| GYG1 encodes glycogenin‑1, a self‑glucosylating glycosyltransferase that initiates cytosolic glycogen biogenesis by generating the short glucose primer required for subsequent elongation by glycogen synthase and branching enzyme. The protein belongs to the glycosyltransferase 8 family and contains a catalytic core that binds UDP‑glucose and an internal tyrosine residue that accepts the first glucose moiety, forming a covalent glucose 1‑O‑tyrosyl linkage from which an α‑1,4‑linked oligosaccharide chain is extended to a length of roughly 8–10 glucose residues, creating a minimal glycogen core particle. Glycogenin‑1 is highly expressed in skeletal muscle and heart, with lower levels in other tissues, and localizes to cytoplasmic glycogen granules where it remains at the center of mature glycogen particles, so the abundance and integrity of GYG1 directly influence the capacity of myofibers to establish and maintain normal glycogen stores for contraction‑linked energy demands. The autoglucosylation reaction of glycogenin‑1 uses UDP‑glucose as the donor substrate, and the generated primer is then handed off to glycogen synthase, which continues α‑1,4 chain elongation, and to the branching enzyme, which introduces α‑1,6 branch points, placing GYG1 at the top of the hierarchical enzymatic cascade that builds the branched glycogen polymer. Mutations in GYG1 that impair autoglucosylation, including substitutions of the essential tyrosine acceptor site or nearby residues, abolish primer formation and lead to glycogen storage disease type XV and related polyglucosan body myopathies, characterized by marked muscle glycogen depletion or abnormal glycogen structure, mitochondrial proliferation, and myopathic or cardiomyopathic phenotypes. Disease‑associated variants identified in patients include nonsense mutations and missense changes such as Thr83Met, which inactivate the glucosyltransferase activity and prevent the initiation of normal glycogen synthesis in skeletal muscle and, in some cases, heart, linking GYG1 defects to exercise intolerance, progressive muscle weakness, cardiomyopathy, and arrhythmias. |
