Glycosyltransferase required for the addition of both glucuronic acid and 4-O-methylglucuronic acid branches to xylan in stem cell walls. In association with GUX1, is responsible for almost all of the substitutions of the xylan backbone in stem glucuronoxylan. No visible phenotype under normal growth conditions, but mutant plants show reduced xylan substitution. Belongs to the glycosyltransferase 8 family. Glycogenin subfamily. Was originally thought to correspond to two different genes At4g33330 and At4g33340. Was originally thought to correspond to two different genes At4g33330 and At4g33340. Was originally thought to correspond to two different genes At4g33330 and At4g33340. Was originally thought to correspond to two different genes At4g33330 and At4g33340.