DSAS-6 Organizes a Tube-like Centriole Precursor, and Its Absence Suggests Modularity in Centriole Assembly

Centrioles are microtubule-based cylindrical structures that exhibit 9-fold symmetry and facilitate the organization of centrosomes, flagella, and cilia [1]. Abnormalities in centrosome structure and number occur in many cancers [1, 2]. Despite its importance, very little is known about centriole biogenesis. Recent studies in C. elegans have highlighted a group of molecules necessary for centriole assembly [1, 3]. ZYG-1 kinase recruits a complex of two coiled-coil proteins, SAS-6 and SAS-5, which are necessary to form the C. elegans centriolar tube, a scaffold in centriole formation [4, 5]. This complex also recruits SAS-4, which is required for the assembly of the centriolar microtubules that decorate that tube [4, 5]. Here we show that Drosophila SAS-6 is involved in centriole assembly and cohesion. Overexpression of DSAS-6 in syncitial embryos led to the de novo formation of multiple microtubule-organizing centers (MTOCs). Strikingly, the center of these MTOCs did not contain centrioles, as described previously for SAK/PLK4 overexpression [6]. Instead, tube-like structures were present, supporting the idea that centriolar assembly starts with the formation of a tube-like scaffold, dependent on DSAS-6 [5]. In DSAS-6 loss-of-function mutants, centrioles failed to close and to elongate the structure along all axes of the 9-fold symmetry, suggesting modularity in centriole assembly. We propose that the tube is built from nine subunits fitting together laterally and longitudinally in a modular and sequential fashion, like pieces of a layered "hollow" cake.