 00:01:41.599 which stands for Soluble NSF Attachment Protein 00:01:41.599 --> 00:01:44.859 Receptors, they are this massive and really ancient 00:01:44.859 --> 00:01:47.019 family of proteins. How massive are we talking? 00:01:47.099 --> 00:01:49.420 Well, you have over 60 different variations in 00:01:49.420 --> 00:01:52.280 a million cells alone. Wow, over 60. OK. Yeah. 00:01:52.459 --> 00:01:54.859 And their primary function, the reason they are 00:01:54.859 --> 00:01:57.579 so critical to complex life, is to solve a very 00:01:57.579 --> 00:02:00.260 specific physics problem. Which is a what? How 00:02:00.260 --> 00:02:02.439 do you get two separate biological membranes 00:02:02.439 --> 00:02:04.579 to actually fuse together? Right, because they 00:02:04.579 --> 00:02:06.879 don't want to. Exactly. For instance, when a 00:02:06.879 --> 00:02:09.699 neuron needs to transmit a signal across a synapse, 00:02:10.199 --> 00:02:12.419 it has its chemical messengers, you know, the 00:02:12.419 --> 00:02:14.620 neurotransmitters. And they're packaged inside 00:02:14.620 --> 00:02:16.580 these little transport vesicles. Like little 00:02:16.580 --> 00:02:19.319 biological cargo ships. Yeah, perfect analogy. 00:02:19.919 --> 00:02:22.319 SNAIR proteins are the mechanical bridge that 00:02:22.319 --> 00:02:25.379 physically forces that vesicle membrane to merge 00:02:25.379 --> 00:02:28.259 with the outer cell membrane which spills those 00:02:28.259 --> 00:02:30.159 chemicals out into the synapse. Right because 00:02:30.159 --> 00:02:33.199 without that fusion event the chemical signal 00:02:33.199 --> 00:02:36.199 just it just sits trapped inside the vesicle. 00:02:36.340 --> 00:02:38.919 Precisely the neuron fires but nothing happens. 00:02:39.120 --> 00:02:43.400 But how do two separate membranes suspended in 00:02:43.400 --> 00:02:45.979 this incredibly crowded chaotic environment to 00:02:45.979 --> 00:02:48.139 the cell, how do they even find the right partner 00:02:48.139 --> 00:02:49.939 to connect with? It's a great question. I mean, 00:02:50.080 --> 00:02:52.379 if there are 60 different snares, what stops 00:02:52.379 --> 00:02:55.349 a neurotransmitter vesicle from accidentally 00:02:55.349 --> 00:02:58.590 fusing with like a mitochondrion. Yeah, that 00:02:58.590 --> 00:03:00.889 would be bad. That is basically the crucial question 00:03:00.889 --> 00:03:03.150 of membrane trafficking. It all comes down to 00:03:03.150 --> 00:03:06.169 a highly specific complementary lock and key 00:03:06.169 --> 00:03:09.270 system. We generally categorize them into two 00:03:09.270 --> 00:03:12.610 main operational groups. So you have V snares, 00:03:12.729 --> 00:03:14.810 which are embedded directly into the membrane 00:03:14.810 --> 00:03:17.009 of the vesicle. The V is for vesicle. Makes sense. 00:03:17.090 --> 00:03:19.530 The transport vehicle itself. Right. Then you 00:03:19.530 --> 00:03:21.830 have T snares, which are anchored to the target 00:03:21.830 --> 00:03:25.189 membrane. The T is for target. Okay, keeping 00:03:25.189 --> 00:03:27.889 it simple. I like it. When the vesicle gets into 00:03:27.889 --> 00:03:30.870 the correct proximity, the specific V -snare 00:03:30.870 --> 00:03:33.129 and the matching T -snares recognize each other. 00:03:33.129 --> 00:03:35.349 They kind of reach out and they intertwine to 00:03:35.349 --> 00:03:37.490 form what's called a trans -snare complex. So 00:03:37.490 --> 00:03:39.710 it's essentially a highly targeted docking sequence. 00:03:40.270 --> 00:03:42.229 The vesicle and the target membrane have to have 00:03:42.229 --> 00:03:44.610 the exact right molecular handshake. Exactly. 00:03:44.689 --> 00:03:47.289 That handshake is key. And this docking core, 00:03:47.330 --> 00:03:49.729 looking at the breakdown here, it's built around 00:03:49.729 --> 00:03:53.419 a 4 -alpha helix bundle. You have a T snare called 00:03:53.419 --> 00:03:56.460 syntax and contributing one helix, a V snare 00:03:56.460 --> 00:03:59.400 called synaptobrevin contributing one, and another 00:03:59.400 --> 00:04:02.520 T snare called snap 25 contributing two helices. 00:04:02.879 --> 00:04:05.639 Yes, that's the classic neuronal snare complex. 00:04:06.060 --> 00:04:08.560 But I want to pause on how these are categorized, 00:04:08.599 --> 00:04:11.280 because honestly, there's a structural detail 00:04:11.280 --> 00:04:13.680 here that seems wildly counterintuitive to me. 00:04:13.879 --> 00:04:16.180 Oh, which part? Well, the source is... These 00:04:16.180 --> 00:04:18.480 proteins are functionally classified as R -snares 00:04:18.480 --> 00:04:21.540 and Q -snares based on literally a single amino 00:04:21.540 --> 00:04:24.579 acid at their core, arginine or glutamine. Ah, 00:04:24.620 --> 00:04:27.740 yes, the ionic layer. Right. If you're telling 00:04:27.740 --> 00:04:30.480 me there's an entire universe of over 60 different 00:04:30.480 --> 00:04:33.040 snare proteins managing millions of distinct 00:04:33.040 --> 00:04:36.279 pathways, how can they all rely on the exact 00:04:36.279 --> 00:04:39.180 same single amino acid to lock together? Doesn't 00:04:39.180 --> 00:04:42.139 that invite massive misfires. I mean it sounds 00:04:42.139 --> 00:04:44.740 like a recipe for biological chaos, I agree, 00:04:45.100 --> 00:04:48.399 but it actually highlights the brilliant efficiency 00:04:48.399 --> 00:04:50.959 of evolution, what you're pointing to is known 00:04:50.959 --> 00:04:54.100 as the zero ionic layer. So when those four helices 00:04:54.100 --> 00:04:56.439 wind together to form that core bundle right 00:04:56.439 --> 00:04:59.000 in the absolute center, they create a highly 00:04:59.000 --> 00:05:01.819 specific, very energetically stable ionic bond. 00:05:02.480 --> 00:05:05.120 The zero layer is always composed of one positively 00:05:05.120 --> 00:05:08.220 charged arginine, that's the R from the V -snare, 00:05:08.399 --> 00:05:11.180 and three glutamines, the Qs from the T -snares. 