Role of Bre2 and Sdc1 in the regulation of histone lysine methylation by Set1 Complex

Abstract

The Set1 complex, often called COMPASS (Complex associated with Set1 subunit), is a multi-protein complex containing the trithorax-related protein, Set1, which methylates histone H3 lysine-4 (H3K4) and additional substrates. The catalytic SET domain of Set1 requires a host of associated subunits to catalyze the transfer of methyl groups from S-adenosylmethionine to lysine residues of target substrates. Bre2 and Sdc1 are two subunits that promote H3K4 tri-methylation, a hallmark modification found at promoters of activatated RNA polymerase II genes. In collaboration with Sharon Dent\u27s group, we have previously demonstrated that Bre2 and Sdc1 are methylated by Set1 in vitro and in vivo. The significance of Bre2 methylation in regulating Set1 catalytic activity is demonstrated by changes in the global levels of H3K4 di-methylation upon blocking methylation of Bre2 at a specific lysine residue (K287). Indirect evidence suggests that demethylation of Bre2 and Sdc1 is possibly mediated by the H3K4 demethylase complex. The mechanism by which Bre2 and Sdc1 regulate Set1\u27s catalytic activity is not well understood, and represents the driving impetus for the current studies. In the present study we show that Sdc1 physically interacts with the C-terminus of Bre2, and that a Bre2/Sdc1 subcomplex can stably interact with the SET domain of Set1. We describe the development of an immobilized methyltransferase activity assay, and use this assay to demonstrate reconstitution of a Set1 complex functional for subunit methylation. The latter result suggests that Bre2/Sdc1 exchange between methyltransferase and demethylase complexes is a tenable hypothesis