FIG 8.
Refined model of AT3-SGNH fused O-antigen acetyltransferases. Periplasmic SGNHext (orange) is structured, therefore positioning the SGNH domain (gray) close to the AT3 domain (purple); this orients the additional helix (teal) in close proximity to the AT3 domain with interactions between the two domains as proposed by the coevolution analysis. These observations result in the current hypothesis: (1) Cytoplasmic acetyl group donor interacts with conserved Arg in TMH1, the acetyl group is processed and transferred to the periplasmic side of the inner membrane, and this process involves catalytic His residue of TMH1. (2) Conserved Asp and Ser mediate transfer of acetate to the SGNH domain. (3) SGNH domain catalyzes addition of the acetate to specific O-antigen monosaccharide. The active site of the SGNH domain is highlighted by an asterisk, and interaction site is highlighted by a plus sign (+).