Properties of carbohydrate‐free recombinant glycogenin expressed in an Escherichia coli mutant lacking UDP‐glucose pyrophosphorylase activity

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Abstract
Glycogenin, the self‐glucosylating primer for glycogen synthesis is expressed in wild‐type E. coli as a recombinant protein in an already partly glucosylated form owing to the presence of its substrate UDP‐glucose. By using an E. coli mutant strain lacking in UDP‐glucose pyrophosphorylase activity, we have succeeded in expressing carbohydrate‐free glycogenin (apo‐glycogenin) in good yield. When provided with UDPxy‐lose, it autocatalytically adds 1 xylose residue. With UDP‐glucose, an average of 8 glucose residues are added. However release of the self‐synthesized maltosaccharide chains with isoamylase reveals them to be a mixture. Chains as long as 11 glucose residues (maltoundecaose) are present. The ability of recombinant apo‐glycogenin to self‐glucosylate is further proof that a separate enzyme is not needed for the addition of the first glucose residue to Tyr‐194 of the protein.