Catalysis of the reaction: UDP-glucose + (1,4)-alpha-D-glucosyl(n) = UMP + (1,4)-alpha-D-glucosyl(n)-glucose-1-phosphate.
Synonyms:
xref:
reactome:R-HSA-3780994 "GYS2 catalyzes the incorporation of phosphoglucose into glycogen-GYG2"; reactome:R-HSA-3781024 "GYS1 catalyzes the incorporation of phosphoglucose into glycogen-GYG1"