This episode examines a cryo-EM study that captures authentic proofreading intermediates of human Polε in complex with PCNA by generating a mismatch in situ. The work reveals how PCNA constrains DNA movement and how a mismatched primer is transferred from the polymerase to the exonuclease site.
0:00Welcome to Base by Base, the papercast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. Imagine, for a 2nd, copying a book.
0:12Okay. But not just any book, like a massive library consisting of 3000000000 letters. Right. Now, imagine you have to copy that entire library by hand every single time a cell in your body divides. It's an absolutely monumental task.
0:27Right. And when you're dealing with a scale that large operating at the, you know, blinding speed of cellular division. Mistakes are completely inevitable. But what happens when the incredibly fast cellular machinery copying your DNA makes a typo.
0:42If those uncaught errors slip through, they become permanent mutations, and those heavily drive things like aggressive cancers and devastating genetic diseases. Which is exactly why this matters so much.
0:52Yeah. So you have this molecular machine flying down a DNA strand. How does it suddenly realize it made a mistake, physically stop its forward momentum, back up and act as literal proofreading scissors to cut out the typo wall without falling off the DNA track entirely.
1:10It is, um, arguably one of the most mechanically beautiful processes in biology. And for a long time, we really only had a very partial picture of how this worked. Just a partial picture. Yeah, I mean, we knew the cellular machine possessed the ability to proofread itself, sure.
1:24But the actual physical choreography, like how it catches a single mismatched atom quarantines it and fixes it while remaining firmly anchored to the genetic code that was massive mystery. And unraveling that mystery is exactly what we are getting into on this deep dive.
1:39Today we celebrate the work of Feng Wang, King He, Michael O'Donnell, and Hulan Lee from the Van Andel Institute and the Rockefeller University, who have advanced our understanding of the proofreading mechanisms of human DNA polymerase.
1:52It's just a remarkable piece of research, really to truly appreciate the scale of what they discovered. We need to look at the machinery itself. Okay, let's look at it. So the specific enzyme doing the heavy lifting here, primarily on the leading strand of your DNA.
2:06It's called DNA polymerase epsilon. Polymerase, epsilon. Right. For simplicity, we'll just call it pull epsilon. Now, pull epsilon is essentially a dual function enzyme. Meaning it does 2 different things.
2:18Exactly. It contains a synthesizing site, which we call the poll site. You can think of that as the printing press, adding the new DNA letters. Got it, the printer. But it also contains a completely separate proofreading site called the exocite.
2:32And that is the shredder, or you know, the molecular scissors. So it is the builder and the editor packed into one single machine. That is the core of it, yeah. But here is the structural problem that has puzzled scientists for decades.
2:45Those 2 active sites are 40 Angstroms apart from each other. 40 Angstroms. I know we're dealing with the atomic scale here, but how far is that relative to the size of the molecules involved? Well, to give you a sense of scale.
2:59And Angstrom is 110 billionth of a meter. Yeah. If we scale this up and imagine the pole epsilon enzyme is the size of a massive automobile factory, 40 angstroms is like needing to physically move a defective car from the assembly line all the way across the factory floor to the scrap crusher.
3:17That's a huge distance internally. Exactly. When the builder makes a mistake, the DNA strand somehow has to bend, travel, and navigate that entire distance over to the editor to get fixed. Okay, let's unpack this.
3:30If the printing press and the Shredder are separated by that much internal space, how does the DNA successfully make the journey? I know previous studies have tried to map this out, but from what I was reading, there was a major flaw in how we historically studied this mechanism.
3:45There was a profound flaw, yes. To understand how the polymerase moves, past researchers would often study it by taking it off its sliding clamp. The sliding clamp? Yes, the sliding clam. It's a rigid, ring shaped protein called PCNA.
3:59It completely encircles the DNA strand. Oh I see. You can think of it like a heavy steel carabiner that locks the polymerase enzyme onto the DNA safety line, so it doesn't float away into the cellular fluid.
4:09Right. But because the entire assembly is so complex, researchers would often study the polymerase in isolation, without that ring. That seems odd. Or, alternatively, they would feed the fully assembled machine, what scientists call the hollow enzyme, a piece of DNA, that already had a mistake prebuilt into it, just to see if the enzyme scissors would recognize and cut it.
4:32Wait, studying proofreading without the sliding clamp seems totally counterintuitive. That is like studying how a train switches tracks by taking the train off the rails entirely. That is a great way to put it.
4:44I mean, if that clamp is always there in real life. Holding the enzyme tightly to the DNA and dictating its angle wouldn't removing the clamp completely change how the DNA bends and moves inside the machine.
4:55You've hit on the exact reason why this new research is so groundbreaking. Yes, it completely changes the trajectory. Wow. In a living cell, pole epsilon does not just let go of the clamp to fix a mistake.
