For 50 years researchers have puzzled over how the animal cell manages to organize a critical component of cell division into a microtubule-rimmed cylinder with a distinctive nine-spoked cross-section. Now, Pierre Gönczy, at the Swiss Federal Institute of Technology, and colleagues have discovered that the three-dimensional structure of a key protein component directly specifies this unusual pattern.
Barrel-shaped organelles, the centrioles organize the spindle fibers that pull paired chromosomes apart during cell division. They also form the basal bodies of cilia and flagella in all eukaryotes possessing these appendages. Centriolar structure has long been a curiosity among researchers because nobody could figure out how the ninefold structure formed. It’s a “big question that has always perplexed people,” says Andrew Fry at the University of Leicester.
About six years ago, researchers discovered five conserved proteins that made up the core of the centriole in C. elegans. But that discovery shed little light on the question of symmetry. Researchers thought that they could get at the question by studying the proteins that make up the core. However, “You don’t get a feel for the three-dimensional architecture until you develop really high-resolution crystal structures,” said Fry, and, until now, nobody had been able to crystallize the proteins inside the core of the centriole.
So Gönczy and colleagues used classical light-scattering techniques to show that one of the core proteins, called SAS-6, has a tadpole shape, with a globular head and a long tail folded into a wound-up coil. They also found that SAS-6 tails bind to each other, form dimers, and that the SAS-6 protein from Chlamydomonas self assembles into hoops at high concentrations. But the breakthrough came when Gönczy’s collaborator and coauthor Michel Steinmetz managed to crystallize and determine the atomic structure of SAS-6’s head domain. The researchers realized that when one head dimerized with a second head domain, a 40° angle formed between the two proteins. The size of this angle allowed nine SAS-6 dimers to self-assemble into a circle about 25 nm in diameter, with nine SAS-6 tails radiating outwards like spokes of a wheel, providing a template for the bundles of microtubules that form the centriole’s “wheel.”
Although SAS-6 alone appears able to build this basic spoked template, Gönczy wants to make sure that this is true in evolutionarily distant animals. He’s also interested in how the other proteins in the centriolar core stabilize and extend the basic template, how the microtubules are attached to the spokes, and set limits on the height of the organelle. Eventually, Gönczy says, he’d like to be able to selectively add proteins to the core, building an entire, synthetic centriole.