Required for the polymorphic O-glycosylation of the serine-rich repeat protein PsrP. A stabilizing protein that is part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat cell wall proteins encoded upstream in the same operon. The GtfA-GtfB complex adds GlcNAc from UDP-GlcNAc to PsrP, attaching the first sugar residue. Stabilizes the glycosylation activity of GtfA (By similarity) (PubMed:24936067, PubMed:28246170). Has no N-acetylglucosaminyl transferase activity on its own (PubMed:24936067). Protein modification; protein glycosylation. Interacts with glycosyltransferase GtfA; probably forms a heterotetramer with 2 subunits each of GtfA and GtfB. Part of the accessory SecA2/SecY2 protein translocation apparatus. The N-terminus of the protein, including the glycosyltransferase 1 domain (residues 1-171), is required but not sufficient for full formation of the stable GtfA-GtfB complex; the complex of GtfA with truncated GtfB has intermediate N-acetylglucosaminyl transferase activity. 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 GtfB family.