Required for polymorphic O-glycosylation of the serine-rich repeat protein PsrP. Catalyzes the second step in glycosylation of the serine-rich repeat protein in this bacteria. Transfers glucose from UDP-glucose to the terminal GlcNAc moiety of 3-O-(N-acetyl-alpha-D-glucosaminyl)-L-seryl-[protein] which results from the first glycosylation step of PsrP (using the short substrate SRR1-GlcNAc) (PubMed:28246170, PubMed:25404702, PubMed:21653318). Has weak hydrolytic activity against UDP-glucose and even weaker activity on UDP-galactose (PubMed:28246170). Does not complement deletion of the gtf3 gene from S.parasanguinis strain FW213 (PubMed:21653318). Protein modification; protein glycosylation. Homotetramer; a dimer of dimers. Binds protein substrate via an exposed loop in the N-terminus. Dimerizes via the C-terminus; dimerization is required for tetramer formation. Binds protein substrate via an exposed loop in the N-terminus. 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 Gtf3 glucosyltransferase family.