Structural Basis of the 9-Fold Symmetry of Centrioles

Abstract

SummaryThe centriole, and the related basal body, is an ancient organelle characterized by a universal 9-fold radial symmetry and is critical for generating cilia, flagella, and centrosomes. The mechanisms directing centriole formation are incompletely understood and represent a fundamental open question in biology. Here, we demonstrate that the centriolar protein SAS-6 forms rod-shaped homodimers that interact through their N-terminal domains to form oligomers. We establish that such oligomerization is essential for centriole formation in C. elegans and human cells. We further generate a structural model of the related protein Bld12p from C. reinhardtii, in which nine homodimers assemble into a ring from which nine coiled-coil rods radiate outward. Moreover, we demonstrate that recombinant Bld12p self-assembles into structures akin to the central hub of the cartwheel, which serves as a scaffold for centriole formation. Overall, our findings establish a structural basis for the universal 9-fold symmetry of centrioles.

Structural characterization of filaments formed by human Xrcc4–Cernunnos/XLF complex involved in nonhomologous DNA end-joining

“…A recent structural study reported that the centriolar homodimeric protein, SAS-6, which belongs to the X4-fold family, forms oligomeric rings which have ninefold symmetry (34). Interestingly, these SAS-6 oligomeric rings are mediated by its N-terminal domain and by loops equivalent to the ones involved in forming the X4 and Cernunnos in the filaments presented here.…”

Quantitative analysis of human centrosome architecture by targeted proteomics and fluorescence imaging

“…Because visualization of cartwheels by electron microscopy has proven difficult in human cells, the number of stacks is unknown. Considering that each stacked hub is predicted to comprise 18 molecules (nine dimers) of Sas‐6 (van Breugel et al , 2011; Kitagawa et al , 2011), our prediction of 276 molecules of Sas‐6 per cartwheel‐containing centriole theoretically allows for the assembly of 15–16 stacks. This likely represents an upper limit, because not all Sas‐6 and STIL proteins are necessarily assembled into cartwheels at all times (Fong et al , 2014; Keller et al , 2014) and we also recognize that the number of stacks may differ between cell types.…”

“…A similar value (60–150 nm) was reported in a recent study using 3D‐STORM imaging of stable Sas‐6‐positive structures, likely reflecting cartwheels, in detergent‐extracted human U2OS cells (Keller et al , 2014). Assuming that stacked hubs display a vertical periodicity of 8.5 nm (Kitagawa et al , 2011; Guichard et al , 2012, 2013; Li et al , 2012), a cartwheel thickness of 60–150 nm would thus allow for ~7–17 stacks. These numbers, albeit tentative, are encouragingly close to our estimate of 15–16 stacked hubs per cartwheel.…”

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

“…Structurally, SAS-6 has an N-terminal globular head domain and a Cterminal coiled-coil domain that mediates protein dimerization. In vitro, SAS-6 dimers self-U n c o r r e c t e d v e r s i o n 6 assemble into ring-like structures where the coiled-coil domains of nine SAS-6 dimers point outwards providing a hub-and-spoke arrangement similar in its dimensions to the cartwheel hub found at the proximal end of the immature (pro-) centrioles (Kitagawa et al 2011). In the absence of SAS-6 assembly of the hub-and-spoke cartwheel fails and although triplet microtubule blades still assemble, nine-fold symmetry is lost (Nakazawa et al 2007).…”