Caenorhabditis elegans centriolar protein SAS-6 forms a spiral that is consistent with imparting a ninefold symmetry

Abstract

Centrioles are evolutionary conserved organelles that give rise to cilia and flagella as well as centrosomes. Centrioles display a characteristic ninefold symmetry imposed by the spindle assembly abnormal protein 6 (SAS-6) family. SAS-6 from Chlamydomonas reinhardtii and Danio rerio was shown to form ninefold symmetric, ring-shaped oligomers in vitro that were similar to the cartwheels observed in vivo during early steps of centriole assembly in most species. Here, we report crystallographic and EM analyses showing that, instead, Caenorhabotis elegans SAS-6 self-assembles into a spiral arrangement. Remarkably, we find that this spiral arrangement is also consistent with ninefold symmetry, suggesting that two distinct SAS-6 oligomerization architectures can direct the same output symmetry. Sequence analysis suggests that SAS-6 spirals are restricted to specific nematodes. This oligomeric arrangement may provide a structural basis for the presence of a central tube instead of a cartwheel during centriole assembly in these species.X-ray crystallography | electron microscopy | structure | centriolar architecture | SAS-5

The Centrosome and Its Duplication Cycle

“…Such a structure could be the underlying reason for the lack of the cartwheel structure in this organism and its replacement by a central tube (Hilbert et al 2013). However, although the detailed arrangements of Sas-6 may vary among different species, its role in dictating centriolar ninefold symmetry seems now indisputable.…”

Eukaryotic Flagella: Variations in Form, Function, and Composition during Evolution

“…Here, SAS-6 still defines a nine-fold symmetry, but assembles as a tubular, rather than radial, spiral illustrating how when even using conserved component parts there can be significant evolutionary variation in the mechanisms through which core flagellar structures assemble (Hilbert et al 2013).…”

Coupling Form and Function: How the Oligomerisation Symmetry of the SAS-6 Protein Contributes to the Architecture of Centriole Organelles

“…X-ray crystallography studies showed that SAS-6 comprises a globular N-terminal domain followed by a long coiled coil ( Figure 1) and a C-terminal disordered region. SAS-6 strongly dimerises in vitro and in vivo via its coiled-coil domain, and protein dimers further interact via their N-terminal domains to form oligomers with 9-fold radial symmetry on average [28][29][30][31]. The SAS-6 N-terminal domain bears strong structural resemblance to similar domains of XRCC4 and XLF (Figure 2), proteins involved in DNA repair and non-homologous end joining.…”

“…For example, Caenorhabditis elegans SAS-6 (CeSAS-6) self-assembles into a spiral [30] rather than the planar rings observed in the Chlamydomonas reinhardtii (CrSAS-6; [31]), Danio rerio (DrSAS-6; [29]) and Leishmania major (LmSAS-6; [28]) variants of the same protein. Furthermore, there is substantial heterogeneity in the intrinsic symmetry of SAS-6 oligomers, both inferred by X-ray crystallography and directly visualised by microscopy.…”