Involved in the initiation of core alpha-(1->6) mannan biosynthesis of lipomannan (LM-A) and multi-mannosylated polymer (LM-B), extending triacylatedphosphatidyl-myo-inositol dimannoside (Ac1PIM2) and mannosylated glycolipid, 1,2-di-O-C16/C18:1-(alpha-D-mannopyranosyl)-(1->4)-(alpha-D-glucopyranosyluronic acid)-(1->3)-glycerol (Man1GlcAGroAc2), respectively. Catalyzes the addition of alpha-(1->6)-mannose residue. Cell wall biogenesis; cell wall polysaccharide biosynthesis. Cells lacking this gene are unable to synthesize lipoglycans (LM-A, LM-B and lipoarabinomannan (LAM)). Belongs to the MptA/B family. Truncated N-terminus.