00:05:11.259 --> 00:05:13.680 But if they all share that exact same R and Q 00:05:13.680 --> 00:05:16.850 core, where does the specificity come from? Any 00:05:16.850 --> 00:05:20.089 R just bind with any Q. Because the zero ionic 00:05:20.089 --> 00:05:22.790 layer isn't responsible for the identity of the 00:05:22.790 --> 00:05:24.670 connection, it's responsible for the stability 00:05:24.670 --> 00:05:27.899 of it. Oh, I see. the specificity, that handshake 00:05:27.899 --> 00:05:30.879 that ensures the right vesicle, fuses with the 00:05:30.879 --> 00:05:34.220 right membrane that comes from the unique amino 00:05:34.220 --> 00:05:37.540 acid sequences on the outer edges of those helices. 00:05:37.860 --> 00:05:40.279 OK, so the outside does the ID check. Exactly. 00:05:40.680 --> 00:05:43.199 But once the correct proteins find each other, 00:05:43.480 --> 00:05:47.019 that R and Q core serves as this universal anchor. 00:05:47.240 --> 00:05:50.420 And to ensure that specific ionic bond doesn't 00:05:50.420 --> 00:05:53.019 get disrupted by the water -rich environment 00:05:53.019 --> 00:05:55.540 of the cell. Because cells are mostly water. 00:05:55.939 --> 00:05:58.819 Right. It is surrounded by layers of hydrophobic 00:05:58.819 --> 00:06:01.860 amino acids, primarily leucines. This creates 00:06:01.860 --> 00:06:04.139 a really cool phenomenon called leucine zippering. 00:06:04.339 --> 00:06:06.560 Leucine zippering. Yeah, the leucines physically 00:06:06.560 --> 00:06:09.639 repel water, which creates a completely watertight 00:06:09.639 --> 00:06:12.319 seal around the zero ionic layer. OK, that makes 00:06:12.319 --> 00:06:14.360 a lot more sense. So the outer surface of the 00:06:14.360 --> 00:06:16.579 protein handles the navigation, but once they're 00:06:16.579 --> 00:06:19.459 cleared to dock, they utilize a universally stable 00:06:19.459 --> 00:06:21.740 watertight anchor to lock in. You've got it. 00:06:21.939 --> 00:06:24.100 So the complex is assembled. The zero ionic layer 00:06:24.100 --> 00:06:27.819 is secure. But docking is infusion. We still 00:06:27.819 --> 00:06:31.220 have two distinct lipid bilayers that are fundamentally 00:06:31.220 --> 00:06:34.199 opposed to touching each other. How do the snares 00:06:34.199 --> 00:06:36.860 actually force a breach? This is where we shift 00:06:36.860 --> 00:06:40.350 from molecular recognition to just sheer mechanical 00:06:40.350 --> 00:06:43.350 force. And you're right to point out the opposition. 00:06:43.709 --> 00:06:46.529 Lipid bilayers are incredibly stable. And they're 00:06:46.529 --> 00:06:49.870 surrounded by this hydration shell of water molecules. 00:06:50.550 --> 00:06:52.649 To force them together, you have to strip away 00:06:52.649 --> 00:06:55.569 that water and overcome a massive electrostatic 00:06:55.569 --> 00:06:57.970 repulsion. The membranes aggressively push back 00:06:57.970 --> 00:07:00.120 against each other. It's the equivalent of trying 00:07:00.120 --> 00:07:03.079 to force the negative poles of two highly powerful 00:07:03.079 --> 00:07:05.680 neodymium magnets together. Yes, perfect example. 00:07:05.839 --> 00:07:07.579 The closer they get, the more violently they 00:07:07.579 --> 00:07:10.339 repel. So how does this delicate protein bundle 00:07:10.339 --> 00:07:13.259 overpower that physics? It requires a highly 00:07:13.259 --> 00:07:15.220 coordinated application of mechanical tension, 00:07:15.319 --> 00:07:17.680 and this is heavily regulated by a chaperone 00:07:17.680 --> 00:07:20.139 protein called Monk 18. Monk 18, what does it 00:07:20.139 --> 00:07:23.360 do? Monkey Teen is fascinating because its initial 00:07:23.360 --> 00:07:27.139 job is actually preventative. It binds tightly 00:07:27.139 --> 00:07:29.779 to syntaxin, that's the T -snare on the target 00:07:29.779 --> 00:07:32.959 membrane, and forces it into a folded, closed 00:07:32.959 --> 00:07:35.560 conformation. Oh, so it hides it? It effectively 00:07:35.560 --> 00:07:38.680 hides syntaxin's binding site so it can't accidentally 00:07:38.680 --> 00:07:41.339 interact with random proteins floating by. It's 00:07:41.339 --> 00:07:43.540 like a safety on a weapon, keeping the system 00:07:43.540 --> 00:07:46.220 disarmed until the exact right moment. Precisely. 