The Cse4/CenH3 Histone Variant

We are studying the functions and regulation of the centromeric histone H3 variant (CenH3) that likely forms a specialized centromeric nucleosome. Because CenH3 is associated with all active kinetochores, it may be the epigenetic mark that specifies the site of kinetochore assembly. We discovered that CenH3 is regulated by ubiquitin-mediated proteolysis and isolated dominant lethal Cse4 mutants that are resistant to proteolysis (Collins, K.A., et al., 2004). Stabilized CenH3 localizes to euchromatin, indicating that proteolysis helps to restrict CenH3 to kinetochores. We are now characterizing the proteolysis machinery that degrades Cse4 to learn more about the regulation of Cse4 degradation and to determine how it targets the euchromatin pool of Cse4. In addition, we have recently developed a new method to analyze the localization of proteins at single nucleosome resolution and have used this technique to demonstrate that there is a single centromeric nucleosome (see below and Furuyama, S. and S. Biggins, 2007). We are now identifying the pathways that ensure that Cse4 deposits at the centromere and those that prevent it from spreading into euchromatin.