Catalyzes the interconversion between glucose-6-phosphate and alpha-glucose-1-phosphate (Probable). This is the first step in the biosynthesis of diglucosyl-diacylglycerol (Glc2-DAG), i.e. the predominant glycolipid found in the S.aureus membrane, which is also used as a membrane anchor for lipoteichoic acid (LTA). 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.