Single-molecule optical tweezers and fluorescence reveal how the S. pombe Pif1-family helicase Pfh1 alternates ATP-dependent unwinding and ATP-modulated rewinding at replication-fork-like substrates, and how mitochondrial SSB spRim1 tunes those activities.
0:00Welcome to Base by Base, the paper cast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. You know, every single time a sell in your body divides.
0:11It undertakes, well, one of the most astonishing logistical operations in the universe. Oh, absolutely. It's a massive undertaking. Right. It has to perfectly copy over 3000000000 base pairs of DNA. And to do this, it relies on a whole fleet of microscopic molecular motors.
0:27Yeah, these tiny biological machines. Exactly. These motors are called hella cases. And they have this very specific incredibly vital job. They have to pry open the intertwined strands of the DNA double helix.
0:40You know, so the genetic code inside can actually be read and copied. Like unzipping a zipper. Yeah, exactly like that. But here's the thing that really blew my mind when I was looking through our sources for this deep dive.
0:51What happens when this tiny motor needs to regulate its speed? Ah, yeah, that's the real problem. Because it's zipping along the DNA, but the cellular environment is incredibly crowded. If it goes too fast, it could literally crash into other cellular machinery.
1:06Which would be catastrophic. Right. A microscopic pileup that could utterly destroy the cell. So my question is, how does a motor that is, you know, 1000000000s of times smaller than a car, know when to push forward and when to pump the brakes?
1:19It is a remarkable engineering problem except, you know, it is playing out at the nanoscale. I mean, these proteins don't have eyes. They don't have a central nervous system to make decisions. Yeah, no little brain telling them what to do.
1:31Exactly. They are governed entirely by physics, chemistry, and structural tension. And the mechanisms they use to prevent those cellular crashes are far more sophisticated, and frankly, um, more dynamic than biologists originally thought.
1:45Today we celebrate the work of the researchers at IMDEA Nanociencia in Madrid, Washington University School of Medicine, and University.com, Lucans, De Madrid, who have advanced our understanding of how molecular motors regulate themselves during DNA replication.
2:02It's really brilliant work. It is. Their work gives us a brilliant front row seat to this microscopic balancing act. So to really understand this balancing act, we need to talk about a specific family of these biological motors, known as the PIF1 family helicases.
2:18Okay, the Piff one family. Right. These are ancient evolutionarily conserved proteins. You find them in bacteria. You find them in yeast, and you find them right now, working away inside human cells. Just doing their thing in our own bodies.
2:32Exactly. And they operate as 5 prime to 3 prime directed motors. Which means what exactly? In practical terms, that means they are strictly directional. They move one way down the track of the DNA strand from the 5 prime end to the 3 prime end.
2:47Got it. One way street. Right. They are also ATT dependent. They use ATP, which you can think of as the universal chemical fuel of the cell, to power this forward movement. Like gasoline for the tiny engine.
3:00Exactly like gasoline. And they are absolutely essential for maintaining the integrity of both the DNA in the cell's nucleus and the DNA inside our mitochondria. So they're versatile. Very. They do a bit of everything to keep the genomes stable.
3:13For instance, they inhibit telomerase, which prevents the ends of our chromosomes from growing out of control, and they process Okazaki fragments. Let me just jump in here. I wanna make sure you listeners have a clear picture of what that means.
3:26When DNA is copied, one strand is copied continuously, but the other strand has to be copied in short, disconnected chunks, right? Just because of the direction the replication machinery moves. Yeah, that's the lagging strand.
3:40Right. And those disconnected chunks are the Okazaki fragments. Exactly right. They are these little fragments formed on the lagging strand of the DNA fork, and they eventually have to be stitched together.
3:50So piff one helicases are critical for cleaning up the edges of those fragments. So the stitching process happens seamlessly. Okay, let's unpack this because in the sources we're examining today. The researchers didn't look at human cells for their primary data.
4:04No, they didn't. And they didn't look at the standard baker's yeast, sacchromyces or Vigier that, you know, biologists seem to use for almost everything. The standard workhorse, yeah. Right. They specifically chose to study fish and yeast.
4:16Schizosechromyces, Pombe. Why study fish and yeast instead of the standard budding yeast? What makes this specific organism, the golden ticket for understanding this machinery? Well, it comes down to biological clarity.
4:29You see, budding yeast actually has 2 different PIF1 family helicases. Two of them. Yeah, named Pif1 and RM3. They are what we call paralogues. A paralog is essentially a duplicate version of a gene that arises during evolution, which, you know, allows the 2 resulting proteins to split the cellular workload.
