BIOSYNTHESIS OF MYCOBACTERIAL LIPOARABINOMANNAN

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 phosphatidyl inositol (PI), and they probably originate in the phosphatidylinositol mannosides (PIMs; PIMans). A novel form of cell-free incubation invol ving in vitro and in situ labeling with GDP-[(14)]Man of the polypreny l-P-mannoses (C35C50-P-Man) and the simpler PIMs of mycobacterial memb ranes, reisolation of the [C-14]Man-labeled membranes, and in situ cha se demonstrated the synthesis of a novel ru(1 --> 6)-linked linear for m of LM at the expense of the C-33/C-50-P-Man. There was little or no synthesis under these conditions of PIMan(5) with its terminal. alpha( 1 --> 6)Man unit or the mature LM or LAM with copious alpha(1 --> 2)Ma n branching. Synthesis of the linear LM, but not of the simpler PIMan( 2), was susceptible to amphomycin, a lipopeptide antibiotic that speci fically inhibits polyprenyl-P-requiring translocases. A mixture of P[H -3]I and P[H-3]IMan(2) 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 glycosylpho sphatidylinositol lipoglycan/glycolipid class. Hence, the simpler PIMs originate in PI and GDP-Man, but further growth of the linear backbon e emanates from C-35-/C-50-P-Man and is amphomycin-sensitive. The orig in of the alpha(1 --> 1)Man branches of mature PIMan(5), LM, and LAM i s not known at this time but is probably GDP-IMan.