Glycogenin is a glycosyltransferase that catalyzes the transfer ofglucose residues from UDP-glucose to itself, creating a linear polymer ofapproximately 12 glucose units bound by α-1,4-glycosidiclinkages. This oligosaccharide chain serves as the primer for the combinedaction of glycogen synthase and glycogen branching enzyme that will completethe polysaccharide synthesis. In humans, there are two forms of glycogenin,glycogenin-1 and glycogenin-2. Glycogenin-1 (HGN1) is mainly expressed in skeletal muscle and heart and to a lesser extent in lung,kidney, brain, pancreas, and placenta. Instead, glycogenin-2 is primarilyexpressed in the liver.

Glycogen storage disease (GSD) XVis a rare metabolic disorder caused by mutations in the GYG1 gene, whichencodes HGN1. To date, eight point mutations have been described in GSD XV patients.One of them was homozygous for an N-terminal missense variant (c.46G>C,p.Ala16Pro) of the protein and exhibited skeletal myopathy with storage ofpolyglucosan in muscle fibers. The mutation was confirmed at the RNA level butthe mutant protein was not detected in the skeletal muscle biopsy of the patient.

Since human and rabbit glycogeninamino acid sequences are 93% identical, we have introduced Ala16Pro mutationinto rabbit enzyme, the most studied member of the family, and expressed themutant in E. coli. We have previouslydescribed that Ala16Pro was inactive for auto- and transglucosylation and has adiminished substrate binding affinity, probably due to a conformational change.In order to explain the absence of the protein in the patient muscle tissue wehave analyzed its stability and oligomerization state by different in vitro techniques.In this work, we show that, in contrast to wild type HGN1, which exists as adimer, the Ala16Pro variant forms soluble high molecular weight oligomers.Besides, our results suggest that the mutant would have a less stableconformation, more prone to proteolytic digestion.