Required for polymorphic O-glycosylation of the serine-rich repeat protein in this bacteria. Catalyzes the second step in glycosylation by transferring N-acetylglucosamine from UDP-GlcNAc to the terminal GlcNAc moiety of the 3-O-(N-acetyl-alpha-D-glucosaminyl)-L-seryl-[protein] resulting from the first glycosylation step. Part of the accessory SecA2/SecY2 system specifically required to export serine-rich repeat cell wall protein(s) encoded upstream in the same operon. 3-O-[N-acetyl-alpha-D-glucosaminyl]-L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-[N-acetyl-beta-D-glucosaminyl-(1->6)-N-acetyl-alpha-D-glucosaminyl]-L-seryl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Homotetramer; a dimer of dimers. Dimerizes via the C-terminus; dimerization is required for tetramer formation. Binds protein substrate via an exposed loop in the N-terminus. L.reuteri is generally considered to be a symbiont rather than pathogenic bacteria. This strain was isolated from pig. Belongs to the Gtf3 glucosyltransferase family.