Abstract
In animal cells, the functions of the microtubule cytoskeleton are coordinated by centriole-based centrosomes via γ-tubulin complexes embedded in the pericentriolar material or PCM.1 PCM assembly has been best studied in the context of mitosis, where centriolar SPD-2 recruits PLK-1, which in turn phosphorylates key scaffolding components like SPD-5 and CNN to promote expansion of the PCM polymer.2-4 To what extent these mechanisms apply to centrosomes in interphase or in differentiated cells remains unclear.5 Here, we examine a novel type of centrosome found at the ciliary base of C.elegans sensory neurons, which we show plays important roles in neuronal morphogenesis, cellular trafficking, and ciliogenesis. These centrosomes display similar dynamic behavior to canonical, mitotic centrosomes, with a stable PCM scaffold and dynamically localized client proteins. Unusually, however, they are not organized by centrioles, which degenerate early in terminal differentiation.6 Yet, PCM not only persists but continues to grow with key scaffolding proteins including SPD-5 expressed under control of the RFX transcription factor DAF-19. This assembly occurs in the absence of the mitotic regulators SPD-2, AIR-1 and PLK-1, but requires tethering by PCMD-1, a protein which also plays a role in the initial, interphase recruitment of PCM in early embryos.7 These results argue for distinct mechanisms for mitotic and non-mitotic PCM assembly, with only the former requiring PLK-1 phosphorylation to drive rapid expansion of the scaffold polymer.
Highlights
Central to PCM assembly is the coiled coil protein SPD-5, which forms a polymeric scaffold around centrioles (Figure 1A).[4,8,9]
SPD-5 spontaneously self-assembles in vitro, a process potentiated by SPD-2 and the polo-like kinase PLK-1.4 In vivo, this process is directed to occur around centrioles, with centriole-localized SPD-2 recruiting both PLK-1 and its activator Aurora A/AIR-1.2 Laser ablation experiments have shown that centrioles are essential for mitotic PCM growth, existing PCM polymer can be maintained by PLK-1 acting within the PCM to counter the destabilizing effect of phosphatases like PP2A.2,11
An acentriolar PCM at the C. elegans ciliary base C. elegans is well known for its stereotypical pattern of development, with precisely 959 somatic cells in the adult hermaphrodite.[12]
Summary
An acentriolar PCM at the C. elegans ciliary base C. elegans is well known for its stereotypical pattern of development, with precisely 959 somatic cells in the adult hermaphrodite.[12]. GFP-tagged SPD-5 forms discrete foci in the amphid and phasmid sensory organs in the head and tail of the animal, respectively, which are the subject of most studies in C. elegans (Figure 1B,18). SPD-5 foci are observed in cephalic and labial neurons, as well as in the sensory neurons of the male tail (Figure S1A). These foci, which are similar in size to interphase centrosomes in the early embryo, are all the more prominent given the near absence of SPD-5 signal in other somatic tissues of the adult worm. Closer inspection shows SPD-5 accumulation to be in the region of the degenerated basal body and extending into the ciliary transition zone, marked by HYLS-1 and MKS-6,6,19 respectively (Figures 1C and 1D). To rule out the possibility that a small population of centriolar proteins below the threshold of detection sustains the continued presence of SPD-5, we performed acut
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