00:07:46.490 --> 00:07:48.910 When the cellular signal arrives, indicating 00:07:48.910 --> 00:07:52.250 it's time to fuse, MONK18 undergoes a conformational 00:07:52.250 --> 00:07:55.550 change. It unclasps syntaxin, allows it to unfold, 00:07:55.750 --> 00:07:57.850 and then actually helps guide it into binding 00:07:57.850 --> 00:08:02.149 with synaptobrevin and SNET25. OK, but the real 00:08:02.149 --> 00:08:04.449 energy, the force that overcomes that membrane 00:08:04.449 --> 00:08:06.670 repulsion? Yeah. Where does that come from? It 00:08:06.670 --> 00:08:08.589 comes from the assembly process itself. This 00:08:08.589 --> 00:08:10.850 is governed by the zippering hypothesis. The 00:08:10.850 --> 00:08:13.350 zippering hypothesis. Walk me through the mechanics 00:08:13.350 --> 00:08:16.149 of that because, frankly, how does simply coiling 00:08:16.149 --> 00:08:18.769 together generate enough outward force to smash 00:08:18.769 --> 00:08:21.290 two membranes together? Well, it's an energetically 00:08:21.290 --> 00:08:23.589 spontaneous process. It starts at the tips of 00:08:23.589 --> 00:08:26.009 the proteins, furthest from the membranes, and 00:08:26.009 --> 00:08:28.529 zippers down toward the membrane anchors. Like 00:08:28.529 --> 00:08:30.889 zipping up a jacket from the top down. Kind of, 00:08:30.930 --> 00:08:33.929 yeah. As these four helices wind around each 00:08:33.929 --> 00:08:36.470 other, they are pulling the vesicle and the cell 00:08:36.470 --> 00:08:39.549 membrane closer. But the regions of the proteins 00:08:39.549 --> 00:08:42.269 connecting the bundle to the membrane are semi 00:08:42.269 --> 00:08:45.070 -rigid. So they don't want to bend. Right. So 00:08:45.070 --> 00:08:48.049 as they twist tighter and tighter, they accumulate 00:08:48.049 --> 00:08:51.830 intense molecular bending stress. So they are 00:08:51.830 --> 00:08:54.929 physically winding up mechanical tension, like 00:08:54.929 --> 00:08:57.289 twisting the rubber band on a toy airplane until 00:08:57.289 --> 00:09:00.870 the wood starts to bow. Exactly that. And thermodynamics 00:09:00.870 --> 00:09:03.269 dictates that the system wants to relieve that 00:09:03.269 --> 00:09:06.409 unfavorable bending stress. The only way to relieve 00:09:06.409 --> 00:09:08.909 it is to allow the rigid linker regions to straighten 00:09:08.909 --> 00:09:12.330 out. And doing that forcibly drives the two repelling 00:09:12.330 --> 00:09:15.169 membranes into each other. The mechanical torque 00:09:15.169 --> 00:09:18.330 of the proteins completely overwhelms the electrostatic 00:09:18.330 --> 00:09:21.549 repulsion of the lipids. That is wild. And there's 00:09:21.549 --> 00:09:23.629 a specific biophysical state mentioned in the 00:09:23.629 --> 00:09:25.710 source that paints such a clear picture of what 00:09:25.710 --> 00:09:28.990 happens at the exact moment of impact, the splayed 00:09:28.990 --> 00:09:31.129 lipid state. Yes, the splayed lipid state. What 00:09:31.129 --> 00:09:32.870 is actually happening to the lipid bilayer there? 00:09:33.169 --> 00:09:35.909 Well, when the mechanical stress jams the membranes 00:09:35.909 --> 00:09:38.990 together, that hydration shell of water we talked 00:09:38.990 --> 00:09:41.409 about is completely squeezed out. Just pushed 00:09:41.409 --> 00:09:44.159 out of the way. Yeah. At that incredibly close 00:09:44.159 --> 00:09:47.240 range, the lipid molecules in the outer leaflet 00:09:47.240 --> 00:09:49.620 of the vesicle and the outer leaflet of the target 00:09:49.620 --> 00:09:53.740 membrane undergo immense stress. To alleviate 00:09:53.740 --> 00:09:55.899 this, some of the lipids will actually splay 00:09:55.899 --> 00:09:59.120 apart, and their hydrophobic tails, the inner 00:09:59.120 --> 00:10:01.539 portion of the lipid that avoids water, will 00:10:01.539 --> 00:10:03.919 physically cross over and insert themselves into 00:10:03.919 --> 00:10:06.019 the opposing membrane. That's incredible. They're 00:10:06.019 --> 00:10:08.779 essentially trading structural material. One 00:10:08.779 --> 00:10:11.220 lipid takes its tail and just embeds it into 00:10:11.220 --> 00:10:13.360 the neighbor's... wall. Yes, and that state of 00:10:13.360 --> 00:10:16.059 cross -contamination is called hemifusion. It 00:10:16.059 --> 00:10:18.360 creates a temporary structure called a fusion 00:10:18.360 --> 00:10:20.580 stalk. That's stalked, okay. From there, the 00:10:20.580 --> 00:10:22.600 inner leaflets of the membranes follow suit, 00:10:23.039 --> 00:10:25.840 merging together to open a fusion pore, and boom, 00:10:26.320 --> 00:10:29.139 the two separate compartments become one continuous 00:10:29.139 --> 00:10:31.799 space. The neurotransmitters spill out into the 00:10:31.799 --> 00:10:34.340 synapse. The signal is successfully sent. Mission 00:10:34.340 --> 00:10:36.860 accomplished. Okay, so the fusion event has occurred. 00:10:37.120 --> 00:10:40.419 The vesicle has emptied its contents. But functionally, 00:10:40.580 --> 00:10:42.960 we now have a massive structural problem, don't 00:10:42.960 --> 00:10:46.179 we? We do. We had a V snare on the vesicle and 00:10:46.179 --> 00:10:48.360 T snares on the target membrane. They zipper 00:10:48.360 --> 00:10:51.059 together to cause the fusion. But now that the 00:10:51.059 --> 00:10:53.919 membranes are fully merged, those snare proteins 00:10:53.919 --> 00:10:55.919 are completely tangled together in the exact 00:10:55.919 --> 00:10:58.279 same membrane. Right. The source refers to this 00:10:58.279 --> 00:11:01.600 as a cis -snare complex. A cis -snare complex. 