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Publication Abstract from PubMed

Glycogen branching enzyme 1 (GBE1) plays an essential role in glycogen biosynthesis by generating alpha-1,6-glucosidic branches from alpha-1,4-linked glucose chains, to increase solubility of the glycogen polymer. Mutations in the GBE1 gene lead to the heterogeneous early-onset glycogen storage disorder type IV (GSDIV) or the late-onset adult polyglucosan body disease (APBD). To better understand this essential enzyme, we crystallized human GBE1 in the apo form, and in complex with a tetra- or hepta-saccharide. The GBE1 structure reveals a conserved amylase core that houses the active center for the branching reaction, and harbors almost all GSDIV and APBD mutations. A non-catalytic binding cleft, proximal to the site of the common APBD mutation p.Y329S, was found to bind the tetra- and hepta-saccharides, and may represent a higher-affinity site employed to anchor the complex glycogen substrate for the branching reaction. Expression of recombinant GBE1-p.Y329S resulted in drastically-reduced protein yield and solubility compared to wild-type, suggesting this disease allele causes protein misfolding and may be amenable to small molecule stabilization. To explore this, we generated a structural model of GBE1-p.Y329S and designed peptides ab initio to stabilize the mutation. As proof-of-principle, we evaluated treatment of one tetra-peptide, Leu-Thr-Lys-Glu, in APBD patient cells. We demonstrate intracellular transport of this peptide, its binding and stabilization of GBE1-p.Y329S, and 2-fold increased mutant enzymatic activity compared to untreated patient cells. Together, our data provide the rationale and starting point for the screening of small molecule chaperones which could become novel therapies for this disease.

Structural basis of glycogen branching enzyme deficiency and pharmacologic rescue by rational peptide design.,Froese DS, Michaeli A, McCorvie TJ, Krojer T, Sasi M, Melaev E, Goldblum A, Zatsepin M, Lossos A, Alvarez R, Escriba PV, Minaissan BA, von Delft F, Kakhlon O, Yue WW Hum Mol Genet. 2015 Jul 21. pii: ddv280. PMID:26199317^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.