Peptide recognition by heterochromatin protein 1 (HP1) chromoshadow domains revisited: Plasticity in the pseudosymmetric histone binding site of human HP1.

Heterochromatin protein 1 (HP1), a highly conserved non-histone chromosomal protein in eukaryotes, plays important roles in the regulation of gene transcription. Each of the three human homologs of HP1 includes a chromoshadow domain (CSD). The CSD interacts with various proteins bearing the PVL motif but also with a region of histone H3 that bears the similar PVL motif. The latter interaction has not yet been resolved in atomic detail. Here we demonstrate that the CSDs of all three human HP1 homologs have comparable affinities to the PVL motif of histone H3. The HP1 C-terminal extension enhances the affinity, as does the increasing length of the H3 peptide. The crystal structure of the human HP1γ CSD (CSD) in complex with an H3 peptide suggests that recognition of H3 by CSD to some extent resembles CSD-PVL interaction. Nevertheless, the prolyl residue of the PVL motif appears to play a role distinct from that of Pro in the known HP1β CSD-PVL complexes. We consequently generalize the historical CSD-PVL interaction model and expand the search scope for additional CSD binding partners.