00:11:01.980 --> 00:11:04.340 And if they stay locked in that highly stable 00:11:04.340 --> 00:11:07.269 water -type bundle, They're useless right? The 00:11:07.269 --> 00:11:09.409 neuron wouldn't be able to fire again. So how 00:11:09.409 --> 00:11:11.980 does the cell clear the jam? It has to forcibly 00:11:11.980 --> 00:11:15.059 reset the system. And because that 4 -helix bundle 00:11:15.059 --> 00:11:17.700 is thermodynamically incredibly stable, remember 00:11:17.700 --> 00:11:20.299 the zero ionic layer and the leucine zippering 00:11:20.299 --> 00:11:22.919 we talked about, it takes a massive injection 00:11:22.919 --> 00:11:25.320 of cellular energy to rip them apart. It can't 00:11:25.320 --> 00:11:27.860 just happen on its own. No, not at all. The cell 00:11:27.860 --> 00:11:30.340 deploys a specialized molecular machine called 00:11:30.340 --> 00:11:33.200 NSF, which is a hexameric ATPase. And it works 00:11:33.200 --> 00:11:35.860 along with an adapter protein called alpha -sesnopay. 00:11:35.940 --> 00:11:38.960 A hexameric ATPase. So we're talking about a 00:11:38.960 --> 00:11:42.100 multi -part motor that's specifically burns ATP, 00:11:42.460 --> 00:11:44.600 the cell's energy currency, to do mechanical 00:11:44.600 --> 00:11:48.409 work. Alpha -synapse binds to the exterior of 00:11:48.409 --> 00:11:51.950 the tangled cis -snare complex, effectively tagging 00:11:51.950 --> 00:11:55.230 it. This recruits the NSF hexamer, which is shaped 00:11:55.230 --> 00:11:57.470 sort of like a microscopic ring. Okay, a ring. 00:11:57.730 --> 00:12:01.070 NSF docks onto the complex, and through the hydrolysis 00:12:01.070 --> 00:12:03.649 of ATP burning that energy, it undergoes rapid 00:12:03.649 --> 00:12:05.929 shape changes. It literally acts like a molecular 00:12:05.929 --> 00:12:08.669 crowbar. A crowbar? Yeah. Prying the tightly 00:12:08.669 --> 00:12:10.950 wound helices apart and unfolding the proteins. 00:12:11.370 --> 00:12:13.289 Once they are separated, they float back into 00:12:13.289 --> 00:12:15.190 the membrane fluid to be recycled for the next 00:12:15.049 --> 00:12:18.649 I love the conceptual framing here. By using 00:12:18.649 --> 00:12:21.970 ATP to forcibly rip them apart, the cell is essentially 00:12:21.970 --> 00:12:24.370 storing potential energy in the separated proteins. 00:12:24.730 --> 00:12:27.059 The source compared it to cocking a gun. It's 00:12:27.059 --> 00:12:29.399 a great analogy. You invest the energy now to 00:12:29.399 --> 00:12:31.919 separate them so that when the next vesicle arrives, 00:12:32.340 --> 00:12:34.600 the zippering process can happen spontaneously 00:12:34.600 --> 00:12:36.980 and instantaneously. Exactly, because you need 00:12:36.980 --> 00:12:39.559 speed when a neuron fires. But looking at the 00:12:39.559 --> 00:12:41.620 individual proteins involved in this violent 00:12:41.620 --> 00:12:44.120 coiling and untangling, there's a mechanical 00:12:44.120 --> 00:12:47.450 anomaly that stands out. Both syntaxin and synaptobrevin 00:12:47.450 --> 00:12:50.169 have standard transmembrane domains, like they 00:12:50.169 --> 00:12:52.470 have physical peptide tails anchored deep into 00:12:52.470 --> 00:12:54.470 the lipid bilayer. Right, they're firmly planted. 00:12:54.889 --> 00:12:58.029 But SNF25, which contributes half of the entire 00:12:58.029 --> 00:13:00.809 core bundle, doesn't have a transmembrane domain. 00:13:01.029 --> 00:13:04.750 It did not. So how does a protein with no anchor 00:13:04.750 --> 00:13:07.590 survive being caught in the middle of a molecular 00:13:07.590 --> 00:13:10.309 crowbar without just floating away into the cytosol? 00:13:10.629 --> 00:13:14.809 That is a brilliant mechanical discrepancy. SNF25 00:13:14.809 --> 00:13:17.309 requires an entirely different anchoring mechanism. 00:13:17.870 --> 00:13:22.309 It's called palmitoylation. After SNF25 is synthesized, 00:13:22.509 --> 00:13:24.929 a specialized enzyme comes along and covalently 00:13:24.929 --> 00:13:28.230 attaches long fatty acid chains palmitoyl groups 00:13:28.230 --> 00:13:30.950 to specific cysteine residues right in the middle 00:13:30.950 --> 00:13:33.460 of the protein. So it glues fat to it. Basically, 00:13:33.720 --> 00:13:36.320 yeah. These fatty acids act as lipid anchors, 00:13:36.399 --> 00:13:38.799 sinking into the membrane to hold SNF25 flush 00:13:38.799 --> 00:13:41.720 against the surface. But why evolve such a complex, 00:13:41.919 --> 00:13:44.379 roundabout way to anchor it? Why not just build 00:13:44.379 --> 00:13:46.600 it with a normal transmembrane tail like the 00:13:46.600 --> 00:13:50.360 others? Because SNF25 needs extreme lateral mobility. 00:13:50.440 --> 00:13:53.159 It has to move around easily. And more importantly, 00:13:53.419 --> 00:13:56.179 it isn't just a passive structural beam. It has 00:13:56.179 --> 00:13:59.259 a secondary, highly critical job. Which is? It 00:13:59.259 --> 00:14:01.740 physically regulates the voltage -gated calcium 00:14:01.740 --> 00:14:04.360 channels in our neurons. Oh, wait. Calcium channels. 