Polo-like kinase 2-dependent phosphorylation of NPM/B23 on serine 4 triggers centriole duplication

Duplication of the centrosome is well controlled during faithful cell division while deregulation of this process leads to supernumary centrosomes, chromosome missegregation and aneuploidy, a hallmark of many Cancer cells. We previously reported that Polo-like kinase 2 (PLK2) is activated near the G1/S phase transition, and regulates the reproduction of centrosomes. In search for PLK2 interacting proteins we have identified NPM/B23 (Nucleophosmin) as a novel PLK2 binding partner. We find that PLK2 and NPM/B23 interact in vitro in a Polo-box dependent manner. An association between both proteins was also observed in vivo. Moreover, we show that PLK2 phosphorylates NPM/B23 on serine 4 in vivo in S-phase. Notably, expression of a non-phosphorylatable NPM/B23 S4A mutant interferes with centriole reduplication in S-phase arrested cells and leads to a dilution of centriole numbers in unperturbed U2OS cells. The corresponding phospho-mimicking mutants have the opposite effect and their expression leads to the accumulation of centrioles. These findings suggest that NPM/B23 is a direct target of PLK2 in the regulation of centriole duplication and that phosphorylation on serine 4 can trigger this process.