HHMI international research scholars in Venezuela and Russia are helping each other piece together the molecular puzzle of muscle contraction.
When it comes to making machines the size of molecules, says Raúl Padrón, a Venezuelan structural biologist who has spent the past two decades studying the molecular basis of muscle contraction, the most miraculous feats of modern day nanotechnologists can't come close to the engines that nature has devised already.
"Inside every muscle cell, there are two sets of filaments arranged like this," explains Padrón, who works at the Venezuelan Institute for Scientific Research in Caracas, as he sketches a series of offset parallel lines. They look a bit like a pair of combs that have been pushed together so that the teeth of one interlock with the teeth of the other. The "teeth" coming in from one side are long, thin filaments that are mostly made of a protein called actin, he says. The teeth coming in from the other side are bulkier structures known as the thick filaments. "In contraction, the filaments slide together," he says, in effect pulling the two combs inward and causing the muscle as a whole to exert a force.
And how does that happen? "Start with the relaxed state," says Padrón, who has made that phase his specialty. "On the surface of each thick filament there are hundreds of myosin molecules—tiny motors about 16.5 nanometers long." Each of these myosin molecules has two identical heads that are linked to the filament by molecular cables, he says. And each head, in turn, is a complex, three-domain protein whose structure is now well understood. "But what we've been studying is the organization of these motors using electron microscopy. We've found that in the relaxed state, when they aren't being used, the myosin heads lock down onto the surface of the large filament so that they can't interact with the actin filaments." Indeed, he adds, each immobilized myosin head links to its neighbor in a helical chain, making the thick filament look something like a submicroscopic barber pole.
However, says Padrón, when a nerve impulse arrives at the muscle with a command to contract, the interior of each cell is suddenly flooded with calcium ions. "We've found that the calcium ions go to a regulatory site on the myosin molecule. This releases the heads, so that they are free to explore and look for the actin fibers."
Once the myosin heads attach to the actin filaments, they can begin the contraction phase, which is the specialty of two other HHMI international research scholars, both from Russia, whom Padrón met at the Institute in June: Andrey Tsaturyan of Moscow State University and Sergey Bershitsky of the Urals Branch of the Russian Academy of Sciences in Yekaterinberg.
Conventionally, scientists have thought that the force of muscle contraction is generated by the myosin heads alone, tugging on the thick filament like so many levers, explains Bershitsky, who has worked with Tsaturyan for two decades. "But our major finding is that muscle force is generated by a realignment of the myosin heads that enables them to lock into position on the actin filaments. It's rather like a zipper; each element has to be in the right position to lock," he says. By flash-freezing muscle cells in various states, which immobilizes the myosin heads, and using techniques known as temperature-jump, flash-photolysis and time-resolved x-ray diffraction to determine the molecules' precise configuration during contraction, he and Tsaturyan have shown that the attached myosin heads wiggle at random for a time, generating no net force, and then suddenly close ranks, locking onto the actin filaments in a precise order. In the process, they draw the filaments together strongly.
Unfortunately, says Bershitsky, "X-ray diffraction picks up only the ordered structure, so you can't say what is going on in the disordered regime." For that, you need electron microscopy—which is precisely why he and Tsaturyan jumped at the opportunity at HHMI to introduce themselves to Padrón, an electron microscopy expert. The three men now hope to collaborate, if possible, forming an unusual biomedical research link between Venezuela and Russia—one with some muscle behind it.