Scm3 interacts with the N-terminal tail of Cse4 to regulate kinetochore assembly in budding yeast

The kinetochore is a multiprotein complex formed at the centromeres and is essential for the faithful chromosome segregation. In most of the organisms the kinetochore is assembled on a specialized centromeric nucleosome where histone H3 is replaced by a variant, named CENP-A. In budding yeast, the CENP-A homolog, Cse4 is recruited to the centromeric nucleosome through an interaction between its C-terminal domain and a specific chaperone, named Scm3. Interestingly, following Cse4 recruitment during S phase, Scm3 persists at the centromeres throughout the cell cycle. Recent in vitro studies have reported that Scm3 also interacts with N-terminal of Cse4 (N-C) that in turn facilitates a better interaction of Ame1-Okp1 (AO) of COMA subcomplex with N-C, which promotes kinetochore assembly. In this work, using genetic and biochemical assays we provide in vivo evidence of the interaction between Scm3 and N-C. Additionally, by artificially tethering Scm3, we show that their association has the potential to stabilize a missegregating chromosome with inactive centromere, perhaps by forming an active kinetochore at an ectopic site. We propose that at the centromeres, Scm3 has two jobs in tandem - Cse4 deposition and stabilization of N-C, which together culminates into proper kinetochore assembly. This work has clinical significance as both CENP-A and its chaperone are known to be upregulated in human cells under disease states which can predispose the cells to aneuploidy due to increased chances of forming ectopic kinetochores.