00:14:04.600 --> 00:14:06.460 This is the trigger for the whole process, right? 00:14:06.899 --> 00:14:08.919 The electrical impulse hits the neuron, calcium 00:14:08.919 --> 00:14:11.600 channels open, calcium floods in, and that sudden 00:14:11.600 --> 00:14:14.220 spike in calcium is the final signal that tells 00:14:14.220 --> 00:14:17.299 the snares to zipper up. You've got it. So how 00:14:17.299 --> 00:14:21.440 does SNAP25 regulate that? SNAP25 physically 00:14:21.440 --> 00:14:24.259 interacts with the intracellular loops of those 00:14:24.259 --> 00:14:27.480 calcium channels. By binding to them, SNAP25 00:14:27.480 --> 00:14:29.659 alters the voltage dependence of the channel. 00:14:30.139 --> 00:14:32.519 It shifts the threshold required to open them, 00:14:32.720 --> 00:14:35.220 making them less responsive. So it acts as a 00:14:35.220 --> 00:14:37.919 biological brake pedal. Yes, ensuring that the 00:14:37.919 --> 00:14:40.200 calcium channels don't just stay open and cause 00:14:40.200 --> 00:14:43.139 the neuron to fire uncontrollably. So it's actively 00:14:43.139 --> 00:14:45.179 managing the very signal that commands it to 00:14:45.179 --> 00:14:48.610 work. If SNAP25 didn't hit the brakes, the neuron 00:14:48.610 --> 00:14:50.950 would exhaust its entire supply of neurotransmitters 00:14:50.950 --> 00:14:54.120 in a continuous spasm. Exactly. And because of 00:14:54.120 --> 00:14:57.019 this dual role in both fusion and calcium regulation, 00:14:57.480 --> 00:15:00.879 any mutation or variation in SNF25 has cascading 00:15:00.879 --> 00:15:03.460 effects on neurological health. It completely 00:15:03.460 --> 00:15:06.080 disrupts the delicate timing of exocytosis. That 00:15:06.080 --> 00:15:08.100 makes sense. The source material highlights that 00:15:08.100 --> 00:15:11.820 in coloboma mutant mice, a deletion in the SNF25 00:15:11.820 --> 00:15:15.179 gene leads to profound hyperactive phenotypes. 00:15:15.200 --> 00:15:18.340 Wow. And in humans, specific genetic variations 00:15:18.340 --> 00:15:21.200 in SNF25 are heavily correlated with the onset 00:15:21.200 --> 00:15:24.700 of PhD, schizophrenia, and early onset bipolar 00:15:24.700 --> 00:15:27.440 disorder. That is just staggering to think about. 00:15:27.620 --> 00:15:30.360 The subtle physics of how a single protein anchors 00:15:30.360 --> 00:15:33.580 via fatty acids to regulate a calcium ion channel 00:15:33.580 --> 00:15:35.879 is functionally the difference between stable 00:15:35.879 --> 00:15:38.139 cognition and severe psychiatric conditions. 00:15:38.399 --> 00:15:40.440 It really is. Biology is incredibly delicate. 00:15:40.539 --> 00:15:43.220 But that level of complex hyperspecific regulation 00:15:43.500 --> 00:15:47.000 also makes it a prime target. Nature is ruthless 00:15:47.000 --> 00:15:49.259 about exploiting structural weak points. And 00:15:49.259 --> 00:15:52.679 that brings us to the toxins. Ah, yes. The neurotoxins, 00:15:52.759 --> 00:15:54.779 the ones that specifically hunt and dismantle 00:15:54.779 --> 00:15:57.059 snare complexes, are arguably the most potent 00:15:57.059 --> 00:15:59.039 and terrifying biological weapons on the planet. 00:15:59.320 --> 00:16:01.580 We're talking about botulinum toxin, known as 00:16:01.580 --> 00:16:06.440 Bon -T, and tetanus toxin, Tent -T. Yes. Botulinum 00:16:06.440 --> 00:16:09.080 is routinely cited as the most acutely lethal 00:16:09.080 --> 00:16:12.330 toxin known to humanity. A dose the size of a 00:16:12.330 --> 00:16:15.789 grain of sand can kill thousands of people. It's 00:16:15.789 --> 00:16:18.230 terrifyingly efficient. How does a bacterial 00:16:18.230 --> 00:16:21.830 byproduct so thoroughly dismantle a system that 00:16:21.830 --> 00:16:23.970 has been perfected over billions of years of 00:16:23.970 --> 00:16:26.549 evolution? It operates through a frighteningly 00:16:26.549 --> 00:16:29.610 methodical multi -stage invasion. The botulinum 00:16:29.610 --> 00:16:32.830 toxin molecule is basically composed of two primary 00:16:32.830 --> 00:16:35.009 parts connected by a chemical bridge called a 00:16:35.009 --> 00:16:37.549 disulfide bond. You have a heavy chain and a 00:16:37.549 --> 00:16:39.559 light chain. Okay, heavy and light. The heavy 00:16:39.559 --> 00:16:41.580 chain acts as the scout and the breaching charge. 00:16:42.000 --> 00:16:44.179 It circulates in the body until it finds the 00:16:44.179 --> 00:16:46.500 highly specific protein receptors on the surface 00:16:46.500 --> 00:16:49.019 of a motor neuron. Right. Once it binds to the 00:16:49.019 --> 00:16:51.080 cell membrane, it actually tricks the neuron 00:16:51.080 --> 00:16:54.039 into swallowing it through a process called endocytosis. 