The mycobacterial
lipoglycans,
lipomannan (LM) and
lipoarabinomannan (
LAM), are potent
immunomodulators in
tuberculosis and
leprosy. Little is known of their biosynthesis, other than being based on
phosphatidylinositol (PI), and they probably originate in the
phosphatidylinositol mannosides (PIMs; PIMans). A novel form of cell-free incubation involving in vitro and in situ labeling with
GDP-[14C]Man of the polyprenyl-P-mannoses (C35/C50-P-Man) and the simpler PIMs of mycobacterial membranes, reisolation of the [14C]Man-labeled membranes, and in situ chase demonstrated the synthesis of a novel alpha(1-->6)-linked linear form of LM at the expense of the C35/C50-P-Man. There was little or no synthesis under these conditions of PIMan5 with its terminal alpha(1-->2)Man unit or the mature LM or
LAM with copious alpha(1-->2)Man branching. Synthesis of the linear LM, but not of the simpler PIMan2, was susceptible to
amphomycin, a
lipopeptide antibiotic that specifically inhibits polyprenyl-P-requiring translocases. A mixture of P[3H]I and P[3H]IMan2 was incorporated into the linear LM, supporting other evidence that, like the PIMs, LM and
LAM, it is a
lipid-linked mannooligosaccharide and a new member of the mycobacterial
glycosylphosphatidylinositol lipoglycan/
glycolipid class. Hence, the simpler PIMs originate in PI and
GDP-Man, but further growth of the linear backbone emanates from C35-/C50-P-Man and is
amphomycin-sensitive. The origin of the alpha(1-->2)Man branches of mature PIMan5, LM, and
LAM is not known at this time but is probably
GDP-Man.