The three-dimensional structure of the lumenal domain of the lectin-like ch
aperone calnexin determined to 2.9 Angstrom resolution reveals an extended
140 Angstrom arm inserted into a beta sandwich structure characteristic of
legume lectins. The arm is composed of tandem repeats of two proline-rich s
equence motifs which interact with one another in a head-to-tail fashion. I
dentification of the ligand binding site establishes calnexin as a monovale
nt lectin, providing insight into the mechanism by which the calnexin famil
y of chaperones interacts with monoglucosylated glycoproteins.