00:16:54.460 --> 00:16:57.240 So the neuron literally wraps a membrane around 00:16:57.240 --> 00:16:59.799 the toxin and pulls it inside, thinking it's 00:16:59.799 --> 00:17:01.200 something useful or at least something it needs 00:17:01.200 --> 00:17:03.379 to process. It just brings the bomb right into 00:17:03.379 --> 00:17:06.809 the bunker. Exactly, but now the toxin is trapped 00:17:06.809 --> 00:17:10.049 inside a vesicle, inside the neuron, as the cell 00:17:10.049 --> 00:17:12.750 begins to acidify that vesicle to break it down. 00:17:13.279 --> 00:17:16.799 The drop in pH triggers a massive conformational 00:17:16.799 --> 00:17:19.500 change in the heavy chain. It activates it. Yeah. 00:17:19.839 --> 00:17:22.380 The heavy chain unfolds, punches a hole through 00:17:22.380 --> 00:17:24.779 the vesicle membrane, and physically channels 00:17:24.779 --> 00:17:27.359 the light chain out into the neuron cytosol. 00:17:27.400 --> 00:17:29.960 Wow. And once the light chain is exposed to the 00:17:29.960 --> 00:17:32.619 cellular environment, a specialized enzyme system 00:17:32.619 --> 00:17:35.420 snaps the disulfide bond, setting the light chain 00:17:35.420 --> 00:17:38.019 free. And what exactly is the light chain doing 00:17:38.019 --> 00:17:40.059 once it's loose in the cell? The light chain 00:17:40.059 --> 00:17:43.549 is a zinc -dependent metalloprotease. meaning 00:17:43.549 --> 00:17:46.829 it requires a zinc ion to act as a catalyst to 00:17:46.829 --> 00:17:49.289 cleave proteins. It's like a highly specialized 00:17:49.289 --> 00:17:52.009 molecular sample. Perfectly stated. The light 00:17:52.009 --> 00:17:54.230 chain is programmed to ignore every other protein 00:17:54.230 --> 00:17:56.589 in the cell and hunt exclusively for snares. 00:17:56.710 --> 00:17:59.190 And it is devastatingly precise. How precise? 00:17:59.430 --> 00:18:01.829 Depending on the specific serotype of modulinum, 00:18:01.950 --> 00:18:04.369 there are several. It will target different members. 00:18:04.930 --> 00:18:07.809 BOTEN -T serotype A, for example, hunts down 00:18:07.809 --> 00:18:11.789 SNF225 and snips a specific peptide bond near 00:18:11.789 --> 00:18:15.140 its tail. Tetanus toxin hunts down synaptobrevin 00:18:15.140 --> 00:18:17.839 and cleaves it directly in half. And if those 00:18:17.839 --> 00:18:19.880 proteins are cleaved, the four -helix bundle 00:18:19.880 --> 00:18:23.059 can't form. The zippering fails. Right. The vesicle 00:18:23.059 --> 00:18:25.400 reaches the membrane but can't generate the force 00:18:25.400 --> 00:18:28.119 to fuse. The neurotransmitters are just trapped. 00:18:28.480 --> 00:18:30.920 Precisely. It induces a complete blockade of 00:18:30.920 --> 00:18:32.980 neurotransmission at the neuromuscular junction. 00:18:33.559 --> 00:18:36.019 The nerve fires but the muscle receives absolutely 00:18:36.019 --> 00:18:38.819 no signal. Which causes profound flaccid paralysis. 00:18:39.019 --> 00:18:42.250 Yes. Eventually, this paralysis reaches the diaphragm 00:18:42.250 --> 00:18:44.230 and you physically cannot command your lungs 00:18:44.230 --> 00:18:47.170 to draw breath, resulting in asphyxiation. Okay, 00:18:47.450 --> 00:18:49.589 understanding that mechanism makes the societal 00:18:49.589 --> 00:18:52.369 application of this so deeply bizarre. I know 00:18:52.369 --> 00:18:54.170 exactly where you're going with this. We are 00:18:54.170 --> 00:18:56.410 talking about an unstoppable molecular scalpel 00:18:56.410 --> 00:18:59.549 that causes asphyxiation. But the source material, 00:18:59.569 --> 00:19:02.329 and just reality, explicitly notes that botulinum 00:19:02.329 --> 00:19:04.529 is the primary ingredient in cosmetic botox. 00:19:04.690 --> 00:19:08.470 Yes, it is. Why on earth are dermatologists willfully 00:19:08.470 --> 00:19:12.430 injecting the most lethal neurotoxin in existence 00:19:12.430 --> 00:19:15.349 into millions of people's foreheads? It's the 00:19:15.349 --> 00:19:17.569 ultimate example of the dose -making the poison. 00:19:18.170 --> 00:19:20.930 The exact mechanism that makes botulinum deadly. 00:19:21.690 --> 00:19:24.990 The absolute localized silencing of neurotransmission 00:19:24.990 --> 00:19:28.210 by destroying snares is precisely why it works 00:19:28.210 --> 00:19:30.450 aesthetically. Because it stops the muscle from 00:19:30.450 --> 00:19:32.869 moving. Exactly. When administered in highly 00:19:32.869 --> 00:19:35.829 purified, vanishingly small quantities directly 00:19:35.829 --> 00:19:38.809 into a specific facial muscle, the toxin never 00:19:38.809 --> 00:19:41.829 circulates systemically. It only enters the local 00:19:41.829 --> 00:19:44.930 neurons driving that specific muscle. OK. It 00:19:44.930 --> 00:19:48.089 destroys their snare complexes, temporarily paralyzing 00:19:48.089 --> 00:19:50.069 the muscle so it can't contract and form wrinkles. 00:19:50.250 --> 00:19:52.990 So you aren't fixing the skin. chemically severing 00:19:52.990 --> 00:19:54.690 the transmission line between your brain and 00:19:54.690 --> 00:19:57.089 your forehead by sabotaging the membrane fusion 00:19:57.089 --> 00:19:58.890 machinery. That is exactly what you're doing. 00:19:58.990 --> 00:20:01.809 You are initiating a highly controlled microscopic 00:20:01.809 --> 00:20:04.569 botulism outbreak in your brow. That is, I mean, 00:20:04.829 --> 00:20:07.930 that is just wild. And it only wears off because 00:20:07.930 --> 00:20:10.470 the neuron eventually realizes the snares are 00:20:10.470 --> 00:20:14.569 destroyed, sprouts new nerve endings, and synthesizes 00:20:14.569 --> 00:20:17.849 entirely new snare complexes from scratch. We 00:20:17.849 --> 00:20:20.869 took an existential biological threat and domesticated 00:20:20.869 --> 00:20:22.869 it for vanity. That's incredible. But I want 00:20:22.869 --> 00:20:24.410 to zoom out from the nervous system for a minute. 