Centriole-less pericentriolar material serves as a microtubule organizing center at the base of C. elegans sensory cilia

During mitosis in animal cells, the centrosome acts as a microtubule organizing center (MTOC) to assemble the mitotic spindle. MTOC function at the centrosome is driven by proteins within the pericentriolar material (PCM), however the molecular complexity of the PCM makes it difficult to differentiate the proteins required for MTOC activity from other centrosomal functions. We used the natural spatial separation of PCM proteins during mitotic exit to identify a minimal module of proteins required for centrosomal MTOC function in C. elegans. Using tissue-specific degradation, we show that SPD-5, the functional homolog of CDK5RAP2, is essential for embryonic mitosis, while SPD-2/CEP192 and PCMD-1, which are essential in the one-cell embryo, are dispensable. Surprisingly, although the centriole is known to be degraded in the ciliated sensory neurons in C. elegans,1–3 we find evidence for “centriole-less PCM” at the base of cilia and use this structure as a minimal testbed to dissect centrosomal MTOC function. Super-resolution imaging revealed that this PCM inserts inside the lumen of the ciliary axoneme and directly nucleates the assembly of dendritic microtubules toward the cell body. Tissue-specific degradation in ciliated sensory neurons revealed a role for SPD-5 and the conserved microtubule nucleator γ-TuRC, but not SPD-2 or PCMD-1, in MTOC function at centriole-less PCM. This MTOC function was in the absence of regulation by mitotic kinases, highlighting the intrinsic ability of these proteins to drive microtubule growth and organization and further supporting a model that SPD-5 is the primary driver of MTOC function at the PCM. [Display omitted] •SPD-5 is physically and functionally central to the MTOC function of the centrosome•SPD-5 and PCMD-1 are maintained at the ciliary base in the absence of a centriole•Centriole-less pericentriolar material at the base of C. elegans cilia is an MTOC•A module of SPD-5 and γ-TuRC drives MTOC function at the ciliary base Magescas et al. use tissue-specific degradation to investigate the role of PCM proteins in the microtubule organizing capacity of the centrosome in C. elegans. They identify a novel MTOC at the base of cilia in sensory neurons and reveal a central role for SPD-5/∼CDK5RAP2 in MTOC function at this centriole-less PCM and at the mitotic centrosome.