Critical determinants for chromatin binding by Saccharomyces cerevisiae Yng1 exist outside of the plant homeodomain finger.

The temporal and spatial regulation of histone post-translational modifications is essential for proper chromatin structure and function. The Saccharomyces cerevisiae NuA3 histone acetyltransferase complex modifies the amino-terminal tail of histone H3, but how NuA3 is targeted to specific regions of the genome is not fully understood. Yng1, a subunit of NuA3 and a member of the Inhibitor of Growth (ING) protein family, is required for the interaction of NuA3 with chromatin. This protein contains a C-terminal plant homeodomain (PHD) finger that specifically interacts with lysine 4-trimethylated histone H3 (H3K4me3) in vitro. This initially suggested that NuA3 is targeted to regions bearing the H3K4me3 mark; however, deletion of the Yng1 PHD finger does not disrupt the interaction of NuA3 with chromatin or result in a phenotype consistent with loss of NuA3 function in vivo. In this study, we uncovered the molecular basis for the discrepancies in these data. We present both genetic and biochemical evidence that full-length Yng1 has two independent histone-binding motifs: an amino-terminal motif that binds unmodified H3 tails and a carboxyl-terminal PHD finger that specifically recognizes H3K4me3. Although these motifs can bind histones independently, together they increase the apparent association of Yng1 for the H3 tail.