The PWAPA cassette: Intimate association of a PHD-like finger and a winged-helix domain in proteins included in histone-modifying complexes.

Polycomb complexes function as enforcers of epigenetically repressed state, balanced by an antagonist state mediated by Trithorax. Using sensitive methods of sequence analysis, we show here that Polycomb-like proteins (PCLs) contain a tandem of intimately associated domains, which we named PWAPA and which is also present in ASH2L, a member of the Trithorax group. Polycomb-like proteins and ASH2L belong to the PCR2 and MLL histone methyltransferase complexes, respectively. A PWAPA cassette is also present in ATAC2, a component of the ATAC histone acetyltransferase complex. The recently solved structure of the PWAPA tandem of ASH2L has revealed that it consist in a PHD-like finger followed by a helix-winged-helix (WH) domain, able to bind DNA. The modeling of the 3D structure of the different members of the PWAPA family suggests that the PHD-like finger might be able, at least for some proteins of the family, to bind methylated marks on histones. The PWAPA PHD/WH cassette might thus be involved in the combined recognition of DNA and specific (perhaps methylation) mark(s) on histones, thereby allowing the recruitment of specific chromatin-modifying activities at these sites. The observations reported here should help to unravel the exact role played by the PWAPA cassette in the different proteins of the PWAPA family, and especially in the antagonistic activities of PcG and TrxG proteins.