Structure of the Sir3 protein bromo adjacent homology (BAH) domain from S. cerevisiae at 1.95 Å resolution

Abstract

Sir3p is a silent‐information‐regulator (SIR) protein required for the assembly of a transcriptionally “silent” chromatin structure at telomeres and the cryptic HM mating‐type loci in Saccharomyces cerevisiae . Sir3p contains a putative “bromo adjacent homology” (BAH) domain at its N terminus that shares strong sequence similarity with the BAH domain of a subunit of the origin recognition complex (ORC), Orc1p. The Orc1p–BAH domain forms a well‐defined complex with the ORC interaction region (OIR) of another Sir protein, Sir1p, which targets formation of silent chromatin to the HM ‐loci. Interestingly, despite sequence similarity of the Sir3p and Orc1p BAH domains and Sir3p's established importance in silencing, Sir3p does not bind the Sir1p–OIR. Here we report the 1.95 Å resolution crystal structure of the Sir3p–BAH domain. The structure reveals two key features that can account for Sir3p–BAH domain's inability to interact with Sir1p. First, several Orc1p–BAH domain residues known to directly contact Sir1p are altered in the Sir3p–BAH domain. Second, a critical OIR‐binding pocket present on the surface of the Orc1p–BAH domain is “filled” in the Sir3p–BAH domain structure, potentially making it inaccessible to Sir1p. These findings imply that the Sir3p–BAH domain structure has evolved for functions distinct from those of the Orc1p–BAH domain.