| Type: | Domain | Name: | SCA7 domain |
| Description: | SAGA (Spt-Ada-Gcn5 acetyltransferase), a coactivator complex involved in chromatin remodelling, harbours both histone acetylation and deubiquitinationactivities. SAGA-associated factor 73 (Sgf734/ATXN7) and Ataxin-7-like protein 3 (ATXN7L3), two subunits of the SAGA deubiquitination module, contain an ~50-residue SCA7 domain characterised by an atypical zinc-finger (Znf) with a Cys-X(9,10)-Cys-X(5)-Cys-X(2)-His motif and a long sequence insertion between the first two zinc coordinating residues.The SCA7 domain is found exclusively in members of the ATXN7 gene family, which includes two distinct subunits of SAGA complexes:ATXN7 and ATXN7L3 orthologues. The analysis of multiple alignments highlights the consensus signature for the SCA7 domain, encompassing the putative zinc-coordinating residues, but also reveals the distinct features of the two proteins. Marked differences are found mostly in the carboxy-terminal of thedomain, suggesting that divergent evolution of the SCA7 Znf domain occurred in order to achieve specific functions in the SAGA complex. Both SCA7 domainscontain disordered regions, albeit not in the same region. Whereas the first and last 10 residues of ATXN7-SCA7 are not folded, the N-terminal region of ATXN7L3-SCA7 is well structured and the last 30 residues of this domain are not folded. In both ATXN7-SCA7 andATXN7L3-SCA7, the large sequence insertion between the first and second zinc- coordinating cysteines corresponds to a protruding extended hairpin structure.The core of the zinc-binding sites shows a conserved structure formed by two short adjacent loops located at the bottom of the hairpin. Although the SCA7domains of both ATXN7 and ATXN7L3 contain two alpha-helices, these are not located at similar positions in the sequences. In ATXN7-SCA7, the two alpha-helices are located downstream from the zinc-binding site and are separated by a loop containing a large number of positively charged residues. In ATXN7L3-SCA7, the two helices lie to either side of the zinc-binding site, leading to a different packing of the two helices. In ATXN7-SCA7, the two helices have analmost perpendicular orientation, the alpha2 helix being anchored to the zinc- binding site. In ATXN7L3-SCA7, the helices alpha1 and alpha2 adopt an anti-parallel orientation defined by hydrophobic interactions. The ATXN7-SCA7 domain binds to the core or the C-terminal ends of the histone H2A and H2Bdimer, a region located on the lateral face of the nucleosome that containsthe ubiquitinated Lys 120 of H2B. This property is lost in the ATXN7-SCA7 domain [, ].This entry represents the SCA7 domain. | Short Name: | SCA7_dom |
