SAS-5 assists in building centrioles of nematode worms Caenorhabditis elegans

We have recently published a paper in eLife describing the structural basis for the role of protein SAS-5 in initiating the formation of a new centriole, called a daughter centriole. But why do we care and why is this discovery important?

We, as humans – a branch of multi-cellular organisms, are in constant demand of new cells in our bodies. We need them to grow from an early embryo to adult, and also to replace dead or damaged cells. Cells don’t just appear from nowhere but undergo a tightly controlled process called cell cycle. At the core of cell cycle lies segregation of duplicated genetic material into two daughter cells. Pairs of chromosomes need to be pulled apart millions of millions times a day. Errors will lead to cancer. To avoid this apocalyptic scenario, evolution supplied us with centrioles. Those large molecular machines sprout microtubules radially to form characteristic asters which then bind to individual chromosomes and pull them apart. In order to achieve continuity, centrioles duplicate once per cell cycle.

Similarly to many large macromolecular assemblies, centrioles exhibit symmetry. A few unique proteins come in multiple copies to build this gigantic cylindrical molecular structure: 250 nm wide and 500 nm long (the size of a centriole in humans). The very core of the centriole looks like a 9-fold symmetrical stack of cartwheels, at which periphery microtubules are vertically installed. We study protein composition of this fascinating structure in the effort to understand the process of assembling a new centriole.

Molecular architecture of centrioles.

SAS-5 is an indispensable component in C. elegans centriole biogenesis. SAS-5 physically associates with another centriolar protein, called SAS-6, forming a complex which is required to build new centrioles. This process is regulated by phosphorylation events, allowing for subsequent recruitment of SAS-4 and microtubules. In most other systems SAS-6 forms a cartwheel (central tube in C. elegans), which forms the basis for the 9-fold symmetry of centrioles. Unlike SAS-6, SAS-5 exhibits strong spatial dynamics, shuttling between the cytoplasm and centrioles throughout the cell cycle. Although SAS-5 is an essential protein, depletion of which completely terminates centrosome-dependent cell division, its exact mechanistic  role in this  process remains  obscure.

IN BRIEF: WHAT WE DID
Using X-ray crystallography and a range of biophysical techniques, we have determined the molecular architecture of SAS-5. We show that SAS-5 forms a complex oligomeric structure, mediated by two self-associating domains: a trimeric coiled coil and a novel globular dimeric Implico domain. Disruption of either domain leads to centriole duplication failure in worm embryos, indicating that large SAS-5 assemblies are necessary for function. We propose that SAS-5 provides multivalent attachment sites that are critical for promoting assembly of SAS-6 into a cartwheel, and thus centriole formation.

For details, check out our latest paper 10.7554/eLife.07410!

@kbrogala

Top panel: cartoon overview of the proposed mechanism of centriole formation. In cytoplasm, SAS-5 exists at low concentrations as a dimer, and each of those dimers can stochastically bind two molecules of SAS-6. Once SAS-5 / SAS-6 complex is targeted to the centrioles, it starts to self-oligomerise. Such self-oligomerisation of SAS-5 allows for the attached molecules of SAS-6 to form a cartwheel. Bottom panel: detailed overview of the proposed process of centriole formation. In cytoplasm, where concentration of SAS-5 is low, the strong Implico domain (SAS-5 Imp, ZZ shape) of SAS-5 holds the molecule in a dimeric form. Each SAS-5 protomer can bind (through the disordered linker) to the coiled coil of dimeric SAS-6. Once SAS-5 / SAS-6 complex is targeted to the site where a daughter centriole is to be created, SAS-5 forms higher-order oligomers through self-oligomerisation of its coiled coil domain (SAS-5 CC – triple horizontal bar). Such large oligomer of SAS-5 provides multiple attachments sites for SAS-6 dimers in a very confied space. This results in a burst of local concentration of SAS-6 through the avidity effect, allowing an otherwise weak oligomer of SAS-6 to also form larger species. Effectively, this seeds the growth of a cartwheel (or a spiral in C. elegans), which in turn serves as a template for a new centriole.

 

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