In human cells, separation of the centrosomes and formation of a bipolar spindle are essential for correct chromosome segregation [1]. During interphase, centrosomes are joined together by the linker proteins C-Nap1 and rootletin [2-4]. At the onset of mitosis, these linker proteins are phosphorylated and displaced from centrosomes by the Nek2A kinase, which is regulated by two Hippo pathway components, Mst2 kinase and the scaffold protein hSav1. The kinesin-5 motor protein Eg5 promotes centrosome separation in a parallel pathway to Nek2A [5]. Here, we report that Polo-like kinase 1 (Plk1) functions upstream of the Mst2-Nek2A kinase module in centrosome disjunction as well as being important for Eg5 localization at centrosomes. Plk1 regulates Mst2-Nek2A-induced centrosome disjunction by phosphorylating Mst2. The absence of Plk1 phosphorylation of Mst2 promotes assembly of Nek2A-PP1γ-Mst2 complexes, in which PP1γ counteracts Nek2A kinase activity. In contrast, Plk1 phosphorylation of Mst2 prevents PP1γ binding to Mst2-Nek2A, allowing Nek2A activity to promote centrosome disjunction. We propose that centrosome disjunction is regulated by Plk1, providing a well-balanced control between the counteracting Nek2A and PP1γ activities on the centrosome linker.

The centrosome is the main microtubule (MT)-organizing centre of animal cells. It consists of two centrioles and a multi-layered proteinaceous structure that surrounds the centrioles, the so-called pericentriolar material. Centrosomes promote de novo assembly of MTs and thus play important roles in Golgi organization, cell polarity, cell motility and the organization of the mitotic spindle. To execute these functions, centrosomes have to adopt particular cellular positions. Actin and MT networks and the association of the centrosomes to the nuclear envelope define the correct positioning of the centrosomes. Another important feature of centrosomes is the centrosomal linker that connects the two centrosomes. The centrosome linker assembles in late mitosis/G1 simultaneously with centriole disengagement and is dissolved before or at the beginning of mitosis. Linker dissolution is important for mitotic spindle formation, and its cell cycle timing has profound influences on the execution of mitosis and proficiency of chromosome segregation. In this review, we will focus on the mechanisms of centrosome positioning and separation, and describe their functions and mechanisms in the light of recent findings.

Centrosomes are required for faithful chromosome segregation during mitosis. They are composed of a centriole pair that recruits and organizes the microtubule-nucleating pericentriolar material. Centriole duplication is tightly controlled in vivo and aberrations in this process are associated with several human diseases, including cancer and microcephaly. Although factors essential for centriole assembly, such as STIL and PLK4, have been identified, the underlying molecular mechanisms that drive this process are incompletely understood. Combining protein proximity mapping with high-resolution structural methods, we identify CEP85 as a centriole duplication factor that directly interacts with STIL through a highly conserved interaction interface involving a previously uncharacterised domain of STIL. Structure-guided mutational analyses in vivo demonstrate that this interaction is essential for efficient centriolar targeting of STIL, PLK4 activation and faithful daughter centriole assembly. Taken together, our results illuminate a molecular mechanism underpinning the spatiotemporal regulation of the early stages of centriole duplication.