Transient, lectin-like association of calreticulin with folding intermediates of cellular and viral glycoproteins.

Abstract

The soluble, calcium-binding protein calreticulin shares high sequence homology with calnexin, a transmembrane chaperone of glycoprotein folding. Our experiments demonstrated that calreticulin, like calnexin, associated transiently with numerous newly synthesized proteins in the endoplasmic reticulum. The population of proteins that bound to calreticulin was partially overlapping with those that bound to calnexin. Hemagglutinin (HA) of influenza virus was shown to associate with both calreticulin and calnexin. Using HA as a model substrate, it was found that both calreticulin- and calnexin-bound HA corresponded primarily to incompletely disulfide-bonded folding intermediates and conformationally trapped forms. Binding of all substrates was oligosaccharide-dependent and required the trimming of glucose residues from asparagine-linked core glycans by glucosidases I and II. In vitro, alpha-mannosidase digestion of calreticulin-bound HA indicated that calreticulin was specific for monoglucosylated glycans. Thus, calreticulin appeared to be a lectin with similar oligosaccharide specificity as its membrane-bound homologue, calnexin. Both are therefore likely to play an important role in glycoprotein maturation and quality control in the endoplasmic reticulum.

Aberrant retention of tyrosinase in the endoplasmic reticulum mediates accelerated degradation of the enzyme and contributes to the dedifferentiated phenotype of amelanotic melanoma cells

“…The 70-kDa doublet from melanoma cells was, however, able to bind a bacterially produced, purified, recombinant glutathione S-transferasecalreticulin fusion protein in vitro (data not shown). The in vitro binding to calreticulin suggested that tyrosinase possessed the necessary monoglucosylated glycans required also for calreticulin binding in vivo (40) but that the microenvironment within the ER did not permit this association.…”

Incomplete endoplasmic reticulum (ER) retention in immature thymocytes as revealed by surface expression of “ER-resident” molecular chaperones

“…Consequently, we asked if other ER proteins (both soluble and integral membrane proteins) were also able to escape from the ER and reach the cell surface. CRT is a multifunctional, peptide and Ca 2ϩ -binding molecular chaperone that is highly homologous to calnexin's lumenal domain (39,40). However, unlike calnexin, CRT is a soluble protein that is found in the ER lumen and that is retained by a distinct motif (KDEL) and a distinct retention receptor (KDEL receptor).…”

ERGIC-53 is a functional mannose-selective and calcium-dependent human homologue of leguminous lectins.

“…We propose that ERGIC-53 recognizes fully folded high-mannose glycoproteins in the ER after they have been released from the proposed quality control machinery for protein folding and N-glycan trimming consisting of the UDP-Glc:glycoprotein glucosyltransferase/calnexin/ER glucosidase cycle (Sousa et al, 1992;Sousa and Parodi, 1995;Hammond et al, 1994;Hebert et al, 1995;Peterson et al, 1995). COP I or COP II coats then bind to the cytoplasmic sorting signals of ERGIC-53 and drive the lectin-glycoprotein complex into budding vesicles.…”