Distinct contributions of the lectin and arm domains of calnexin to its molecular chaperone function.
Calnexin is a Ca2+-binding transmembrane chaperone of the endoplasmic reticulum that recognizes Glc1Man5-9GlcNAc2 oligosaccharides on folding glycoproteins as well as non-native elements of the polypeptide backbone. This latter mode of recognition enables calnexin to suppress the aggregation of both glycosylated and nonglycosylated substrates. The luminal portion of calnexin (S-Cnx) consists of two domains, a globular lectin domain and an extended arm domain. To understand the function of these domains during the interaction of calnexin with non-native protein conformers, we tested deletion mutants of S-Cnx for their abilities to suppress the aggregation of nonglycosylated firefly luciferase. The arm domain alone exhibited no capacity to suppress aggregation. However, stepwise truncation of the arm domain in S-Cnx resulted in a progressive reduction in aggregation suppression potency to the point where the globular domain alone exhibited 25% potency. To characterize the polypeptide-binding site, we used hydrophobic peptides that were competitors of the ability of S-Cnx to suppress luciferase aggregation. Direct binding experiments revealed a single site of peptide binding in the globular domain (Kd = 0.9 microm) at a location distinct from the lectin site. Progressive truncation of the arm domain in S-Cnx had no effect on the binding of small peptides but reduced the binding affinity of S-Cnx for large, non-native protein substrates. Because protein substrates exhibited no binding to the isolated arm domain, our findings support a model in which calnexin suppresses aggregation through a polypeptide-binding site in its globular domain, with the arm domain enhancing aggregation suppression by sterically constraining large substrates.
Brockmeier
Achim
A
Department of Biochemistry and Immunology, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
Brockmeier
Ulf
U
Williams
David B
DB
D000818:Animals; D037281:Calnexin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004285:Dogs; D037102:Lectins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018832:Molecular Chaperones / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D009844:Oligosaccharides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology
D037102:Lectins; D018832:Molecular Chaperones; D009844:Oligosaccharides; D037281:Calnexin