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

Abstract

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.

DrosophilaBld10 Is a Centriolar Protein That Regulates Centriole, Basal Body, and Motile Cilium Assembly

“…Mutations in genes encoding centriolar and basal body proteins often lead to defects in the biogenesis of centrioles and in the assembly of functional centrosomes at mitosis, loss of locomotion due to impaired basal bodies of ciliated neurons, and immotile sperm due to axoneme dysfunction Martinez-Campos et al, 2004;Bettencourt-Dias et al, 2005;Basto et al, 2006;Rodrigues-Martins et al, 2007;Varmark et al, 2007;Blachon et al, 2008;Giansanti et al, 2008).…”

“…In Caenorhabditis elegans there appears to be no bld10 ortholog (Dutcher, 2007), and centriole biogenesis involves assembly of a precursor tube-like structure that requires sas-6 (Pelletier et al, 2006). Examination of centrioles from Drosophila sas-6 mutants has led to a model whereby centriole assembly involves incorporation of "enatosomes," modules that comprise one-ninth of the centriole's rotational architecture (Rodrigues-Martins et al, 2007). These findings have led to the hypothesis that the precursor tube that Sas-6 regulates provides ninefold assembly instruction similar to the cartwheel in Chlamydomonas (Pelletier et al, 2006;RodriguesMartins et al, 2008).…”

“…In Drosophila several approaches have identified evolutionarily conserved centriole proteins required for centriole biogenesis including Sak, Sas4, Sas6, Ana1, Ana2, and Asterless (Bettencourt-Dias et al, 2005;Basto et al, 2006;Goshima et al, 2007;Rodrigues-Martins et al, 2007;Blachon et al, 2008). In flies, mutations in these genes abolish centrosome and cilium/flagellum assembly (except mutations in ana1 and ana2, which have not been described yet).…”

“…In most differentiated cell types, centrioles transform into basal bodies, membrane-embedded centrioles that template cilium and flagellum axoneme assembly. The requirement of cilia and flagella for many developmental and physiological processes, together with the growing list of human diseases that result from defects in basal bodies and cilia (Badano et al, 2006;Bisgrove and Yost, 2006;Bettencourt-Dias and Glover, 2007;Fliegauf et al, 2007;Marshall, 2008), drives the need to understand the basic processes involved in centriole, basal body, cilium assembly, and motility.In Drosophila several approaches have identified evolutionarily conserved centriole proteins required for centriole biogenesis including Sak, Sas4, Sas6, Ana1, Ana2, and Asterless (Bettencourt-Dias et al, 2005;Basto et al, 2006;Goshima et al, 2007;Rodrigues-Martins et al, 2007;Blachon et al, 2008). In flies, mutations in these genes abolish centrosome and cilium/flagellum assembly (except mutations in ana1 and ana2, which have not been described yet).…”

Methods to Study Centrosomes and Cilia in Drosophila

“…Note: This table is not an exhaustive list of all centrosome and cilia components mapped. Due to space constrain, only references for mutants and/or markers for each gene are mentioned in the list [15,16,19,21,31,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Also, for more information about the genes visit www.flybase.org.…”