Reversibly catalyzes the synthesis and degradation of trehalose from glucose and alpha-D-glucose 1-phosphate. The equilibrium lies in the direction of trehalose synthesis (By similarity). alpha,alpha-trehalose + phosphate = alpha-D-glucose + alpha-D-glucose 1-phosphate Homodimer. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.