Catalyzes the transfer of a glucuronic acid (GlcA) residue from UDP-glucuronate to mannose-alpha-1,3-glucose-beta-1,4-glucose-P-P-polyisoprenyl to form the lipid-linked tetrasaccharide GlcA-Man-Glc(2)-PP-Pol, with a glucuronic acid-beta-mannose linkage. Is involved in the biosynthesis of the exopolysaccharide xanthan, since it catalyzes the fourth glycosylation step in the assembly of the pentasaccharide-P-P-polyisoprenyl repeating unit of xanthan. Is unable to use the trisaccharide acceptor freed from the pyrophosphate lipid moiety. Does not show specificity for the lipidic portion of the acceptor. Shows diminished activity when tested with 6-O-acetyl-mannose-alpha-1,3-glucose-beta-1,4-glucose-P-P-polyisoprenyl, a putative intermediate in the synthesis of xanthan; this could indicate that acetylation of the internal mannose takes place after the formation of the GumK product. alpha-D-Man-(1->3)-beta-D-Glc-(1->4)-alpha-D-Glc-1-di-trans,octa-cis-undecaprenyl diphosphate + UDP-alpha-D-glucuronate = beta-D-GlcA-(1->2)-alpha-D-Man-(1->3)-beta-D-Glc-(1->4)-alpha-D-Glc-di-trans,octa-cis-undecaprenyl diphosphate + H(+) + UDP Glycan biosynthesis; xanthan biosynthesis. Cells lacking this gene accumulate Man-alpha-1,3-Glc-beta-1,4-Glc-PP-polyisoprenyl, are unable to produce GlcA-Man-Glc(2)-PP-Pol, and show a decrease in xanthan production of more than 95%. Belongs to the glycosyltransferase 70 family. Truncated N-terminus.