Catalyzes the interconversion between glucose-6-phosphate and alpha-glucose-1-phosphate. This is the first step in the biosynthesis of diglucosyl-diacylglycerol (Glc2-DAG), i.e. the predominant glycolipid found in B.subtilis membrane, which is also used as a membrane anchor for lipoteichoic acid (LTA). Has a role in the biosynthesis of all phosphate-containing envelope polymers, since glucose-1-phosphate is the precursor of UDP-glucose, which serves as a glucosyl donor not only for the biosynthesis of LTA but also for wall teichoic acids (WTAs). Is required for biofilm formation. This is likely due to another role of UDP-glucose, which might also act as a metabolic signal regulating biofilm formation or may be involved in some unknown biosynthetic pathway essential for biofilm formation, e.g. the synthesis of an exopolysaccharide. alpha-D-glucose 1-phosphate = alpha-D-glucose 6-phosphate Binds 1 Mg(2+) ion per subunit. Glycolipid metabolism; diglucosyl-diacylglycerol biosynthesis. Belongs to the phosphohexose mutase family.