00:20:24.509 --> 00:20:27.750 Sure. We focused heavily on neurons, exocytosis, 00:20:27.950 --> 00:20:30.390 and muscle movement. But the literature makes 00:20:30.390 --> 00:20:33.849 it very clear that snare proteins are not exclusive 00:20:33.849 --> 00:20:36.730 to the brain. They are operating in almost every 00:20:36.730 --> 00:20:39.509 cell in your body, managing entirely different 00:20:39.509 --> 00:20:41.730 logistics. Right, they're everywhere. Specifically, 00:20:41.930 --> 00:20:44.750 they double as the cell's internal hazard containment 00:20:44.750 --> 00:20:48.250 and waste disposal mechanism. Yes, through a 00:20:48.250 --> 00:20:51.529 vital pathway known as macrotophagy. Your cells 00:20:51.529 --> 00:20:53.349 are essentially high -performance factories, 00:20:53.569 --> 00:20:55.930 and over time, factory parts break down. Natural 00:20:55.930 --> 00:20:58.890 wear and tear. Exactly. Mitochondria become damaged, 00:20:59.369 --> 00:21:01.650 proteins misfold, and toxic aggregates build 00:21:01.650 --> 00:21:04.119 up. If the cell doesn't clear this debris, it 00:21:04.119 --> 00:21:08.119 undergoes apoptosis program cell death. To survive, 00:21:08.460 --> 00:21:11.160 the cell initiates macroautophagy. How does that 00:21:11.160 --> 00:21:14.519 work? It begins constructing a specialized double 00:21:14.519 --> 00:21:17.380 membrane structure called a phagophore. Think 00:21:17.380 --> 00:21:20.019 of it as building a quarantine tent around a 00:21:20.019 --> 00:21:22.779 hazardous material spill. It physically walls 00:21:22.779 --> 00:21:25.160 off the damaged organelles from the rest of the 00:21:25.160 --> 00:21:28.779 healthy cytosol. Exactly. The tent closes, completely 00:21:28.779 --> 00:21:31.000 engulfing the waste, forming what's called an 00:21:31.000 --> 00:21:33.799 autophagosome. But simply trapping the waste 00:21:33.799 --> 00:21:35.920 isn't enough, right? It has to be neutralized. 00:21:36.019 --> 00:21:38.200 Right, you can't just leave it there. To do that, 00:21:38.339 --> 00:21:41.660 this isolation tent has to fuse with a lysosome, 00:21:41.779 --> 00:21:44.539 which is an acidic organelle packed with degradative 00:21:44.539 --> 00:21:47.119 enzymes. Essentially, the cells hazmat team. 00:21:47.339 --> 00:21:49.759 And getting those two membranes to fuse requires 00:21:49.759 --> 00:21:52.420 the exact same machinery. Precisely. The snare 00:21:52.420 --> 00:21:55.140 mechanism is universally deployed here. The source 00:21:55.140 --> 00:21:58.039 highlights a specific T snare called Syntaxin 00:21:58.039 --> 00:22:01.160 17. Syntaxin 17. Once the autophagosome is fully 00:22:01.160 --> 00:22:03.500 formed and the quarantine is sealed, Syntaxin 00:22:03.500 --> 00:22:06.319 17 is recruited to its outer membrane. It acts 00:22:06.319 --> 00:22:08.990 as the beacon. It seeks out matching snares on 00:22:08.990 --> 00:22:11.470 the lysosome, zippers up, forces the hemifusion 00:22:11.470 --> 00:22:13.970 of the membranes, and dumps the hazmat enzymes 00:22:13.970 --> 00:22:16.569 directly into the quarantine zone. Dismantling 00:22:16.569 --> 00:22:19.049 the waste down to base amino acids that the cell 00:22:19.049 --> 00:22:21.950 can reuse. Exactly. It's beautiful, really. Without 00:22:21.950 --> 00:22:24.900 that specific snare interaction. The cell essentially 00:22:24.900 --> 00:22:27.839 fills with its own garbage and dies, which makes 00:22:27.839 --> 00:22:30.480 a section in the research on flexible substitutions 00:22:30.480 --> 00:22:33.259 so fascinating. Oh, the redundancy. Yeah. Because 00:22:33.259 --> 00:22:35.559 this machinery is so critical, you'd assume a 00:22:35.559 --> 00:22:37.759 single mutation would be instantly fatal to the 00:22:37.759 --> 00:22:40.950 organism. But there's a robustness to it. The 00:22:40.950 --> 00:22:43.170 research points out that if a yeast cell suffers 00:22:43.170 --> 00:22:45.609 a genetic deletion and loses a crucial snare 00:22:45.609 --> 00:22:49.650 protein, like the r -snare sick 22p, the cell 00:22:49.650 --> 00:22:52.430 doesn't just die. No, it adapts. It naturally 00:22:52.430 --> 00:22:55.750 upregulates a homologous protein called YKT6P 00:22:55.750 --> 00:22:58.049 to take its place. It detects a failure in a 00:22:58.049 --> 00:23:00.670 critical component and hot swaps a structurally 00:23:00.670 --> 00:23:02.890 similar backup part to keep the factory running. 00:23:03.130 --> 00:23:05.609 It's an incredible fail safe, and it is not unique 00:23:05.609 --> 00:23:08.829 to yeast. We see the exact same functional redundancy 00:23:08.829 --> 00:23:11.990 in Drosophila fruit flies. If they lose SNAP25, 00:23:12.130 --> 00:23:14.910 they can compensate with a homolog called SNAP24. 