Required for the polymorphic O-glycosylation of serine-rich repeat protein PsrP. Catalyzes the first step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to serine residues in PsrP (PubMed:24936067, PubMed:28246170). Part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat cell wall proteins encoded upstream in the same operon (Probable). The GtfA-GtfB complex adds GlcNAc from UDP-GlcNAc to PsrP (experimentally characterized with truncated PsrP-SSR1 constructs); this subunit alone has weak N-acetylglucosaminyl transferase activity that is 10-fold stimulated by GtfB. The complex requires at least a 25 residue-long peptide for activity; the in vitro assay has only been seen to glycosylate Ser residues (PubMed:24936067). The alpha linkage was shown in L.reuteri. L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-[N-acetyl-alpha-D-glucosaminyl]-L-seryl-[protein] + H(+) + UDP kcat for hydrolysis of UDP-N-acetyl-alpha-D-glucosamine with transfer of GlcNAc to PsrP-SSR1 and generation of UDP is 76.0 min(-1) for GtfA-GtfB and 7.35 min(-1) for GtfA alone. Protein modification; protein glycosylation. Monomer (PubMed:24936067). Interacts with stabilizing protein GtfB, probably as a heterotetramer with 2 subunits each of GtfA and GtfB, part of the accessory SecA2/SecY2 protein translocation apparatus (PubMed:24936067). 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 and is required for recognizing both PsrP substrate and coactivator GtfB. UDP and N-acetyl-D-glucosamine bind between the Rossmann-like folds. Encoded in RD10, a pathogenicity island with an atypical GC content that is associated with invasive pneumococcal disease. Pathogenicity islands account for greater than half the genomic diversity observed between isolates (PubMed:11463916, PubMed:16861665). The main function of this island seems to be correct synthesis and export of pneumococcal serine-rich repeat protein PsrP (Probable). Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.