Structural and functional divergence of the newly identified GtrIc from its Gtr family of conserved Shigella flexneri serotype-converting glucosyltransferases

Abstract

Glucosyltransferases (Gtrs) and O-acetyltransferase (Oac) are integral membrane proteins embedded within the cytoplasmic membrane of Shigella flexneri. Gtrs and Oac are responsible for unidirectional host serotype conversion by altering the epitopic properties of the bacterial surface lipopolysaccharide (LPS) O-antigen. In this study, we present the membrane topology of a recently recognized Gtr, GtrIc, which is known to mediate S. flenxeri serotype switching from 1a to 1c. The GtrIc topology is shown to deviate from those typically seen in S. flexneri Gtrs. GtrIc has 11 hydrophilic loops, 10 transmembrane helices, a double intramembrane dipping loop 5, and a cytoplasmic N- and C-terminus. Along with a unique membrane topology, the identification of non-critical Gtr-conserved peptide motifs within large periplasmic loops (N-terminal D/ExD/E and C-terminal KK), which have previously been proven essential for the activity of other Gtrs, challenge current opinions of a similar mechanism for enzyme function between members of the S. flexneri Gtr family.