We show that overexpression of Polo-like kinase 4 (Plk4) in human cells induces centrosome amplification through the simultaneous generation of multiple procentrioles adjoining each parental centriole. This provided an opportunity for dissecting centriole assembly and characterizing assembly intermediates. Critical components were identified and ordered into an assembly pathway through siRNA and localized through immunoelectron microscopy. Plk4, hSas-6, CPAP, Cep135, gamma-tubulin, and CP110 were required at different stages of procentriole formation and in association with different centriolar structures. Remarkably, hSas-6 associated only transiently with nascent procentrioles, whereas Cep135 and CPAP formed a core structure within the proximal lumen of both parental and nascent centrioles. Finally, CP110 was recruited early and then associated with the growing distal tips, indicating that centrioles elongate through insertion of alpha-/beta-tubulin underneath a CP110 cap. Collectively, these data afford a comprehensive view of the assembly pathway underlying centriole biogenesis in human cells.

Centrioles function as the major components of centrosomes, which organize microtubule (MT) arrays in proliferating cells, and as basal bodies for primary cilia formation in quiescent cells. Centrioles and basal bodies are structurally similar, barrel-shaped organelles composed of MTs. In proliferating cells, two new centrioles, termed procentrioles, form during the S phase of the cell cycle in close proximity to the proximal ends of the two preexisting parental centrioles, often at a near-orthogonal angle. Considerable progress has been made toward understanding the biogenesis of centrioles, but the mechanisms that determine their lengths remain unknown. Here we show that overexpression of the centriolar protein CPAP in human cells enhances the accumulation of centriolar tubulin, leading to centrioles of strikingly increased length. Consistent with earlier work, we also find that elongated MT structures can be induced by depletion of the distal end-capping protein CP110 from centrioles. Importantly, though, these structures differ from genuine primary cilia. We thus propose that CPAP and CP110 play antagonistic roles in determining the extent of tubulin addition during centriole elongation, thereby controlling the length of newly formed centrioles.

Centrosomes in animal cells are dynamic organelles with a proteinaceous matrix of pericentriolar material assembled around a pair of centrioles. They organize the microtubule cytoskeleton and the mitotic spindle apparatus. Mature centrioles are essential for biogenesis of primary cilia that mediate key signalling events. Despite recent advances, the molecular basis for the plethora of processes coordinated by centrosomes is not fully understood. We have combined protein identification and localization, using PCP-SILAC mass spectrometry, BAC transgeneOmics, and antibodies to define the constituents of human centrosomes. From a background of non-specific proteins, we distinguished 126 known and 40 candidate centrosomal proteins, of which 22 were confirmed as novel components. An antibody screen covering 4000 genes revealed an additional 113 candidates. We illustrate the power of our methods by identifying a novel set of five proteins preferentially associated with mother or daughter centrioles, comprising genes implicated in cell polarity. Pulsed labelling demonstrates a remarkable variation in the stability of centrosomal protein complexes. These spatiotemporal proteomics data provide leads to the further functional characterization of centrosomal proteins.

Centrioles are the main constituents of the mammalian centrosome and act as basal bodies for ciliogenesis. Centrosomes organize the cytoplasmic microtubule network during interphase and the mitotic spindle during mitosis, and aberrations in centrosome number have been implicated in chromosomal instability and tumor formation. The centriolar protein Polo-like kinase 4 (Plk4) is a key regulator of centriole biogenesis and is crucial for maintaining constant centriole number, but the mechanisms regulating its activity and expression are only beginning to emerge. Here, we show that human Plk4 is subject to βTrCP-dependent proteasomal degradation, indicating that this pathway is conserved from Drosophila to human. Unexpectedly, we found that stable overexpression of kinase-dead Plk4 leads to centriole overduplication. This phenotype depends on the presence of endogenous wild-type Plk4. Our data indicate that centriole overduplication results from disruption of Plk4 trans-autophosphorylation by kinase-dead Plk4, which then shields endogenous Plk4 from recognition by βTrCP. We conclude that active Plk4 promotes its own degradation by catalyzing βTrCP binding through trans-autophosphorylation (phosphorylation by the other kinase in the dimer) within homodimers.

The localization of B cells to lymphoid organs where they can become activated and differentiate into antibody-secreting plasma cells is controlled by multiple chemoattractants that promote cell migration and integrin-mediated adhesion. CXCL13 and sphingosine 1-phosphate (S1P) are two important chemoattractants that control the trafficking of B cells. CXCL13 directs B lymphocytes to lymphoid follicles where they receive survival signals and, if activated, undergo a germinal center response. In contrast, S1P allows B cells and plasma cells to exit lymphoid organs and re-enter the circulation. The Rap1 GTPase is a key regulator of cell adhesion and cell migration in a number of systems. We now show that Rap activation is required for CXCL13 and S1P to induce B cell migration as well as adhesion to ICAM-1 and VCAM-1. We also show that Pyk2, a tyrosine kinase involved in cytoskeleton rearrangements and B cell migration, is a downstream target of both CXCL13 and S1P signaling and that Rap activation is important for CXCL13 and S1P to stimulate tyrosine phosphorylation of Pyk2, a modification that increases Pyk2 kinase activity. This suggests that the ability of CXCL13 and S1P to direct the trafficking and localization of B cells in vivo may be dependent on Rap activation.