RNA Export Is Like Basketball, Only Better
Imagine playing basketball blindfolded, but having an assistant pull the ball through the net when you get close. Now you have an idea how messenger RNA molecules make it out of the nucleus.
One of the most memorable parts of Illustra Media's intelligent-design documentary Unlocking the Mystery of Life is the animation of DNA transcription and translation. If you've seen it, you may remember the messenger RNA molecule with its transcript of the DNA code reaching the nuclear pore complex, pausing, then waiting for the net-like gate to open before it moves out like a freight train.
In real life, it's more like a basketball game. Since Unlocking was released in 2003, we've learned more about RNA export. Some of the latest findings have just been reported on PhysOrg. It's been known (as portrayed in the film) that the nuclear pore complex has a net-like structure resembling a basketball hoop. Researchers at the University of Bonn (Siebrasse, Kaminski and Kubitscheck), publishing in PNAS, found out that messenger RNA (mRNA) doesn't always get a swish. Sometimes it hits the inside wall of the nuclear membrane as it tries to find the net. It only scores on about 1/4 of its attempts:
Interestingly enough, only about every fourth collision between arriving messenger RNA and the cell nucleus leads to a successful export. Here, two kinds of processes can be distinguished: On the one hand, brief collisions with the nuclear membrane where presumably no pore is hit, and, on the other hand, those transports that are slowly aborted perhaps on account of a deficient quality control.This is not surprising, considering that mRNA is blind, trying to shoot hoops at random in the dark. But no worries; helper molecules are at the ready to get the ball through the net.
First, proteins package the mRNA in a kind of suitcase. Then, helpers on the outside (in the cytoplasm), may be involved in pulling the mRNA through the net.
The RNA is packed in a type of "suitcase" made of proteins for transporting. "And it is quite a chunk," grins Prof. Kubitscheck. This is why some of his colleagues presume there are helpers on the outside of the cell's nucleus which pull the "suitcase" through the pores, a theory which the professional physicist together with the molecular biologist Jan Peter Siebrasse are currently investigating.We need to pause and realize that photographing these nano-size molecules in real time is quite a challenge. Hats off to the researchers who get to be some of the first spectators of the game and can relay the action for us. They did, indeed, see the mRNA pause before going through the pore -- and they have an idea why:
The working group has found out that the messenger RNA lingers briefly at the pores in the membrane of the nucleus before it is finally transported out -- presumably for a final "quality control" or simply because it has to adjust in order to leave via the pore exit. The export process lasts in total only a few hundredths of a second to several seconds. "In all likelihood, the process needs much longer for larger, voluminous messenger RNA molecules than for smaller ones," adds Prof. Ulrich Kubitscheck, head of the working group Biophysical Chemistry and senior author of the publication.In short, like basketball, this game has referees. And the hoop is no ordinary basketball net. It's a controlled-access gate that protects the nucleus from intruders and also inspects the cargo that's leaving.
That is why the mRNA needs to be checked, like an athlete having his credentials inspected, before being allowed on the court. Imagine playing on a court with a basketball hoop that did quality control on the ball, and could re-shape it if it were too big to pass through. For a cell to get all this done in a few hundredths of a second to several seconds is enough to give Jeremy Lin a run for his money.
The short article, which did not mention evolution once, underscores why Dr. Dean Kenyon, at the end of the animation in Unlocking, exclaimed with obvious delight, "This is absolutely astounding to perceive, at this scale of size, a device -- an apparatus -- that bears the hallmarks of intelligent design and manufacture!" That was in 2003; now we know it's even better, almost as much fun as watching a fast-moving professional basketball game.
Photo credit: Wikicommons.