4:49So they share the job. Right. So in budding yeast, if a cell loses one of those helicases, it can often still survive because the other parallog picks up the slack. It is a backup. Exactly. But fish and yeast, just like us humans, only has one single PF1 family helicase, which they call PFH1.
5:06Oh, wow. So it's a soloist. There is no backup system. Precisely. Because PF1 and Fish Knees has to do the combined jobs of both of those paralogs all on its own, it is absolutely essential for the survival of the cell.
5:17Yeah, if you want to understand how this vital machinery operates and regulates itself without any backup systems muddying your data, fish and yeast is the perfect model. The mechanics of how it handles that immense solo responsibility can tell us a lot about our own cellular engines.
5:34So we have our perfect biological model. But, you know, that brings up a massive logistical question for the researchers. We are talking about a single protein. A very, very small protein. Right. This thing is smaller than the wavelength of visible light.
5:48We can't just put it under a standard laboratory microscope and watch it move. So how do we actually see it working? How do you measure the speed of a single motor prying apart a single strand of DNA? What's fascinating here is the sheer ingenuity of the methodology.
6:03To solve this visibility problem, the researchers used an incredibly advanced technique called dual beam counterpropagating optical tweezers. See that again, optical tweezers? Optil tweezers, yeah. And they simultaneously combine that with convocal fluorescence microscopy.
6:18Okay, let's break that down for the listeners because optical tweezers sound like something straight out of science fiction. You are physically manipulating microscopic objects using nothing but light.
6:29You literally are. I mean, light carries momentum. If you tightly focus a laser beam through a microscope objective. You can actually create an optical trap. That is wild. It is. The forces of the light physically hold onto a microscopic object, but the optical tweezers are only half the setup.
6:47They paired this with convocal fluorescence microscopy. Okay, so what does the microscope add to the lasers? Well, while the lasers of the tweezers act as the physical hands holding the structure, the confocal microscope acts as the eyes.
7:00I see. Yeah, it uses highly focused lasers to excite specific fluorescent markers. allowing the researchers to pinpoint the exact location of single molecules with incredible resolution. It completely eliminates the background blur you'd get with a normal microscope.
7:16So they have the hands to hold it and the eyes to see it. What exactly are they holding in this laser trap? They set up this incredibly precise molecular stage. They created a DNA hairpin, a hairpin. Think of a tiny strand of DNA, exactly 559 base pairs long, that is folded back on itself and zipped shut.
7:33And they tethered the 2 ends of this DNA hairpin between 2 microscopic polystyrene beads. So if I'm picturing this right, you have two tiny plastic beads suspended in liquid, being held perfectly still by these invisible optical laser traps.
7:48Yes. And strung between those beads like a highly tensioned tightrope, is this single folded piece of DNA. That is the exact setup. And because the researchers control the lasers, they can pull the beads apart slightly, applying a very specific controlled physical force to the DNA tightrope.
8:06How much force are we talking about? In this experiment, they applied forces ranging from 5 to 11 Pika Newtons. I want to pause on that unit of measurement for you listening, because Pika Newton is hard to wrap your head around.
8:17A Newton is already a small amount of force, and a Pika Newton is 100000000th of a Newton. It's vanishingly small. To put that in perspective, the downward force of a single tiny grain of salt resting on the palm of your hand is about 10,000 pike Newtons.
8:32So the force they are applying with these lasers is an unimaginably delicate whisper of physical tension. It is an incredibly delicate environment, but at the molecular scale, those few pike and Newtons are enough to keep the DNA hairpin taut and completely closed.
8:47Once the stage is set, they introduce the PFH1 helicase and its ATP fuel into the liquid. As the helicase engages the DNA and begins to unwind the hairpin, the folded tightrope opens up. And what happens to the beads?
9:00As it opens, the tether gets slightly longer, and the microscopic beads physically drift further apart. By measuring that exact distance between the beads in real time, they can infer the exact biological action of the enzyme, they can literally watch a single PFH1 helicase, unwind the DNA, base pair by base pair.
9:19So we are tracking the motor's progress by measuring the subtle drifting of laser tracked beads. That is an astonishing way to gather data. It's beautiful system. So let's talk about what they actually saw when they turned the lasers on and watch this thing go.
