HCP-4, a CENP-C-like protein in Caenorhabditis elegans, is required for resolution of sister centromeres

The centromere plays a critical role in the segregation of chromosomes duri ng mitosis. In mammals, sister centromeres are resolved from one another in the G2 phase of the cell cycle. During prophase, chromosomes condense with sister centromeres oriented in a back to back configuration enabling only one chromatid to be captured by each half spindle. To study this process, w e identified a centromere protein (CENP)-C-like protein, holocentric protei n (HCP)-4, in Caenorhabditis elegans based on sequence identity, loss of fu nction phenotype, and centromeric localization. HCP-4 is found in the cytop lasm during interphase, but is nuclear localized in mitosis, where it local izes specifically to the centromere. The localization of HCP-4 to the centr omere is dependent on the centromeric histone HCP-3; in addition, HCP-3 and HCP-4 are both required for localization of a CENP-F-like protein, HCP-1, indicating an ordered assembly pathway. Loss of HCP-I expression by RNA-med iated interference resulted in a failure to generate resolution of sister c entromeres on chromosomes, suggesting that HCP-4 is required for sister cen tromere resolution. These chromosomes also failed to form a functional kine tochore. Thus, the CENP-C-like protein HCP-4 is essential for both resoluti on sister centromeres and attachment to the mitotic spindle.