Sos7, an essential component of the conserved Schizosaccharomyces pombe Ndc80-MIND-Spc7 complex, identifies a new family of fungal kinetochore proteins.

Chromosome segregation is powered by the kinetochore, a large macromolecular structure assembled on centromeric chromatin. Attachment of sister chromatids to microtubules is mediated by the highly conserved tripartite KMN (acronym for KNL-1-Mis12-Ndc80) kinetochore network. In the fission yeast Schizosaccharomyces pombe, the equivalent complex is called NMS (Ndc80-MIND-Spc7). Here, we show that not all components of the NMS complex had been identified previously. A 10th NMS component exists, the essential Sos7 protein, which is a genetic and physical interaction partner of Spc7. The analysis of sos7 kinetochore-null mutant yeast strains demonstrated that Sos7 is central to NMS function. In particular, Sos7 is required for kinetochore targeting of Spc7 as well as components of the MIND complex. sos7 mutant strains show severe chromosome missegregation phenotypes and have compromised microtubule-kinetochore interactions. Sos7 is the founding member of a functionally conserved fungal kinetochore family not present in the point centromere carrying Saccharomycotina clusters, suggesting that the new Sos7 family might be a signature motif of fungi with regional centromeres.