9:30When I think of a motor opening up a path, my brain naturally goes straight to a snowplow. A snowplow? Yeah, you know, you fire it up, it hits the double helix and it just steadily plows down the road, shoving the 2 strands of DNA apart in one smooth, continuous push until the job is done.
9:47Did the data show a microscopic snowplow? The data strongly rejected the snowplow analogy, actually. Really? Yeah. The hella case does not just plow forward indefinitely. Instead, PH1 operates in what the researchers define as unwinding, rewinding cycles.
10:04Unwinding and rewinding. Right. It moves forward, chemically separating the DNA strands, but only for an average burst of about 20 base pairs. And then it completely stops and it snaps back to where it started.
10:16Wait, so it's artificially governed. It does 20 steps of hard work and then it just gives up and reverses course. It reverses course, allowing the 2 separated strands of DNA to rezip or rewind back together, and then it starts the entire process over.
10:29That seems so inefficient. Well, what is crucial to understand here is that this 20 base pair limit, which biochemists refer to as its processivity, is an intrinsic, hardwired physical limit of the enzyme.
10:41Hardwired. Yeah. The research has tried everything they could to break that limit. They increased the force on the optical tweezers, literally pulling harder to pre-open the DNA and help the motor along.
10:52Did that work? No, they flooded the surrounding liquid with massive excess amounts of ATP fuel. Nothing changed it. It hit that 20 base pair wall every single time. That is fascinating. But what about its speed?
11:06Did flooding the injury with fuel or pulling on the track change how fast it covered those 20 base pairs before it hit the wall? Yes, the dynamics of speed and force are very revealing about how the motor functions.
11:16As you would logically expect for an engine if you give it more fuel, like more ATP, the speed of unwinding increases. It scales up from about 10 base pairs per 2nd all the way to a maximum velocity, or VMAX, of about 75 base pairs per second.
11:30But the force dynamics were entirely counterintuitive. Let me think through this. If you're using the tweezers to apply pulling force on the DNA strams, you are essentially pulling the track apart for the motor.
11:42I would assume that pretension makes the motor's job easier, right? It should be more efficient. You would certainly think so, but the data showed the exact opposite. Applying that pulling force actually decreased the helicase's catalytic efficiency.
11:56Pulling the track apart made the motor worse at unwinding it. Why? How does that mechanism even work? It comes down to the immensely delicate physical grip the protein maintains on the DNA structure. The researchers calculated that the applied pulling force causes a tiny conformational change in the whole system.
12:14Like the protein changes shape. Yes, specifically, it causes a structural shift of about .8 nanometers. Right. That tiny shift corresponds to the helicase being forced to release at least 2 single stranded DNA nucleotides that it normally holds tightly within its grip.
12:30So it's like a rock climber gripping a rope. If you suddenly pull the rope perfectly taut from both ends. The climber's hand is physically forced open slightly, that .8 nanometer shift. That is an excellent way to visualize it.
12:43And because the protein's hand is forced slightly open, the internal pocket of the protein, the specific place where it binds and burns its ATP fuel, gets physically distorted. Oh, the fuel tank gets crushed.
12:56Sort of yeah. The whole architecture of the enzyme warps just enough to reduce its chemical affinity for ATP. It literally makes it physically harder for the motor to grasp and process its energy source.
13:07Here's where it gets really interesting. In biology. context is everything. So far, we've been talking about the helicase wedging its way into a closed hairpin, a structure called a DNA fork. Right, the fork.
13:19But what happens if the motor is just driving along a simple flat road? How does unwinding a complex DNA fork compared to just moving along a single strand of DNA. That contrast was one of the most revealing parts of the entire study.
13:32Suggest this, the researchers took the hella case and placed it on a long single strand of DNA. There was no fork to pry apart, no double helix to wedge into, just a straight unencumbered track. On this single stranded DNA, its maximum speed was actually much faster, clocking in at about 109 nucleotides per second, compared to the maximum of 75 we saw when it was fighting the fork.
13:54Okay, so it drives faster on an open highway. That makes perfect logical sense. It does, but top speed isn't the whole story. We have to look at catalytic efficiency, which is basically a measure of how effectively the enzyme converts its ATP fuel into forward motion.
14:10Even though it's raw top speed is higher on a single open strand, its catalytic efficiency is roughly 35 fold higher when it is actively unwinding a fork compared to when it's just cruising along single stranded DNA.
14:23Wait, 35 times more efficient? Yes, 35 fold. That's not a marginal difference. So it moves faster on the open highway, but it burns fuel much more efficiently when it's actively wedging the fork apart.