5:06It acts processively, meaning it stays locked onto the DNA for 1000000s of base pairs at a time. Millions. And this is incredibly relevant globally. When Paul Epsilon makes an error while walked onto the DNA, specifically a mismatch that forces a cytosine to guanine transversion.
5:24It creates the single most prevalent type of mutation found in human cancers. So understanding how this enzyme operates in its natural, constrained state, locked tightly to its clamp, is absolutely essential for understanding how our genome protects itself from cancer.
5:39Precisely. It is life or death on a cellular level. So the team needed a way to look at this microscopic machine while it was still fully assembled on the tracks, clamp and all. How did they actually manage to capture that level of detail?
5:51They utilized revolutionary imaging technique called cryoelectron microscopy for cryo EM. Essentially, you take your molecular sample and plunge it into liquid ethane to flash freeze it in a very thin layer of vitreous or non-crystalline ice.
6:07Wait, why does the ice need to be non-crystalline? Well, normal ice forms sharp, jagged crystals that would physically pierce and shred these delicate protein structures? Oh, that makes sense. Yeah. Vitreous ice forms so fast that the water molecules do not have time to crystallize, creating a glass like suspension.
6:24This traps the enzymes in their natural active states without damaging them. Fascinating. Then, you shoot a beam of electrons at them to take 1000s of high resolution snapshots. This team managed to refine those snapshots down to a resolution of 3.11 Engstroms.
6:393.11 Engstroms. Yeah, at that level of detail. You can literally see individual amino acids and the exact helical twist of the DNA ladder. But wait, if CryoEM is basically taking a static freeze frame, How do you catch the machine, the exact millisecond it makes a mistake?
6:55Wouldn't it just fix the error and move on before you can get the picture? That was the massive hurdle. Yes. To overcome it, they introduce 2 highly innovative steps. First, to ensure that the PCNA sliding clamps stayed attached securely to the DNA, they used a much longer piece of DNA than previous studies.
7:10How long? A 35 base pair, double stranded region. This long track gave the client plenty of physical room to hold on tight while the polymers did its job. Okay, that covers the clamp and the 2nd step. Here's where it gets really interesting.
7:22Okay. They force the enzyme to make a mistake live. What scientists call an in situ mismatch. Live on camera, basically. Exactly. Instead of handing the machine a DNA strand with a typo already in it, they essentially fed the molecular builder the wrong building block.
7:39Oh, sneaky. Right. instead of giving it the correct letter, DCTP. They flooded the cellular environment with the wrong letter, DTTP. So the mismatch occurred completely naturally, deep inside the printing site.
7:53Wouldn't the Shredder just instantly cut out the wrong letter the moment it was printed. Not if you break the shredder first. What? Yeah. The researchers used a mutated version of the enzyme. An XO minus mutant.
8:05It had all the moving parts. It could sense the mistake, and it could physically move the DNA into the shredder, but the actual cutting mechanism was deactivated. Oh, wow. So it just gets stuck. Exactly.
8:15This engineered a molecular traffic jam. The enzyme tried to fix the air, but got stuck holding the DNA in the proofreading position, allowing the researchers to freeze the machine right in the exact middle of the act.
8:27That is a brilliant experimental design, which brings us to what they actually saw when they developed those electron snapshots. And from what I understand, their very 1st control observation completely upended decades of textbook science.
8:42It was a massive plot twist in the field of structural biology. Tell me about it So as a baseline control. The researchers did what old studies used to do. They fed the fully assembled machine, clamp it all, a piece of DNA with a pre-existing mismatch.
8:58Okay. And the result was stunning. The rigid ring of the PCNA sliding clamp literally blocked the mismatched DNA from entering the proofreading site. Wait, I want to make sure I am visualizing this correctly.
9:10If the DNA already has a mistake. The sliding clamp physically gets in the way and stops the editor from fixing it. Why would it do that? Because of the angle of entry. The sliding clamp holds the DNA in a very specific rigid trajectory.
9:22When a premade mismatch approaches the assembled hollow enzyme from the outside, the clamp does not allow the DNA to bend at the severe angle required to thread the needle into the internal proofreading chamber.
9:32So it just crashes. Yeah, the mismatched DNA just physically collides with the outer structural wall, the enzyme. So decades of past studies, which used premade mismatches without that stably bound clamp, were basically looking at an artificial process.
9:47Precisely. Those older studies do not reflect physiological reality. A premade mismatch, just bumps against the outside. To trigger the true proofreading mechanism. The mistake must originate from within the builder site itself.
10:01Because this team forced the mistake in Situ, live on camera, they were finally able to watch the authentic internal process unfold. Yes, and they captured 3 distinct, never before seen states of this molecular choreography.
