Catalytic subunit 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. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + UDP-N-acetyl-alpha-D-glucosamine = a 6-(N-acetyl-alpha-D-glucosaminyl)-1-phosphatidyl-1D-myo-inositol + H(+) + UDP Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Component of the glycosylphosphatidylinositol-N-acetylglucosaminyltransferase (GPI-GnT) complex composed at least by PIGA, PIGC, PIGH, PIGP and PIGQ, DPM2. Interacts with PIGC, PIGH, PIGP, PIGQ and DPM2. The latter is not essential for activity. Interacts directly with PIGY; this interaction regulates glycosylphosphatidylinositol-N-acetylglucosaminyltransferase activity. Interacts with PIGQ. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.