Required for polymorphic O-glycosylation of serine-rich repeat protein Fap1. Catalyzes the first step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to serine residues in Fap1. Part of the accessory SecA2/SecY2 system specifically required to export Fap1, a serine-rich fimbrial adhesin encoded upstream in the same operon. The GtfA-GtfB (Gtf1-Gtf2 in this bacteria) complex adds GlcNAc from UDP-GlcNAc to Fap1, attaching the first sugar residue. Cannot use not UDP-Glc as substrate. This subunit has very low glycosyltransferase activity; the GtfB stabilizing protein enhances membrane association, protease resistance and glycosyltransferase activity. 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 (Gtf1), probably via the N-terminus of this protein; probably 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 (Gtf2), protein is more active and protected from trypsin (PubMed:21862581). No glycosylation of serine-rich fimbrial adhesin Fap1, an unglycosylated version of Fap1 accumulates that is larger than the wild-type protein. No further defect in a double gtfA/gtfB disruption mutant, but biofilm formation is further decreased than in a gtfB deletion. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.