Catalytic subunit of the glycosylphosphatidylinositol- mannosyltransferase I complex which catalyzes the transfer of the first mannose, via an alpha-1,4 bond from a dolichol-phosphate-mannose (Dol- P-Man) to a 2-acyl-6-alpha-D-glucosaminyl-1-(1-radyl,2-acyl-sn-glycero- 3-phospho)-1D-myo-inositol (glucosaminyl acyl phosphatidylinositol, GlcN-(acyl)PI) intermediate to generate 2-acyl-6-(alpha-D-mannosyl- (1->4)-alpha-D-glucosaminyl)-1-(1-radyl,2-acyl-sn-glycero-3-phospho)- 1D-myo-inositol (also termed H2) and participates in the sixth step of the glycosylphosphatidylinositol-anchor biosynthesis. . This is the function of Ensembl gene identifier ENSG00000143315 (PIGM, phosphatidylinositol glycan anchor biosynthesis class M).