Transfers N-acetylgalactosamine onto mannose groups of carbohydrate substrates. Required for susceptibility to pore-forming crystal toxins in conjunction with bre-1, bre-2, bre-3, and bre-4. Involved in resistance to the nematotoxic C.cinerea galectin Cgl2 (PubMed:20062796). Protein modification; protein glycosylation. Expressed in the gut. Worms exhibit resistance to the Cry5B and Cry14A toxins produced by Bacillus thuringiensis. This is thought to be due to mutants having reduced population of glycolipids which are targeted by the Cry proteins. Belongs to the glycosyltransferase 31 family.