Separation of function mutations in microcephaly protein CPAP/CENPJ, reveal its role in regulating centriole number and length.

Centriole are microtubule-based cylindrical structures characterized by their definite size, and stable, slow growing microtubules. The centriole core protein CPAP/CENPJ is known to act as a molecular cap regulating centriole length by interacting with microtubule/tubulin via the conserved microtubule destabilizing, PN2-3, and microtubule stabilizing, A5N, domains. The C-terminus of CPAP has a conserved glycine-rich G-box/TCP domain (1050-1338 amino acids). This region is involved in centriole cartwheel assembly by interacting with the cartwheel protein STIL. However, previously reported primary microcephaly mutation mapped in the G-box of CPAP, i.e., E1235V (MCPH6) affects centriole length via an unknown mechanism. Recently, another primary microcephaly mutation has been mapped to this region of CPAP, i.e., D1196N. However, the effect of D1196N on CPAP functioning is not known. We simultaneously characterized these two MCPH mutations in the G-box of CPAP. We identified that despite affecting the same domain of CPAP, they affect distinct CPAP functions at the centriole. The E1235V mutation caused an overly long centriole, and the D1196N mutation increased the centriole number. Interestingly, both these mutations affect CPAP direct interaction with the cartwheel protein STIL, which is involved in CPAP recruitment to the centriole. Accordingly, the CPAP E1235V centriole localization is significantly affected at the centriole. However, CPAP D1196N can still localize to centriole at levels comparable to the wild-type CPAP. We show that CPAP utilizes an alternate CEP152-dependent route for centriole recruitment. Importantly, our work highlights the importance of the CPAP region outside direct microtubule/tubulin interacting domains in influencing CPAP activity in cartwheel assembly and centriole length. Perhaps, this is why deleterious naturally occurring missense mutations are frequently occurring in this particular region of CPAP in primary microcephaly.