2.A.129.  The Lipid-linked Sugar Translocase (LST) Family

Shigella flexneri bacteriophage SfX, SfV and SfII each has a 3-gene operon encoding a glucosyltransferase (GtrX), which is involved in O antigen modification (serotype Y to serotype X). This operon is responsible for full O antigen conversion. Besides the gtrX gene, the other two genes in the gtr locus of SfX are also involved in the O antigen modification process. The first gene in the cluster (gtrA) encodes a small hydrophobic protein involved in the translocation of lipid-linked glucose across the cytoplasmic membrane. The second gene in the cluster (gtrB) encodes an enzyme catalysing the transfer of the glucose residue from UDP-glucose to a lipid carrier. The third gene (gtrX) encodes a bacteriophage-specific glucosyltransferase which is largely responsible for the final step, i.e. attaching the glucosyl molecules onto the correct sugar residue of the O antigen repeat unit. A three-step model for the glucosylation of bacterial O antigen has been proposed (Guan et al. 1999).  Salmonella phage P22 also has genes involved in serotype conversion, and they are homologous to the Shigella phage operons cited above (Vander Byl and Kropinski 2000). E. coli also has these genes, proabably because they were incorporated  into the bacterial chromosome (Adams et al. 2001). The Shigella SfV and SfX phage GtrX proteins have 4 TMSs (Korres et al. 2005). The 12-2 antigen is a S. enterica subspecies I-specific LPS modification enhances long-term intestinal colonization (Bogomolnaya et al. 2008).

The Mycobacterium tuberculosis homologue GtrA (Rv3789) (TC# 2.A.129.1.2) may be an anchor protein recruiting AftA, the first arabinosyl transferase involved in arabinogalactan biosynthesis, rather than a lipid flippase (Kolly et al. 2015).