00:23:15.089 --> 00:23:18.150 Wow. We see massive, highly complex snare networks 00:23:18.150 --> 00:23:20.269 in all plant life, too, managing the intricate 00:23:20.269 --> 00:23:22.289 vesicle traffic required to build plant cell 00:23:22.289 --> 00:23:25.049 walls. Which really speaks to the sheer evolutionary 00:23:25.049 --> 00:23:27.480 age of this mechanism. Exactly. This isn't some 00:23:27.480 --> 00:23:30.599 recent biological invention. The fact that the 00:23:30.599 --> 00:23:33.480 four helix bundle and the zero ionic layer operate 00:23:33.480 --> 00:23:37.480 on the exact same biophysics in yeast, in a fruit 00:23:37.480 --> 00:23:40.500 fly's gut, in an oak tree, and in the human brain, 00:23:41.200 --> 00:23:44.059 it indicates that this mechanism evolved billions 00:23:44.059 --> 00:23:46.880 of years ago in our last universal common ancestor. 00:23:47.039 --> 00:23:49.859 That's profound. It is so fundamental to the 00:23:49.859 --> 00:23:52.819 definition of eukaryotic life that nature has 00:23:52.819 --> 00:23:56.400 built deep overlapping layers of redundancy to 00:23:56.400 --> 00:23:58.680 ensure it rarely fails. Well, let's pull all 00:23:58.680 --> 00:24:00.420 of this together because the sheer scale of what 00:24:00.420 --> 00:24:02.660 we've covered is mind -bending. We started with 00:24:02.660 --> 00:24:05.039 a fundamental physics problem, overcoming the 00:24:05.039 --> 00:24:07.700 massive electrostatic repulsion of lipid membranes. 00:24:08.220 --> 00:24:10.119 We looked at how evolution solved this with a 00:24:10.119 --> 00:24:12.460 four -helix bundle built around a perfectly conserved 00:24:12.460 --> 00:24:15.660 arginine and glutamine core, shielded by a watertight 00:24:15.660 --> 00:24:18.279 leucine zipper. We examined the sheer mechanical 00:24:18.279 --> 00:24:21.019 torque of the zippering hypothesis, forcing lipids 00:24:21.019 --> 00:24:23.619 into a splayed state to achieve hemifusion. The 00:24:23.619 --> 00:24:25.940 physics of it is just incredible. And then we 00:24:25.940 --> 00:24:28.400 talked about the ATP -powered molecular crowbar, 00:24:28.539 --> 00:24:31.980 NSF, resetting the system. We saw how a lipid 00:24:31.980 --> 00:24:35.180 -anchored protein like SNAP25 acts as a brake 00:24:35.180 --> 00:24:37.640 pedal for our psychological stability, and how 00:24:37.640 --> 00:24:41.019 a single atom of zinc in a bacterial toxin can 00:24:41.019 --> 00:24:44.319 snip that exact protein to cause lethal paralysis 00:24:44.319 --> 00:24:46.660 or, you know, erase a wrinkle. who's building 00:24:46.660 --> 00:24:49.599 it. Exactly. All while these same proteins are 00:24:49.599 --> 00:24:52.019 quietly building quarantine tents to recycle 00:24:52.019 --> 00:24:54.640 cellular waste. The elegance of the system is 00:24:54.640 --> 00:24:57.990 absolute, but... Looking at how universally conserved 00:24:57.990 --> 00:25:00.069 and how strictly mechanical this fusion engine 00:25:00.069 --> 00:25:03.029 is, I think there is a deeply provocative implication 00:25:03.029 --> 00:25:05.130 for the future of medicine that you have to consider. 00:25:05.750 --> 00:25:08.089 Where does this lead us? Well, we've spent this 00:25:08.089 --> 00:25:10.589 entire time talking about how nature utilizes 00:25:10.589 --> 00:25:12.910 and occasionally hijacks snares. But now that 00:25:12.910 --> 00:25:15.490 we understand the precise biophysics of the zero 00:25:15.490 --> 00:25:18.430 ionic layer and the leucine zippering, we can 00:25:18.430 --> 00:25:21.529 start engineering them ourselves. Synthetic snares. 00:25:21.990 --> 00:25:24.190 Exactly. Imagine creating an artificial lipid 00:25:24.190 --> 00:25:26.750 vesicle, loading it with a highly toxic chemotherapy 00:25:26.750 --> 00:25:29.210 drug or maybe a CRISPR gene editing sequence. 00:25:29.369 --> 00:25:32.529 If we can engineer synthetic T -snares into the 00:25:32.529 --> 00:25:35.349 membrane of a specific cancer cell and put the 00:25:35.349 --> 00:25:38.049 matching synthetic V -snare on our drug -loaded 00:25:38.049 --> 00:25:41.309 vesicle, we bypass the entire immune system. 00:25:41.329 --> 00:25:44.369 Oh, wow. We aren't relying on passive drug absorption 00:25:44.369 --> 00:25:48.029 anymore. We are mechanically forcing an artificial 00:25:48.029 --> 00:25:51.099 membrane to zipper open and dump a payload directly 00:25:51.099 --> 00:25:53.660 and exclusively into the targeted diseased cell, 00:25:54.039 --> 00:25:56.640 leaving every healthy cell completely untouched. 00:25:56.960 --> 00:26:00.200 That is revolutionary. The same mechanism that 00:26:00.200 --> 00:26:02.279 a neuron uses to spark a thought could become 00:26:02.279 --> 00:26:04.500 the most precise targeted delivery system in 00:26:04.500 --> 00:26:07.200 the history of medicine. Hacking the core biological 00:26:07.200 --> 00:26:09.619 machinery of life to force it to heal itself. 00:26:09.839 --> 00:26:12.059 I genuinely don't think I will ever look at a 00:26:12.059 --> 00:26:13.920 simple action the same way again. Next time you 00:26:13.920 --> 00:26:16.019 snap your fingers or just have a passing thought. 00:26:16.460 --> 00:26:18.859 Remember the millions of snare complexes quietly 00:26:18.859 --> 00:26:20.960 winding up and violently zippering together in 00:26:20.960 --> 00:26:22.220 the dark to make you who you are.