A Pocket on the Surface of the N-Terminal BRCT Domain of Mcph1 Is Required to Prevent Abnormal Chromosome Condensation

Mcph1 is mutated in autosomal recessive primary microcephaly and premature chromosome condensation (PCC) syndrome. Increased chromosome condensation is a common feature of cells isolated from patients afflicted with either disease. Normal cells depleted of Mcph1 also exhibit PCC phenotype. Human Mcph1 contains three BRCA1-carboxyl terminal (BRCT) domains, the first of which (Mcph1N) is necessary for the prevention of PCC. The only known disease-associated missense mutation in Mcph1 resides in this domain (T27R). We have determined the X-ray crystal structure of human Mcph1N to 1.6 Å resolution. Compared with other BRCT domain structures, the most striking differences are an elongated, ordered β1–α1 loop and an adjacent hydrophobic pocket. This pocket is in the equivalent structural position to the phosphate binding site of BRCT domains that recognize phospho-proteins, although the phosphate-binding residues are absent in Mcph1N. Mutations in the pocket abrogate the ability of full-length Mcph1 to rescue the PCC phenotype of Mcph1−/− mouse embryonic fibroblast cells, suggesting that it forms an essential part of a protein–protein interaction site necessary to prevent PCC.