Required for polymorphic O-glycosylation of the serine-rich repeat protein Fap1. 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 Fap1; does not use other sugar nucleotides as substrates. In vitro glycosyltransferase activity is metal-independent. Protein modification; protein glycosylation. Homotetramer; a dimer of dimers. Dimerizes via the C-terminus; dimerization is required for tetramer formation (PubMed:21653318). Binds protein substrate via an exposed loop in the N-terminus (PubMed:25404702). The Fap1 protein no longer reacts with a glycan-specific antibody, and migrates as a much larger than wild-type protein. Fap1 is not able to undergo the second glycosylation step. Has defects in biofilm mass accumulation, the biofilm that forms is not as thick as wild-type (PubMed:20164186). Deletion of this gene can be complemented by the ortholog from S.agalactiae strain COH1, but not from S. pneumoniae strain TIGR4 or from S. sanguinis strain SK36 (PubMed:21653318). Belongs to the Gtf3 glucosyltransferase family.