Overly long centrioles and defective cell division upon excess of the SAS-4-related protein CPAP

The centrosome is the principal microtubule organizing center (MTOC) of animal cells. Accurate centrosome duplication is fundamental for genome integrity and entails the formation of one procentriole next to each existing centriole, once per cell cycle. The procentriole then elongates to eventually reach the same size as the centriole. The mechanisms that govern elongation of the centriolar cylinder and their potential relevance for cell division are not known. Here, we show that the SAS-4-related protein CPAP is required for centrosome duplication in cycling human cells. Furthermore, we demonstrate that CPAP overexpression results in the formation of abnormally long centrioles. This also promotes formation of more than one procentriole in the vicinity of such overly long centrioles, eventually resulting in the presence of supernumerary MTOCs. This in turn leads to multipolar spindle assembly and cytokinesis defects. Overall, our findings suggest that centriole length must be carefully regulated to restrict procentriole number and thus ensure accurate cell division

Functional characterization of the SAS-4-related protein CPAP in centrosome biology of human cells

Gregor Kohlmaier

The centrosome is an organelle that resides at the center of most animal cells and comprises two microtubule-based centriole cylinders surrounded by pericentriolar material (PCM). The centrosome plays a fundamental role for nucleating and organizing a radial array of cytoplasmic microtubules during interphase and promoting the assembly of the mitotic spindle during mitosis. In proliferating cells, the centrosome duplicates once per cell cycle, a process that involves notably the formation of one procentriole at the base of each centriole. The procentriole then grows until it reaches the same size of its associated centriole. The formation of supernumerary centrosomes causes severe problems, since this can lead to multipolar spindle formation, chromosome missegregation and genomic instability, which are hallmarks of cancer cells. The mechanisms regulating centrosome duplication are still incompletely understood. We have studied the role of the centrosomal P4.1-associated protein (CPAP) in human cells. We found that this protein is required for efficient growth of cytoplasmic microtubules from centrosomes as well as for centrosome duplication. There, CPAP is required at an early step during procentriole assembly, after the incorporation of HsSAS-6 but before that of Centrin. Importantly, we found also that the overexpression of CPAP leads to abnormal elongation of procentrioles and centrioles, indicating that the levels of CPAP determine centriole length. Excess CPAP levels furthermore lead to the formation of multiple procentrioles along overly elongated centrioles, multipolar spindle formation and cell division errors. Therefore, proper regulation of proteins such as CPAP that set centriole length contributes to ensure genome integrity. Overall, we gained important insights into the functions of CPAP at the centrosome and identified a novel control mechanism of centriole length. This will be relevant for a better understanding of how centrosomes function in the progression of cancer and other diseases.

EPFL2009

SAS-4 is essential for centrosome duplication in C elegans and is recruited to daughter centrioles once per cell cycle

Pierre Gönczy

The mechanisms governing centrosome duplication remain poorly understood. We identified a gene called sas-4 that is essential for this process in C. elegans. SAS-4 encodes a predicted coiled-coil protein that localizes to a tiny dot in the center of centrosomes throughout the cell cycle. FRAP experiments with GFP-SAS-4 transgenic embryos reveal that SAS-4 is recruited to the centrosome once per cell cycle, at the time of organelle duplication. Additional evidence indicates that SAS-4 is recruited to the daughter centriole or a closely associated structure. These findings identify SAS-4 recruitment as a key step in the centrosome duplication cycle.

2003

PP2A Phosphatase Acts upon SAS-5 to Ensure Centriole Formation in C. elegans Embryos

Pierre Gönczy, Debora Keller

Centrosome duplication occurs once per cell cycle and ensures that the two resulting centrosomes assemble a bipolar mitotic spindle. Centriole formation is fundamental for centrosome duplication. In Caenorhabditis elegans, the evolutionarily conserved proteins SPD-2, ZYG-1, SAS-6, SAS-5, and SAS-4 are essential for centriole formation, but how they function is not fully understood. Here, we demonstrate that Protein Phosphatase 2A (PP2A) is also critical for centriole formation in C. elegans embryos. We find that PP2A subunits genetically and physically interact with the SAS-5/SAS-6 complex. Furthermore, we show that PP2A-mediated dephosphorylation promotes centriolar targeting of SAS-5 and ensures SAS-6 delivery to the site of centriole assembly. We find that PP2A is similarly needed for the presence of HsSAS-6 at centrioles and for centriole formation in human cells. These findings lead us to propose that PP2A-mediated loading of SAS-6 proteins is critical at the onset of centriole formation.

Elsevier2011

Functional characterization of the SAS-4-related protein CPAP in centrosome biology of human cells

Gregor Kohlmaier

EPFL2009