Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the first alpha-1,4-mannose to GlcN-PI during GPI precursor assembly (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. In T.brucei, PIGM has a different substrate specificity compared to mammalian. Given that the protozoan parasite evades the immune response of mammalian hosts and digestion in the gut of the insect vector by means of its coat proteins tethered to the cell surface via GPI-anchors, it may be a good candidate for chemotherapy against African trypanosomiasis. Belongs to the PIGM family.