Required for polymorphic O-glycosylation of GspB, a serine-rich repeat cell wall protein encoded upstream in the same operon. Catalyzes the first step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to serine residues in GspB. Part of the accessory SecA2/SecY2 system specifically required to export GspB. Upon coexpression in E.coli with GtfB glycosylates GspB constructs. Glycosylation probably occurs intracellularly (PubMed:15489421). Requires GtfB for glycosylation activity, it has no activity alone. Does not use UDP-glucose as substrate. Has a fast, probably processive glycosylation phase followed by a slower, non-processive phase. The enzyme probably modifies its tertiary conformation by opening and closing its intersubunit interfaces to accomodate the increasingly glycosylated substrate; protein substrate recognition is provided by GtfB (PubMed:26884191). 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 (PubMed:26884191). Part of the accessory SecA2/SecY2 protein translocation apparatus required to export cell wall protein GspB (Probable). Cell membrane association requires GtfB. Has 2 Rossmann-like domains, called R-fold-1 and R-fold-2. R-fold-1 is interrupted by an extended beta-sheet domain that loops out of the structure. UDP and N-acetyl-D-glucosamine bind between the Rossman-like folds, which move closer together upon binding. Loss of export of cell wall protein GspB; transcription should not be affected. Non-glycosylated GspB may form aggregates. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.