The molecular mechanism of centromeric chromatin assembly and recognition
Final Report Abstract
Centromeres are epigenetically defined by the presence of the centromere-specific histone H3 variant CENP-A. A specialized loading machinery, including the histone chaperone HJURP/Scm3, participates in CENP-A nucleosome assembly. Surprisingly, Scm3/HJURP is missing from multiple lineages, including nematodes, which rely on a CENP-A-dependent centromere. We showed that the extended N-terminal tail of C. elegans CENP-A contains a predicted structured region that is essential for centromeric chromatin assembly. A CENP-A tail mutant does not localize on chromatin, resulting in a failure of kinetochore assembly and defective chromosome condensation; in contrast, the tail mutant localizes normally in the presence of endogenous CENP-A. A portion of the tail containing a structured region interacts with KNL-2, a Myb domaincontaining conserved protein specifically involved in CENP-A nucleosome assembly. This interaction is conserved in the related nematode C. briggsae, despite divergence of the N-tail and KNL-2 primary sequences. These results suggest that the extended N-tail of CENP-A is essential for CENP-A chromatin assembly in C. elegans, and is partially analogous to Scm3/HJURP in that it mediates an interaction of the specialized histone fold of CENP-A with the KNL-2 protein family. These results highlight an evolutionary variation on centromeric chromatin assembly in the absence of a dedicated CENP-A- specific chaperone of the Scm3/HJURP family.
