Functional Redundancies, Distinct Localizations and Interactions Among Three Fission Yeast Homologs of Centromere Protein-B

Several members of protein families that are conserved in higher eukaryotes are known to play a role in centromere function in the fission yeast Schizosaccharomyces pombe, including two homologs of the mammalian centromere protein CENP-B, Abp1p and Cbh1p. Here we characterize a third S. pombe CENP-B homolog, Cbh2p (CENP-B homolog 2). cbh2Δ strains exhibited a modest elevation in minichromosome loss, similar to cbh1Δ or abp1Δ strains. cbh2Δ cbh1Δ strains showed little difference in growth or minichromosome loss rate when compared to single deletion strains. In contrast, cbh2Δ abp1Δ strains displayed dramatic morphological and chromosome segregation defects, as well as enhancement of the slow-growth phenotype of abp1Δ strains, indicating partial functional redundancy between these proteins. Both cbh2Δ abp1Δ and cbh1Δ abp1Δ strains also showed strongly enhanced sensitivity to a microtubule-destabilizing drug, consistent with a mitotic function for these proteins. Cbh2p was localized to the central core and core-associated repeat regions of centromeric heterochromatin, but not at several other centromeric and arm locations tested. Thus, like its mammalian counterpart, Cbh2p appeared to be localized exclusively to a portion of centromeric heterochromatin. In contrast, Abp1p was detected in both centromeric heterochromatin and in chromatin at two of three replication origins tested. Cbh2p and Abp1p homodimerized in the budding yeast two-hybrid assay, but did not interact with each other. These results suggest that indirect cooperation between different CENP-B-like DNA binding proteins with partially overlapping chromatin distributions helps to establish a functional centromere.