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J. Biol. Chem., Vol. 268, Issue 20, 14687-14693, 07, 1993
Y Cao, AM Mahrenholz, AA DePaoli-Roach and PJ Roach
The biogenesis of glycogen involves a specific initiation event mediated by
the initiator protein, glycogenin, which undergoes self- glucosylation to
generate an oligosaccharide primer from which the glycogen molecule grows.
Rabbit muscle glycogenin was expressed at high levels in Escherichia coli
and purified close to homogeneity in a procedure that involved binding to a
UDP-agarose affinity column. The resulting protein had subunit molecular
weight of 38,000 as judged by polyacrylamide gel electrophoresis in the
presence of sodium dodecyl sulfate. Analysis of peptide fragments by mass
spectroscopy indicated that the recombinant glycogenin was already
glucosylated at Tyr-194 and contained from 1 to 8 glucose residues
attached. The enzyme was active as a glucosyl transferase and could
incorporate a further approximately 5 mol of glucose/mol. The apparent Km
for the glucosyl donor UDP- glucose was 4.5 microM, and the pH optimum was
pH 8. Of a number of nucleotides and related compounds surveyed, UDP and
UTP were the most effective inhibitors. There was also a correlation
between inhibition and the presence of a pyrophosphate group. Of several
oligosaccharides of glucose, only maltose caused significant inhibition.
The glucosylation reaction was first order with respect to glycogenin
suggesting that it was intramolecular. The efficacy of the purified
glycogenin as a substrate for the elongation reaction catalyzed by glycogen
synthase was significantly enhanced if glycogenin was first allowed to
undergo self-glucosylation. The length of the priming oligosaccharide is
thus important for glycogen synthase action. A mutant of glycogenin, in
which Tyr-194 was changed to Phe, behaved identically to the wild-type
through purification and in particular bound to the UDP-agarose affinity
matrix. Despite these indications of the protein's overall structural
integrity, it was unable to self- glucosylate. This result indicates that
Tyr-194 is necessary for glycogenin function and is consistent with Tyr-194
being the sole site of glucosylation.
Characterization of rabbit skeletal muscle glycogenin. Tyrosine 194 is essential for function
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indiana 46202-5122.
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