Cultured quail embryo muscle has proven to be an excellent model system for studying the synthesis of macromolecular glycogen from, and its degradation to, glycogenin, the autocatalytic, self-glucosylating primer for glycogen synthesis. We recently demonstrated that proglycogen, a low-M(r) form of glycogen, is an intermediate in the synthesis. Here we show that proglycogen also functions as an intermediate in macroglycogen degradation and, in one set of circumstances, represents an arrest point in glycogen breakdown, which does not continue to glycogenin. We suggest that in the nutritionally dependent turnover of glycogen in tissues, the molecules cycle between proglycogen and macromolecular glycogen and are not normally degraded to glycogenin. Nevertheless, when this does happen, the released glycogenin is active, capable of re-initiating glycogen synthesis. Under culture conditions where the conversion of proglycogen into glycogenin does take place, the intermediates lying between form a discrete rather than a continuous series, suggestive of a cluster structure for proglycogen and indicating that breakdown is stepwise. Evidence of post-translational modification of glycogenin was obtained by the finding that, in glycogen from cultured muscle, glycogenin is phosphorylated.
- glycogen synthase
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