Required for alpha-1,3-glucan and alpha-1,4-glucan production which are required for cell wall synthesis. [(1->3)-alpha-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->3)-alpha-D-glucosyl](n+1) + H(+) + UDP Interacts with sad1. Belongs to the glycosyltransferase group 1 family.