Involved in the biosynthesis of the exopolysaccharide xanthan, a polymer that is comprised of repeating pentasaccharide units with the structure of a beta-(1,4)-linked D-glucose backbone with trisaccharide side chains composed of mannose-beta-(1,4)-glucuronic acid-beta-(1,2)-mannose attached to alternate glucose residues in the backbone by alpha-(1,3) linkages. Xanthan is involved in pathogenicity but has also been used in a variety of applications as a specialty polymer for commercial applications, including food additives, where they act as viscosifying, stabilizing, emulsifying, or gelling agents. beta-D-Glc-(1->4)-alpha-D-Glc-di-trans,octa-cis-undecaprenyl diphosphate + GDP-alpha-D-mannose = alpha-D-Man-(1->3)-beta-D-Glc-(1->4)-alpha-D-Glc-1-di-trans,octa-cis-undecaprenyl diphosphate + GDP + H(+) Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.