Required for polymorphic O-glycosylation of the serine-rich repeat protein (SRRP) in this bacteria. Catalyzes the first step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to serine residues in the substrate protein. Part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat cell wall proteins encoded in the same operon. The GtfA-GtfB complex adds GlcNAc from UDP-GlcNAc to SRRP (experimentally characterized with a truncated SSR1 construct); the alpha linkage was shown for this enzyme but not the residues glycosylated on SRRP. L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-[N-acetyl-alpha-D-glucosaminyl]-L-seryl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Forms a heterotetramer with 2 subunits each of GtfA and GtfB. Part of the accessory SecA2/SecY2 protein translocation apparatus. Cell membrane association requires GtfB. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.