This family of proteins encompasses the E. coli WcaJ protein involved in colanic acid biosynthesis, the Methylobacillus EpsB protein involved in methanolan biosynthesis, as well as the GumD protein involved in the biosynthesis of xanthan. All of these are closely related to the well-characterized WbaP (formerly RfbP) protein, which is the first enzyme in O-antigen biosynthesis in Salmonella typhimurium. The enzyme transfers galactose from UDP-galactose (NOTE: not glucose) to a polyprenyl carrier (utilizing the highly conserved C-terminal sugar transferase domain, pfam02397) a reaction which takes place at the cytoplasmic face of the inner membrane. The N-terminal hydrophobic domain is then believed to facilitate the "flippase" function of transferring the liposaccharide unit from the cytoplasmic face to the periplasmic face of the inner membrane. Most of these genes are found within large operons dedicated to the production of complex exopolysaccharides such as the enterobacterial O-antigen. Colanic acid biosynthesis utilizes a glucose-undecaprenyl carrier, knockout of EpsB abolishes incorporation of UDP-glucose into the lipid phase, and the C-terminal portion of GumD has been shown to be responsible for the glucosyl-1-transferase activity.
Jiyao W et al.(2023). "The conserved domain database in 2023.",