CNX and CRT, together with the oxidoreductase ERp57, promote the correct folding and maturation of G1M9 containing glycoprotein substrates ( Oliver et al. 2015), whilst further trimming to G1M9 enables binding to calnexin (CNX) and calreticulin (CRT) ( Helenius and Aebi 2004 Tannous et al. Trimming to G2M9 facilitates binding to malectin ( Tannous et al. This processing involves the sequential removal, or “trimming”, of glucose residues from the G3M9 N-glycan moiety initially added to newly synthesized glycoproteins either during or after protein synthesis (Figure (Figure1). 2016) are resident within the lumen of the endoplasmic reticulum (ER), where they process newly synthesized glycoproteins. Due to their structural mimicry of natural substrates, iminosugars have therapeutic potential in several areas of disease and have been evaluated for inhibitory activity towards a variety of α- and β-glycosidases ( de Melo et al. Widely distributed in plants, iminosugars represent a structurally diverse group of compounds, comprised of both monocyclic (piperidines and pyrrolidines) and bicyclic scaffolds (indolizines, pyrrolizidines and nortropanes) and their isolation from natural sources, chemical syntheses and biological evaluation are the subject of several comprehensive reviews ( Stütz 1999 Asano et al. Taken together, our work identifies the C-3 substituted pyrrolizidines casuarine and 3,7a-di epi-alexine as promising “second-generation” iminosugar inhibitors. The other active iminosugars all inhibit α-glucosidase II and, having identified 1,4-dideoxy-1,4-imino- D-arabinitol (DAB) as the most effective of these compounds, we use in silico modeling to understand the molecular basis for this enhanced activity. Comparison of their inhibitory activities in ER microsomes against their effects on purified ER α-glucosidase II, suggests that 3,7a-di epi-alexine acts as a selective inhibitor of ER α-glucosidase I. Using ER microsomes to recapitulate authentic protein N-glycosylation and oligosaccharide processing, we identify five iminosugars that selectively inhibit N-glycan trimming. In this study, we investigate the effects of a panel of nine iminosugars on the actions of ER luminal α-glucosidase I and α-glucosidase II. By acting as structural mimetics, iminosugars can selectively inhibit these ER localized α-glycosidases, preventing N-glycan trimming and providing a molecular basis for their therapeutic applications. The endoplasmic reticulum (ER) contains both α-glucosidases and α-mannosidases which process the N-linked oligosaccharides of newly synthesized glycoproteins and thereby facilitate polypeptide folding and glycoprotein quality control.
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