Required for polymorphic O-glycosylation of the serine-rich repeat protein Srr2. Catalyzes the first step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to serine residues of Srr2. Part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat proteins, probably Srr2 in this organism. The GtfA-GtfB (Gtf1-Gtf2 in this bacteria) complex adds GlcNAc from UDP-GlcNAc to Srr2 substrate. This subunit has low glycosyltransferase activity; GtfB enhances glycosyltransferase activity in vitro. Upon expression in S.parasanguis GtfA/GtfB restores expression of serine-rich repeat protein Fap1 and complements a biofilm formation defect. 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. Interacts with stabilizing protein GtfB (Gtf2), probably as 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.