Pericentriolar matrix expansion does not fully account for the function of centrosome-localized PLK-1. (A) Current model for centrosome maturation in C. elegans. PLK-1 is recruited to centrosomes by binding of its C-terminal polo boxes (PB1 and PB2) to a site in SPD-2 (CEP192 homologue) thought to be primed by Cdk1 (Thr233). PLK-1 controls PCM expansion by phosphorylating residues, including S653 and S658, in the central region of the PCM matrix component SPD-5 that promote its self-assembly (red circled P indicates phosphorylation). PCM expansion enables docking of a larger number of γ-tubulin complexes (black rings), thus increasing the capacity of the centrosome to nucleate microtubules. The previously defined ASH (ASPM, SPD-2, Hydin; Ponting, 2006) and SPD-2 (Pelletier et al., 2004) domains in SPD-2 are indicated. (B and C) Images of embryos expressing in situ GFP-tagged PLK-1 (green) and in situ mCherry (mCh)-tagged SPD-5 (magenta) in the presence of WT or PD^mut SPD-2 after endogenous SPD-2 depletion and WT or expansion-defective (Exp^mut) GFP::SPD-5 after endogenous SPD-5 depletion (C). Graphs on right show quantification of centrosomal signals at nuclear envelope breakdown (NEBD). In B, the protein sequence between amino acids 232 and 234 was changed from STP to TAP in PD^mut SPD-2 as indicated. Error bars are mean with 95% confidence intervals. P values are from two-tailed t tests. Times in seconds relative to NEBD are shown in top left of large panels and below the smaller panels, which show SPD-5 signal at one centrosome over time. In B, PLK-1 localization to centrosomes (arrow) and kinetochores (arrowhead) in the presence of WT SPD-2 is indicated. (D) Chromosome segregation errors and embryonic lethality for the indicated conditions. Top images: example of normal versus defective chromosome segregation, defined as visible chromatin bridges between anaphase chromosome masses in live imaging data. n refers to number of embryos imaged. For embryonic lethality analysis, the mean ± SD is plotted; N refers to number of worms and n to number of embryos scored, respectively. All scale bars, 2 µm.