Synthetic mannosides act as acceptors for mycobacterial α1-6 mannosyltransferase

A series of synthetic mannosides was screened in a cell-free system for their ability to act as acceptor substrates for mycobacterial mannosyltransferases. Evaluation of these compounds demonstrated the incorporation of [¹⁴C]Man from GDP-[¹⁴C]Man into a radiolabeled organic-fraction and analysis by thin layer chromatography and autoradiography revealed the formationof two radiolabeled products. Each synthetic acceptor was capable of accepting one or two mannose residues, resulting in a major and a minor mannosylated product. Both products from each acceptor were isolated and their mass was confirmed by fast-atom bombardment-mass spectrometry FABMS). Characterization of each mannosylated product by exo-glycosidase digestion, acetolysis and linkage analysis by gas chromatography-mass spectrometry of partially per-O-methylated alditols, revealed only α1-6-linked products. In addition, the antibiotic amphomycin selectively inhibited the formation of mannosylated products suggesting polyprenolmonophosphate-mannose C_35/50-P-Man) was the immediate mannose donor in all mannosylation reactions observed. The ability of synthetic disaccharides to act as acceptor substrates in this system, is most likely due to the action of a mycobacterial polyprenol-P-Man:mannan α1-6 mannosyltransferase involved in the biosynthesis of linear α1-6-linked lipomannan.