STRUCTURE-FUNCTION RELATIONSHIP OF A RECOMBINANT HUMAN GALACTOSIDE-BINDING PROTEIN

A galactoside-binding lectin (hL-31) containing a collagen-like sequen ce was identified in human r cells. It was found to be the homologue o f the IgE-binding protein, the macrophage cell-surface Mac-2 antigen, and the murine CBP35, RL-29, and mL-34 lectins. Here we report on the expression in Escherichia coli and functional analysis of recombinant hL-31 (rhL-3 1). The rhL-31 was purified in one step through an asialo fetuin affinity column. The rhL-31 was reactive to anti-lectin antibod ies and retained its lactose-dependent hemagglutination of trypsin-tre ated glutaraldehyde-fixed rabbit erythrocytes. The rhL-31 elutes from an affinity column as a 31-kDa monomer and undergoes homodimerization at relatively high protein concentrations, comparable to those used to mediate hemagglutination. Electron microscopy showed that the rhL-31 appears as a Y-shaped structure. Lactoperoxidase-catalyzed iodination of murine tumor cell-surface proteins followed by collagenase treatmen t revealed that the lectin is probably a peripheral membrane protein w hereby both the amino and the carboxy termini are exposed on the outer cell membrane. These results point to the membrane disposition and or ientation of the lectin and suggest a mechanism for a structure-functi on relationship of lectin activity.