10:16Let's walk through them. What happens the exact millisecond the wrong letter is printed? We enter the 1st intermediate. the mismatch locking state. The enzyme immediately senses the physical distortion of the wrong letter.
10:26The standard Watson Crick base pairing fails. Just to clarify, Watson Crick pairing is the standard puzzle piece rule of DNA, right? Where adenine only fits perfectly with thymine, and cytosine only fits perfectly with guanine.
10:38Correct. When the enzyme accidentally forces a mismatched pair together, the puzzle pieces physically clash. Which creates a bulge. Exactly. It creates a microscopic bulge in the DNA ladder. The enzyme detects this bulge, and in response, a part of its structure called the fingers domain physically opens up.
10:55Like it's dropping its tools. Yes. This structural shift instantly halts the printing press. But here is the surprising action. The entire enzyme actually moves one base pair forward along the DNA. Forward.
11:10If it just printed a mistake and wants to back up to cut it out, why take a step forward? It steps forward to lock the mismatch in place. By shifting the newly made error, one position down into what is called the post insertion site, it ensures the active printing press cannot accidentally grab another nucleotide and extend the DNA past the mistake.
11:29Oh, so it essentially quarantines the typo so it cannot be permanently buried under new letters. Exactly. It isolates it. Okay, so the typos quarantine. The fingers are open. The printing has stopped, but it still has to somehow get the typo to the scissors, 40 Engstroms away.
11:43This initiates the 2nd intermediate. The pull backtracking state. The enzyme utilizes a part of its structure called the thumb domain, along with a specialized anchor loop. You could picture these as tiny mechanical livers.
11:56Levers pushing on the DNA? Yes. They physically push against the DNA backbone. The DNA unwinds by exactly one base pair, separating the 2 strands, and the entire DNA helix starts sliding backward, away from the printing site and toward the exocyte.
12:11So it actively throws the machinery in reverse, but it still has a long distance to cross. Which brings us to the 3rd intermediate, the mismatch editing state. What's fascinating here is the sheer mechanical force the enzyme exerts.
12:24Force, like pulling it. Twisting it. A highly unique domain of pole epsilon called the p domain tilts by 12 degrees. Simultaneously, a structural connector called the Linker Helix violently rotates 36 degrees.
12:3736 degrees. On a molecular scale for a protein structure holding onto a rigid DNA molecule, that sounds like a massive, violent structural shift. It is a huge structural torque. This violent twisting motion forces the DNA to backtrack even further.
12:52And in the process of that violent twist. The enzyme physically melts apart, a massive 6 base pairs of DNA. Six. Six base pairs. That is the longest stretch of unwinding ever seen in any proofreading study.
13:04For context, previous studies on other types of polymer races usually only saw about 3 base pairs unwind. Hang on, 6 space pairs unwound. Inside that tiny molecular chamber, if you forcibly unzip 6 rungs of the DNA ladder in such a confined space, wouldn't the 2 strands just wildly tangle up?
13:23Or worse, wouldn't they just snap right back together before the machine can navigate the typo into the shredder? In almost any other biological system, that is exactly what would happen. They would snap back together.
13:33But Paul Epsilon possesses a unique structural feature in its proofreading domain called a beta hairpin loop. This loop is uniquely short compared to other polym races. And a short loop means what? Because it's so short, it cannot cleanly keep the separated DNA strands apart like a wedge.
13:50So the unwound DNA actually scrunches up inside the chamber, and the strands begin to bind back together incorrectly. Wait, they bind together incorrectly? Yes, forming what we call out of register-based pairs.
14:03Oh, I see. So it is essentially like a buttoning a shirt, but you miss a buttonhole. You keep buttoning all the way up, but the fabric scrunches up awkwardly and the collar is totally misaligned at the top.
14:13That is a perfect visualization. It is exactly like buttoning a shirt incorrectly. The DNA strands repair with each other, but entirely out of their proper alignment. So they form these weird, scrunched up shapes.
14:26Yeah, they form these bizarre, scrunched up, mismatched pairs lower down the strand, but this molecular scrunching serves a beautiful, highly specific purpose. Which is? By scrunching up the fabric of the DNA, it shortens the physical distance required to reach the shredder.
14:41It protects the rest of the genetic code, leaving only the single wrong, mismatched nucleotide, dangling straight into the molecular scissors. Wow. It deliberately creates a tangled, scrunched up mess of the surrounding DNA just to perfectly isolate the one bad letter in the crosshairs of the Shredder.
14:59That is unbelievable engineering. So what does this all mean? Well, on a fundamental level, It requires us to completely rethink polymerase mechanics. How so? We now know that the PCNA sliding clamp is not just a passive carabiner holding the enzyme to the track.
