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, PIGQ, PIGY and DPM2 (PubMed:16162815, PubMed:9463366). Interacts with PIGC, PIGH, PIGP, PIGQ and DPM2 (PubMed:16162815). Interacts directly with PIGY; this interaction regulates glycosylphosphatidylinositol-N-acetylglucosaminyltransferase activity (PubMed:16162815). Interacts with PIGQ (PubMed:9463366). Disease susceptibility is associated with variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily. Phosphatidylinositol N-acetylglucosaminyltransferase subunit A