Effector proteins for methylated histones: an expanding family.

Methylation of histone lysine residues in eukaryotic chromatin has been an exciting area of research ever since the first histone methyltransferase enzyme, Suv39h, was found to methylate lysine 9 of histone H3 in 2000. Only a year later, the HP1 chromodomain polypeptide was identified as a recognition module for this histone modification. Similar to bromodomain-containing proteins that recognize histone acetylation sites and subsequently stabilize large complexes to chromatin, effector proteins can also be recruited and stabilized by histone methylation. Although histone acetylation generally correlates with active transcription, histone methylation is associated with both the activation and silencing of transcription, depending on which lysine residue is modified. The list of proteins that may in fact directly associate with specific methylated histone lysines is expanding. Since the finding of HP1, many additional proteins have been shown to bind methylated histone residues. For instance, Polycomb, Chd1, 53BP1, and Crb2/Rad9 proteins all associate with methylated chromatin in a unique manner governed by their respective recognition motifs. Here we highlight recent data on the recognition specificity and biological significance of proteins that associate with methylated histone lysines.