Required for polymorphic O-glycosylation of the serine-rich repeat protein Srr2. 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 Srr2; does not use other sugar nucleotides as substrates (PubMed:25404702, PubMed:21653318). Complements deletion of the gtf3 gene from S.parasanguinis strain FW213 (PubMed:21653318). Protein modification; protein glycosylation. Homotetramer; a dimer of dimers. Dimerizes via the C-terminus; dimerization is required for tetramer formation (By similarity). Binds protein substrate via an exposed loop in the N-terminus (PubMed:25404702). Dimerizes via the C-terminus; dimerization is required for tetramer formation. Binds protein substrate via an exposed loop in the N-terminus. Altered modification (probably glycosylation) of Srr2. Belongs to the Gtf3 glucosyltransferase family.