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.

Formation of one procentriole next to each pre-existing centriole is essential for centrosome duplication, robust bipolar spindle assembly and maintenance of genome integrity. However, the mechanisms maintaining strict control over centriole copy number are incompletely understood. Here we show that Plk4 and STIL, the key regulators of centriole formation, form a protein complex that provides a scaffold for recruiting HsSAS-6, a major component of the centriolar cartwheel, at the onset of procentriole formation. Furthermore, we demonstrate that phosphorylation of STIL by Plk4 facilitates the STIL/HsSAS-6 interaction and centriolar loading of HsSAS-6. We also provide evidence that negative feedback by centriolar STIL regulates bimodal centriolar distribution of Plk4 and seemingly restricts occurrence of procentriole formation to one site on each parental centriole. Overall, these findings suggest a mechanism whereby coordinated action of three critical factors ensures formation of a single procentriole per parental centriole.

The number of centrioles is tightly controlled to ensure bipolar spindle assembly, which is a prerequisite to maintain genome integrity. However, our understanding of the fundamental principle that governs the formation of a single procentriole per parental centriole is incomplete. Here, we show that the local restriction of Plk4, a master regulator of the procentriole formation, is achieved by a bimodal interaction of STIL with Plk4. We demonstrate that the conserved short coiled-coil region of STIL binds to and protects Plk4 from protein degradation at the site of procentriole formation. On the other hand, the conserved C-terminal region of STIL named truncated in microcephaly (TIM) domain promotes autophosphorylation and degradation of adjacent Plk4 by the direct interaction. Thus, we propose that positive and negative regulation based on the bimodal binding of Plk4 and STIL ensures the formation of a single procentriole per parental centriole.

SummaryPatients with MCPH (autosomal recessive primary microcephaly) exhibit impaired brain development, presumably due to the compromised function of neuronal progenitors. Seven MCPH loci have been identified, including one that encodes centrosome protein 4.1 associated protein (CPAP; also known as centromere protein J, CENPJ). CPAP is a large coiled-coil protein enriched at the centrosome, a structure that comprises two centrioles and surrounding pericentriolar material (PCM). CPAP depletion impairs centriole formation, whereas CPAP overexpression results in overly long centrioles. The mechanisms by which CPAP MCPH patient mutations affect brain development are not clear. Here, we identify CPAP protein domains crucial for its centriolar localization, as well as for the elongation and the formation of centrioles. Furthermore, we demonstrate that conditions that resemble CPAP MCPH patient mutations compromise centriole formation in tissue culture cells. Using adhesive micropatterns, we reveal that such defects correlate with a randomization of spindle position. Moreover, we demonstrate that the MCPH protein SCL/TAL1 interrupting locus (STIL) is also essential for centriole formation and for proper spindle position. Our findings are compatible with the notion that mutations in CPAP and STIL cause MCPH because of aberrant spindle positioning in progenitor cells during brain development.