This is a cysteine-rich domain termed ADD (ATRX-DNMT3-DNMT3L, AD-DATRX) found in ATRX proteins. Chromatin-associated human protein ATRX was originally identified because mutations in the ATRX gene cause a severe form of syndromal X-linked mental retardation called ATR-X syndrome. Mutations or knockdown of ATRX expression cause diverse effects, including altered patterns of DNA methylation, a telomere-dysfunction phenotype, aberrant chromosome segregation, premature sister chromatid separation and changes in gene expression. ATRX localizes predominantly to large, tandemly repeated regions (such as telomeres, centromeres and ribosomal DNA) associated with heterochromatin, and studies show that it directs H3.3 deposition to pericentric and telomeric heterochromatin. The ADD domain of ATRX, in which most syndrome-causing mutations occur, engages the N-terminal tail of histone H3 through two rigidly oriented binding pockets, one for unmodified Lys4 and the other for di- or trimethylated Lys9. Mutations in the ATRX ADD domain cause mislocalization of ATRX protein to heterochromatin, and this may contribute to understanding the underlying etiology of ATRX syndrome. Structure analysis of the ADD domain of ATRX revealed that it contains a PHD zinc-finger domain packed against a GATA-like zinc finger. Same structure is also found in the DNMT3 DNA methyltransferases and DNMT3L [, , ].
One of the largest protein families in the human genome is the zinc fingerfamily that contains members involved in the regulation of transcriptionprocesses. The zinc finger domains have been classified based on the order ofcysteine (C) and histidine (H) residues. Zinc fingers are thought to mediateprotein-DNA and protein-protein interactions. The ADD (ATRX, DNMT3, DNMT3L)domain is a cysteine-rich region that consists of a C2C2-type zinc finger anda closely located domain of an imperfect PHD-type zinc finger with C4C4. The region between the two subdomains has a constant length, andit contains identical and conserved amino acids [, ]. The ADD domain binds to the histone H3 tail that is unmethylated at lysine 4 [, ].The ADD domain is present in chromatin-associated proteins that play a role inestablishing and/or maintaining a normal pattern of DNA methylation:DNMT3A, DNMT3B, DNA methyltransferases.DNMT3L, a DNMT3-like enzymatically inactive regulatory factor.ATRX, a large nuclear protein predominantly localized to heterochromatinand nuclear PML bodies. At the C terminus is a helicase/ATPase domain, which characterises ATRX as a member of the SNF2 (SWI/SNF) family of chromatin-associated proteins. The ADD domain is composed of three clearly distinguishable modules that packtogether through extensive hydrophobic interactions to form a single globulardomain. Packed against this GATA-like finger is a second subdomain,which binds two zinc ions and closely resembles the structure reported forseveral PHD fingers. Finally, there is a long C-terminal α-helix that runsout from the PHD finger and makes extensive hydrophobic contacts with the N-terminal GATA finger, bringing the N- and C-termini of the ADD domain closetogether. This combination of fused GATA-like and PHD fingers within a singledomain is thus far unique [, ].
