Fig. 6: Illustration of the proposed functions of SccN, SccP, SccM, SccQ and a hypothetical glucosyltransferase in modification of the polyrhamnose backbone with Glc side-chains.

In this proposed model, α-Glc-P-Und and β-Glc-P-Und are synthesized on the cytoplasmic surface of the plasma membrane from Und-P and UDP-Glc, catalyzed by GT-A type glycosyltransferases encoded by SccP and SccN, respectively. Following synthesis, the Und-P linked intermediates diffuse transversely to the exoplasmic surface of the plasma membrane, mediated by hypothetical flippase(s)/scramblase(s) (indicated by question marks), where they function as glucosyl donors in glucosylation reactions catalyzed by GT-C type glycosyltransferases, SccQ, SccM and an additional unidentified transferase. SccM uses β-Glc-P-Und to catalyze α-glucosylation of the 2-position of 3-Rha; SccQ uses α-Glc-P-Und to catalyze β-glucosylation of the 4-position of 3-Rha and of 2,3-Rha. α-glucosylation of the 4-position of 2-Rha appears to be catalyzed by a glycosyltransferase encoded outside of the SCC genetic locus, using β-Glc-P-Und, synthesized by SccN, as glucosyl donor. Created with BioRender. Korotkov K. (2024) https://BioRender.com/a74t849.