Part of the glycosylphosphatidylinositol-N- acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to phosphatidylinositol and participates in the first step of GPI biosynthesis. {ECO:0000269|PubMed:16162815, ECO:0000269|PubMed:27694521, ECO:0000269|PubMed:8806613}. This is the function of ENSG00000135845 (PIGC, phosphatidylinositol glycan anchor biosynthesis class C).