Catalyzes the formation of Forssman glycolipid via the addition of N-acetylgalactosamine (GalNAc) in alpha-1,3-linkage to GalNAcb-1,3Gala-1,4Galb-1,4GlcCer (Gb4Cer) (PubMed:14573676). Forssman glycolipid (also called Forssman antigen; FG) probably serves for adherence of some pathogens (PubMed:14573676). Conversely, it diminishes Shiga toxins susceptibility (PubMed:14573676). globoside Gb4Cer (d18:1(4E)) + UDP-N-acetyl-alpha-D-galactosamine = globoside Forssman (d18:1(4E)) + H(+) + UDP globoside Gb4Cer + UDP-N-acetyl-alpha-D-galactosamine = globoside IV3GalNAc-Gb4Cer + H(+) + UDP Binds 1 Mn(2+) ion per subunit. Protein modification; protein glycosylation. The conserved DXD motif is involved in cofactor binding. The manganese ion interacts with the beta-phosphate group of UDP and may also have a role in catalysis (By similarity). Belongs to the glycosyltransferase 6 family. Globoside alpha-1,3-N-acetylgalactosaminyltransferase 1