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Different routes to building symmetry in cell structures

Researchers from the department and from Institutes in Switzerland have provided a molecular explanation for a puzzling aspect of the centriole, a cellular component crucial for many aspects of cell function.

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 recent paper published by Dr John Vakonakis and colleagues describes how centrioles appear to be built on similar principles across the animal kingdom despite differences in the structural details of component proteins (1).

Centrioles are large barrel-shaped structures which are found in all animal cells. Most centrioles appear to be organised around a nine-pointed cartwheel with central ring and spokes. Attached to the spokes are nine blades composed primarily of microtubules. This ninefold radial symmetric arrangement of microtubules is seen universally in all centrioles even where the cartwheel structure is not observed.

In the cell, 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.

Duplication of centrioles, which happens after each cell division, presents an interesting problem since only a handful of proteins are needed to generate this massive structure. Key amongst these is the SAS-6 protein.

Previous work by Dr Vakonakis and his collaborators showed that SAS-6 from the algae Chlamydomonas reinhardtii provides the basis of centriolar nine-fold symmetry by self-assembling to form a structure akin to the ring and spokes of the centriole cartwheel (see Single protein holds key to building up cell structures). A group in Cambridge showed that SAS-6 from the zebrafish Danio rerio organised into the same structure.

Although centriolar proteins are generally well conserved, there is a notable exception to this in the nematode worm C. elegans. The sequence divergence of centriolar proteins in this organism which is widely used as a model system in centriole cell biology, along with the apparent absence of a cartwheel structure, has led some researchers to suggest that not all centrioles are constructed along the same principles.

Dr Vakonakis and his colleagues set out to investigate whether this is the case.

The three groups combined their expertise to tackle the problem. The Oxford group used crystallographic and biophysical methods, the Swiss groups used electron microscopy and molecular dynamics simulations as well as cell biology.

Because of fragility of the structures formed by the C. elegans SAS-6 protein, the researchers were initially unable to visualise them by electron microscopy. They eventually overcame this by engineering a stabilised mutant of SAS-6 to strengthen oligomerisation and persevering with the microscopy.

Ring or spiral? Chlamydomonas reinhardtii SAS-6 forms rings in vitro (top: electron micrograph on left, crystallographic model on right), whereas SAS-6 from C. elegans forms long spiral structures (bottom: top: electron micrograph on left, crystallographic model on right).

The researchers found that in vitro the C. elegans SAS-6 is indeed different from SAS-6 of other organisms. ‘Instead of forming rings like the Chlamydomonas reinhardtii and Danio rerio SAS-6 variants, the C. elegans SAS-6 forms spirals’, explains Dr Vakonakis.

But despite this difference in underlying structure, the group found that the overall symmetry of the centrioles is the signature ninefold. ‘We also show that the mechanistic function of SAS-6 - determining 9-fold symmetry - can be achieved just as well using this spiral form’, he adds. ‘So despite the in vitro difference, the mechanistic principle of what SAS-6 does is similar across organisms.’

Dr Vakonakis points out that this is very much an in vitro study where they are proposing models for what happens in vivo.

‘Obviously we would like to see if there are spirals in vivo. How to do that is not clear yet, but some ideas relate to making SAS-6 mutants that change the oligomeric form in a defined manner, and then testing these in vivo.’

The collaboration between Dr Vakonakis’ lab and the Swiss labs is a long-standing one with a common goal of building a blueprint of how centrioles assemble. ‘Knowing the differences in this blueprint across organisms is just as important as knowing the similarities’, comments Dr Vakonakis.

Dr Michèle Erat, the postdoc in the lab who performed most of the work in Oxford, comments that she surprised a senior scientist in the field when she first started to work on this project. ‘“Why would you work on C. elegans centrioles?” I was asked. “They are just bizarre.”’

‘It is true that in many ways C. elegans turns out to be a bit of an outlier. But if our hypothesis is correct, these nematode worms essentially achieve the same nine-fold symmetric centriole architecture as other species in a different yet very elegant way.'

Reference

1. Hilbert M, Erat MC, Hachet V, Guichard P, Blank ID, Flückiger I, Slater L, Lowe ED, Hatzopoulos GN, Steinmetz MO, Gönczy P, Vakonakis I. Caenorhabditis elegans centriolar protein SAS-6 forms a spiral that is consistent with imparting a ninefold symmetry (2013).Proc Natl Acad Sci U S A. Jul 9;110(28):11373-8.

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