Abstract 2158 Novel interactions within a heterochromatin complex potentiate inter-subunit communication and gene repression

Heterochromatin is the epigenetically-maintained, repressed genomic structure that is characterized by a contiguous architecture indicative of spreading from an initiation site. Heterochromatic regions bear distinctive histone modification patterns that are marked during formation, the most common being deacetylated histones. To achieve stable repression, heterochromatin structure incorporates additional proteins that facilitate compaction and exclusion of transcriptional machinery. In S. cerevisiae the Silent Information Regulator complex (SIRc) carries out the core functions of heterochromatin, driving first the initiation and spreading of histone deacetylation and subsequently incorporating stably with nucleosomes for compaction and repression of the genome. These two distinct functions of SIRc have each been characterized in much detail, but the mechanism by which the dynamic spreading state switches to stable compaction are not well-understood. This incomplete knowledge of intra-complex communication is partly due to a lack of structural information of the complex as a whole, since only structures of fragments have been determined to date. Using cross-linking mass spectrometry in solution, we identified a novel inter-subunit interaction in SIRc that physically connects the two states of SIRc. Mutation of this interaction disrupts heterochromatic repression, potentially through disrupting a conserved mechanism that communicates completion of deacetylation to switch to compaction. Our work highlights how a single multi-functional chromatin regulatory complex can stage a step-wise mechanism that requires a major shift in activity. This work is supported by NSF Grant 2002231.