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© The Rockefeller University Press, 0021-9525/2000//1123 $5.00
The Journal of Cell Biology, Volume 148, Number 6, , 2000 1123-1130

Brief Report

Conformational Requirements for Glycoprotein Reglucosylation in the Endoplasmic Reticulum



E. Sergio Trombettaa and Ari Heleniusb

a Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520-8002
b Institute of Biochemistry, ETH-Zurich CH-8092 Switzerland
Institute of Biochemistry, ETH-Zurich, Universitatstrasse 16, CH-8092 Switzerland.41-1-632-126941-1-632-6817

ari.helenius{at}bc.biol.ethz.ch

Newly synthesized glycoproteins interact during folding and quality control in the ER with calnexin and calreticulin, two lectins specific for monoglucosylated oligosaccharides. Binding and release are regulated by two enzymes, glucosidase II and UDP-Glc:glycoprotein:glycosyltransferase (GT), which cyclically remove and reattach the essential glucose residues on the N-linked oligosaccharides. GT acts as a folding sensor in the cycle, selectively reglucosylating incompletely folded glycoproteins and promoting binding of its substrates to the lectins. To investigate how nonnative protein conformations are recognized and directed to this unique chaperone system, we analyzed the interaction of GT with a series of model substrates with well defined conformations derived from RNaseB. We found that conformations with slight perturbations were not reglucosylated by GT. In contrast, a partially structured nonnative form was efficiently recognized by the enzyme. When this form was converted back to a nativelike state, concomitant loss of recognition by GT occurred, reproducing the reglucosylation conditions observed in vivo with isolated components. Moreover, fully unfolded conformers were poorly recognized. The results indicated that GT is able to distinguish between different nonnative conformations with a distinct preference for partially structured conformers. The findings suggest that discrete populations of nonnative conformations are selectively reglucosylated to participate in the calnexin/calreticulin chaperone pathway.

Key Words: folding • glucosyltransferase • glucosidase II • calnexin • calreticulin

© 2000 The Rockefeller University Press

Abbreviations used in this paper: EndoH, endoglycosidase H; GT, UDP-Glc:glycoprotein:glycosyltransferase; SBA, soybean agglutinin.


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J. Cell Biol. 2000 148: 1-2. [Full Text] [PDF]





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