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Single protein holds key to building up cell structures

Cartoon showing how the nine-fold symmetric cartwheel model determines the overall symmetry of centrioles, drawn as ensembles of microtubule blades. An electron micrograph of C.reinhardtii SAS-6 with clearly visible ring structures is shown in the background

Cartoon showing how the nine-fold symmetric cartwheel model determines the overall symmetry of centrioles, drawn as ensembles of microtubule blades. An electron micrograph of C.reinhardtii SAS-6 protein with clearly visible ring structures is shown in the background

Researchers from the department and from institutes in Switzerland have described for the first time how a cellular component crucial for many aspects of cell function is constructed.

Published in the latest issue of the journal Cell1, the work from departmental researcher Dr John Vakonakis and colleagues provides insight into how a single protein lies at the root of this component, known as the centriole. The research will contribute to a better understanding of how centriole aberrations can lead to a number of disease conditions including male sterility and cancer.

The centriole is a large barrel-shaped structure which is found in all animal cells. It is organised around a nine-pointed cartwheel with central ring and spokes. Attached to the spokes are nine blades composed primarily of microtubules.

Centrioles provide a hub for the construction of a wide range of internal and external structures such as centrosomes, which are important for accurate cell division, and cilia and flagella. Like the centriole's own core, many of these structures have nine-fold symmetry.

At cell division, the centrioles must duplicate themselves so that each daughter cell will also contain an appropriate centriole number. As Dr Vakonakis explains, little is known about how this happens: 'It's interesting from a mechanistic point of view - a massive structure is created out of relatively few proteins. We know that 5 proteins are essential for early centriole duplication in the worm, C. elegans. We also know the order in which these proteins come together to produce new centrioles. But until now, we've not known how these proteins dictate the ultimate structure of the centriole.'

The crystal structure of the N-terminal domain of C. elegans SAS-6. One dimer of the protein is shown in two different representations. The single amino acid I154 highlighted is critical for SAS-6 dimer assembly and thus for centriole formation

The crystal structure of the N-terminal domain of C. elegans SAS-6. One dimer of the protein is shown in two different represent-ations. The single amino acid highlighted in yellow is critical for SAS-6 dimer assembly and thus for centriole formation

Dr Vakonakis and colleagues show in their paper that a single centriole protein, SAS-6, known to be essential for the earliest steps in centriole formation, can self-assemble to form a structure akin to the ring and spokes of the centriole cartwheel.

In their study, they looked at SAS-6 protein from C.elegans and from the algae Chlamydomonas reinhardtii. They subjected the protein to a battery of structural and biophysical techniques including X-ray crystallography and analytical ultracentrifugation, to determine its structure and details about how the protein molecules fit together.

Using this information, they showed that the structures obtained were biologically significant, being crucial for centriole assembly in the cell. In addition, the structural models they built of multiple protein molecules assembling together revealed a structure that strongly resembled the cartwheel's ring and spokes.

Cartoon representation of the C. reinhardtii cartwheel model. Nine SAS-6 dimers interact to form a closed ring structure. This model was derived from crystal structures and validated by electron microscopy data

Cartoon representation of the C. reinhardtii cartwheel model. Nine SAS-6 dimers interact to form a closed ring structure. This model was derived from crystal structures and validated by electron microscopy data

The results indicated that the SAS-6 protein is capable of self-assembling into the centriole's cartwheel structure without any additional factors. To establish this beyond doubt, the group turned to electron microscopy. They found that C. reinhardtii SAS-6 protein was able to assemble into the centriole's distinctive cartwheel shape, forming clear ring-like structures with spokes protruding at nine points.

This is the first time that researchers have shown how the nine-fold symmetry, which is found in all centrioles from amoeba to man, is generated.

Dr Vakonakis highlights that the group's multidisciplinary approach was key to its success. The broad range of techniques pursued, firstly in Switzerland and subsequently in the Biochemistry department, enabled the group to establish how this initial step in centriole formation takes place.

One of many challenges following on from this work will be to understand the role of other essential centriole proteins in forming a fully functional centriole.

Reference

  1. Structural Basis of the 9-Fold Symmetry of Centrioles. Kitagawa, D., Vakonakis, I. et al. Cell doi:10.1016/j.cell.2011.01.008

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