Mechanism of H2B ubiquitylation-dependent H3K4 methylation

Abstract

Genetic and cell-based studies have documented a cross-talk between H2B ubiquitylation (H2Bub) and H3K4 methylation, but little, if any, direct biochemical evidence exists explaining the mechanism underlying H2Bub-dependent H3K4 methylation on chromatin templates. To gain valuable insights, we have turned to an in vitro histone methyltransferase assay employing a reconstituted/purified yeast Set1 complex (ySet1C) and a recombinant chromatin template containing fully ubiquitylated H2B. In conjunction with genetic analyses, we demonstrate that the n-SET domain within Set1, but not Swd2, is essential for H2Bub-dependent H3K4 methylation. Spp1, a homolog of human CFP1, is conditionally involved in this cross-talk, and our findings extend to the human Set1 complex, underscoring the highly conserved nature of this disease-relevant, cross-talk pathway. As not all members of the known H3K4 methyltransferase family contain n-SET domains, our studies call attention to the n-SET domain as being a predictor of H2B ubiquitylation ‘sensing’ in bringing about downstream H3K4 methylation