Essential role of MptB in the biosynthesis of phosphatidylinositol mannosides, lipomannan and lipoarabinomannan in mycobacteria

Lipomannan (LM) and lipo-arabinomannan (LAM) are important components of the cell envelope of all mycobacteria that have been extensively studied for their roles in mycobacterial physiology and host-pathogen interactions. Despite the considerable progress made in deciphering the structure and biosynthesis of these lipoglycans over the last few decades, some of the key steps leading to their assembly and export to the cell surface remain ill-defined. We report on the characterization of a conserved and essential polyprenyl phosphate mannose-dependent mannosyltransferase named MptB, involved in the initial steps of the elongation of the mannan domain of LM and LAM from a phosphatidylinositol mannoside (PIM) anchor. Genetic silencing of mptB in Mycobacterium smegmatis led to the arrest of LM, LAM, and PIM synthesis beyond di-mannosylated forms of these glycolipids. In cell-free assays, mptB overexpression led to the increased production of tetra-mannosylated forms of PIMs by M. smegmatis membranes, whereas reduced mptB expression resulted in the dramatically decreased synthesis of phosphatidylinositol tri-, tetra-, and hexa-mannosides. Together with structural modeling predictions, the results of these assays support MptB as the α-(1,6)-mannosyltransferase elongating the mannan backbone of LM from a di- and/or tri-mannosylated PIM primer.