14:34The physical obstacle itself is the key to the engine's efficiency. Exactly. It tells us that PFH1 is not just a generic brute force motor. It is a highly specialized tool, biologically optimized for the fork structure.
14:47That's incredible. Yeah, the physical act of engaging with the intersection of both DNA strands actually changes the protein's internal conformation, locking it into a shape that makes it incredibly efficient at binding and utilizing ATP.
15:01It's a beautiful example of form fitting function, but I want to transition casually here. It's fascinating that the fork structure itself acts as a key to its efficiency. But inside a living cell, specifically inside the mitochondria, This motor isn't operating on a bare scheral track, is it?
15:18Oh, definitely not. There's a whole crew of other proteins crowding the area. If you add its natural biological partner proteins to the mix, does that added bulk slow the motor down? It's a great question because inside the mitochondria, as the helicase unwinds the DNA, those newly exposed single strands, don't just sit there naked.
15:36Right, they'd be vulnerable. They're immediately coated and protected by specific proteins. In this phase of the experiment, the researchers introduce the mitochondrial single stranded DNA binding protein known as sprim one.
15:47Okay, sprim one. So if we introduce sprim one, does it help the motor break that stubborn 20 base pair limit? Does the processivity change? It does not. The intrinsic 20 base pair processivity limit remained exactly the same, proving it really is hardwired into the motor.
16:04Unbreakable. Right. However, adding sprim one significantly boosted the motor's maximum unwinding velocity, pushing its top speed at the fork up to 115 base pairs per second. Wait, if the distance limit didn't change, and you're adding more bulky proteins to the track, how did it make the motor go faster?
16:22It has to do with how the helicase physically interacts with the DNA, it's actively pushing out of its way. The displays strand. Yes. As PH1 moves forward and wedges the strands apart, the display strand tends to flop back and interact with the back of the helicase.
16:37This creates a sort of molecular friction or physical drag that slows the engine down. Oh, I see. When sperm one is introduced to the environment, it immediately binds to that displaced DNA strand. By binding the strand, Sprim one essentially competes with the helicase's own internal interaction.
16:53It gets it out of the way. It actively sweeps that displaced strand out of the way, completely removing the physical drag. The motor can spin faster because its molecular exhaust pipe, so to speak, is clear.
17:05is so elegant. And as a fascinating side note, the researchers found that substituting the human version of this binding protein stimulated the yeast helicase to a similar extent. Really? A human protein worked on the yeast motor.
17:19It did, which proves that this speed boost is all about managing the physical architecture of the DNA, not relying on some highly specific evolutionary protein to protein handshake. So what does this all mean?
17:33We have a motor that is incredibly efficient when wedged into a fork, can go even faster when its partner proteins clear its exhaust path, but absolutely refuses to unwind more than 20 base pairs at a time without throwing itself in reverse.
17:46Yep, that's the picture. Why does the helicase do this repetitive 20 base pair dance of moving forward and backwards? If we connect this to the bigger picture, this unwinding and rewinding cycle is not a glitch.
17:57It is a critical biological safety feature. A safety feature. Yes. It is called a sliding back rewinding mechanism. Here's the mechanism the data suggests. As PFH1 wedges forward. The physical tension of holding that fork open builds up.
18:12Like stretching a rubber band. Exactly. When it hits that intrinsic 20 base pair limit, the tension becomes too much. The enzyme's grip physically distorts and transiently loosens. Because the 2 separated DNA strands naturally want to snap back together, a chemical process called reannealing, this powerful snapping force literally drives the loosened motor backward along its track.
18:35So it's like a wind up toy, where the spring is firmly attached to a heavy door. You can only wind the toy forward so far before the sheer tension of the door overpowers the internal spring and violently unwinds the toy back to a starting position.
18:49That captures the physics perfectly. The tension overpowers the forward drive. But we have to be careful because there was a major competing theory in the field that the researchers needed to disprove.
18:58Oh, there was another idea. Yes. Some scientists hypothesized that the hella case wasn't passively slang backwards due to tension, but rather executing an active maneuver called strand switching. Strand switching.
19:09The theory was that upon stalling, the motor would jump across the fork to the opposite DNA strand and actively drive in the other direction, plowing away from the intersection. That sounds difficult to prove or disprove if both actions result in the motor ending up back where it started.
19:24So how did they definitively prove it was sliding back passively, and not actively strand switching. They proved it through a brilliantly clever molecular substitution. They built a hybrid fork where the track it was driving on was normal DNA, but the display strand, the one it would supposedly jump to, was made of RNA.
