A specialized nucleosome has a "point" to make

Abstract

Three recent papers, including Mizuguchi et al. (2007) in this issue, show that the nonhistone protein Scm3 is required for the recruitment of the histone H3 variant Cse4 to centromeres in budding yeast. Scm3 forms a chromatin component with Cse4:histone H4 tetramers that appear to lack H2A/H2B histones. These studies provide key insights into the pathway that recruits Cse4 to centromeres and have important implications for other functions of chromatin.

Figure 1. Scm3 in the Assembly of Yeast Centromeric Chromatin

(A) A specialized “nucleosome” at budding yeast centromeres. S. cerevisiae CEN DNA is comprised of three conserved DNA elements: CDE I, CDE II, and CDE III. The CBF3 complex binds to CDEIII and recruits additional kinetochore proteins, such as Cse4, to centromeres. Scm3 physically associates with Cse4:H4 tetramers and could form a centromeric “nucleosome” that lacks histone H2A and H2B. Scm3 also interacts with Ndc10, part of the CBF3 complex. The proteins involved in connecting the centromere to a microtubule are not shown. (B) A new model for assembly of centromeric nucleosomes. Scm3 and Ndc10 are codependent with respect to their centromere binding through CDEII and CDEIII, respectively, and could act cooperatively to promote the nucleation of Cse4:H4:Scm3 “nucleosomes” at centromeres.