15:16It's an active constraint that forcefully dictates the exact trajectory the DNA must take during proofreading. It's basically steering the whole process. Exactly. Furthermore, that pedomain we discussed earlier.
15:28Biologists previously assumed it just helped the enzyme grip the DNA. Now we have proven it has a critical 3rd function. It actively powers the violent melting and repositioning of the DNA strands. And there has to be a direct clinical application here.
15:42We mentioned earlier that errors made by this specific enzyme are a massive driver of human cancers. There is a profound clinical link, yeah. Because the researchers could see the enzyme down to the individual amino acid.
15:53They mapped out the exact structures involved in this scrunching process. They found a specific amino acid building block in the enzyme called proline 286. Proling 286. Yes. Prolene is a very structurally rigid molecule.
16:07During this scrunched out of register state, proline 286 acts as a physical wedge to keep the strands separated just enough right at the top to expose the typo to the scissors. A physical wedge. So if something happens to that specific proline molecule, the whole proofreading system breaks down.
16:23Completely breaks down. Medical oncologists have known for a long time that genetic mutations occurring at this exact spot, specifically mutations where prolene 286 is swapped out for a lycine or an arginine, known as P286K, or P286R, devastate the enzymes proofreading fidelity.
16:40These specific mutations are documented, well-known drivers of highly aggressive human cancers, particularly colorectal and endometrial cancers. Because swapping the rigid prolene for a bulkier molecule changes the shape of the wedge.
16:52Exactly. For years, we knew the mutation caused cancer, but thanks to this structural snapshot, we finally know why. The wedge fails. Right. If that proline wedge is mutated and loses its rigid shape, the scrunch DNA cannot be properly stabilized.
17:06The strand slip, the typo escapes the scissors, it becomes permanently embedded in the genome, and a cancer causing mutation is born. Connecting a microscopic atomic wedge directly to a colorectal cancer diagnosis is staggering.
17:19Are there other types of genetic errors this structural scrunching helps explain? It might finally solve a long-standing mystery regarding a phenomenon called replication slippage. What's? Sometimes when a DNA sequence has a long run of the exact same letter, let's say, 10 atomines in a row, the plumber race enzyme gets confused during copying and slips, accidentally adding an extra letter or missing one.
17:43Like losing your place while reading. Yeah. If the DNA utilizes this out of register buttoning mechanism to scrunch up during proofreading, you can easily imagine how a stretch of identical letters might snap back together.
17:54Because they are all the same letter. They pair up perfectly, but they are completely out of alignment. Oh, the machine buttons the shirt perfectly. But because all the buttons look identical, it doesn't realize it started on the wrong buttonhole until it loses its place entirely.
18:09Exactly. That makes so much sense. But it also makes me wonder. If this highly unique pedomain and this short beta hairpin loop are specific to pull epsilon, do other machines in our body use totally different tricks to proofread?
18:23I mean, could they actually drop the clamp, or do they have their own ways of scrunching the DNA? You've identified the exact next frontier for this field of research. Really? Yeah. Pol Epsilon handles the continuous leading strand of DNA.
18:37But we have a whole host of other polymerases. We have pole delta working on the lagging strand. We have completely different polymerases operating side, our mitochondria. Do they scrunch too? Do they also scrunch their DNA into out of register pairs?
18:49Do they maintain constant contact with their own sliding clamps? The reality is we simply do not know yet. The researchers make it explicitly clear that the immediate next step is to apply this exact in situ methodology, forcing the live mistake on camera to other polymerases.
19:06We need to find out if this violent scrunching mechanism is a universal law physics for DNA replication across the board, or a highly specialized trick, unique to this one specific leading strand machine.
19:18It is incredible how one beautifully designed study can answer a decades old structural question, while simultaneously opening up 10 new avenues of inquiry. That is the enduring beauty of structural biology.
19:31So to synthesize what we have unpacked today. Please do. By observing the human DNA polymerase tightly bound to its sliding clamp and forcing it to make a mistake naturally. Researchers uncovered an entirely new proofreading mechanism.
19:43To fix a single typo, the enzyme must violently twift and melt 6 base pairs of DNA, scrunching the strands into an out of register alignment, to perfectly position that single error for removal. A violent, beautiful microscopic ballet happening 1000000000s of times inside you right now, just to keep your code intact.
20:00If we connect this to the bigger picture. What does this mean for our understanding of how other massive molecular machines handle errors and could targeting these scrunched intermediate states be the key to stopping cancer mutations before they even start?
20:13It is a question that will undoubtedly drive the next decade of genomic and oncological research. This episode was based on an open access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description.
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20:42Thanks for listening and join us next time as we explore more science, base by base.