19:44RNA instead of DNA. Right. Now, it is a proven biological fact that this specific PFH1 helicase is completely structurally incapable of translocating, of moving directionally on single stranded RNA. It simply cannot grip or drive on it.
19:59Ah. So if the strand switching theory was correct. The motor would hit its 20 step limit, jump across the gap to the RNA strand, realize it was on a track it couldn't drive on, and the whole rewinding process would completely fail.
20:11It would just be stuck. Exactly, but that isn't what happened. It wasn't stuck. When they observed the motor. On the RNA DNA hybrid fork using the optical tweezers, PSH1 rewound identically. The backward sliding happened at the exact same speed and the exact same frequency as it did on normal DNA.
20:29Because it could successfully rewind on a track, it is biologically incapable of driving on, it proved definitively that the motor isn't driving backwards. It is being pushed backwards by the tension of the double helix snapping shut.
20:42That RNA hybrid test is such a clever piece of scientific detective work. It really is. But let's bring this whole mechanism back to the living cell. Why does a cell want a motor that constantly slips backward?
20:53Why expend precious ATP fuel to build a machine that works so hard to make 20 steps of progress, only to immediately slide 20 steps back? Think back to the collision problem we discussed at the very beginning of our conversation.
21:05The microscopic pile up. Right. This sliding back mechanism acts as a highly sensitive built-in break. During DNA replication. This hella case is clearing the path for an even larger, vastly more complex piece of cellular machinery called the repless sum.
21:20Okay, the replica. The replicum contains the leading strand polymerase, which is the actual enzyme copying the DNA letters. If the helicase were a snowplow that just burned ATP and sped forward uncontrollably, it would severely outpace the polymeries.
21:36And that's bad. Very bad. It would expose far too much vulnerable, single stranded DNA to damage, or worse, it could crash into other regulatory protein roadblocks ahead of it. By constantly moving forward exactly 20 base pairs, checking the structural tension of the track and sliding back.
21:53It ensures that PSH1 can promote the forward progression of the replication fork without ever accidentally speeding ahead. It acts as a governor that preserves the entire structural organization of the reposum.
22:05It's essentially a biological scheme. It runs ahead 20 paces, clears a little bit of brush, checks the tension of the environment, and falls back to the main group. A scout, yeah, exactly. It repeats this endlessly.
22:15Keeping the path open, but never, ever getting separated from the convoy. That's a great way to put it. So to distill this all down for you listening. PFH1 does not plow blindly through our genetic code.
22:27It operates through highly coordinated unwinding and rewinding cycles, strictly limited to short 20 base pair bursts. And by utilizing an ATP dependent sliding back mechanism, which is tightly regulated by structural tension and partner proteins like sprim one, the helicase dynamically balances moving forward with backing off, ensuring smooth DNA replication without causing fatal molecular collisions.
22:51Got it perfectly. Which leads me to wonder, what does this mean for our understanding of molecular motors and human diseases where DNA replication goes terribly wrong? Because if this tension-based sliding is how healthy cells prevent genomic pileups, the implications for when things break seem massive.
23:07This raises an important question, and it is exactly where this fundamental physics research intersects with the future of medicine. Well, we know that in many cancers, DNA replication is completely dysregulated.
23:19The tumor cells are copying their DNA at a frantic, highly unstable pace, leading to uncontrolled cellular division. If we now know that this vital hella case relies on physical tension, specifically that .8 nanometer confirmational shift we talked about, to trigger its built in sliding back break, could future synthetic drugs be designed to artificially induce that exact behavior.
23:42Imagine a targeted therapeutic molecule that slips into the helicase and slightly alters its grip, forcing that .8 nanometer shift permanently. jamming the brake pedal, exactly. It would lock the motor into a permanent state of sliding back, effectively freezing the runaway DNA replication in cancer cells without directly damaging the DNA strands themselves.
24:05By understanding of how the brake title works at the Pika Newton level, we might one day learn exactly how to press it. That is a staggering thought. Designing a molecule to leverage a cell's own tension-based safety mechanisms, turning the brakes against the diseases that threaten it.
24:20It's the ultimate goal of understanding these tiny machines. 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.
24:32If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating. If you'd like to support our work, use the donation link in the description. Now stay with us for an original track created especially for this episode and inspired by the article you've just heard about.
24:47Thanks for listening and join us next time as we explore more science base by base.
