A nonhistone protein-protein interaction required for assembly of the SIR complex and silent chromatin

Budding yeast silent chromatin, or heterochromatin, is composed of histones and the Sir2, Sir3, and Sir4 proteins. Their assembly into silent chromatin is believed to require the deacetylation of histones by the NAD-dependent deacetylase Sir1 and the subsequent interaction of Sir3 and Sir4 with these hypoacetylated regions of chromatin. Here we explore the role of interactions among the Sir proteins in the assembly of the SIR complex and the formation of silent chromatin. We show that significant fractions of Sir2, Sir3, and Sir4 are associated together in a stable complex. When the assembly of Sir3 into this complex is disrupted by a specific mutation on the surface of the C-terminal coiled-coil domain of Sir4, Sir3 is no longer recruited to chromatin and silencing is disrupted. Because in sir4 mutant cells the association of Sir3 with chromatin is greatly reduced despite the partial Sir2-dependent deacetylation of histones near silencers, we conclude that histone deacetylation is not sufficient for the full recruitment of silencing proteins to chromatin and that Sir-Sir interactions are essential for the assembly of heterochromatin.