Mechanism of glycogen synthase inactivation and interaction with glycogenin

Abstract

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and 18 structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen 19 synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by 20 phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated 21 human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated 22 N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress 23 against GS regulatory helices. This keeps GS in an inactive conformation mediated by 24 phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided 25 mutagenesis perturbing interactions with phosphorylated tails led to increased 26 basal/unstimulated GS activity.