Pallidin. Purification and characterization of a carbohydrate-binding protein from Polysphondylium pallidum implicated in intercellular adhesion

Abstract

A carbohydrate-binding protein from Polysphondylium pallidum, a species of cellular slime mold, was purified to homogeneity by adsorption to formalized erythrocytes and elution with d-galactose. The protein, for which we propose the name PALLIDIN, is assayed by its activity as an agglutinin of erythrocytes. It was previously shown to have different carbohydrate-binding specificities than discoidin, a carbohydrate-binding protein from Dictyostelium discoideum, another species of slime mold. Evidence has been presented previously that each of these proteins is detectable on the cell surface. In the present report we show that the physico-chemical properties of pallidin are different from discoidin. Pallidin has a subunit molecular weight of 24 800 ± 1100 determined by polyacrylamide electrophoresis in the presence of dodecyl sulfate and 2-mercaptoethanol, compared to 26 100 ± 1000 for discoidin. The weight-average molecular weight of pallidin is 250 000 ± 50 000 determined by equilibrium sedimentation in the presence of d-galactose compared to 100 000 ± 2000 for discoidin. In equilibrium sedimentation studies, pallidin exhibited some heterogeneity at equilibrium while discoidin was homogeneous. The amino acid composition of pallidin is generally similar but clearly different from the composition of discoidin. The isoelectric point of pallidin is 7.0 compared to 6.1 for discoidin. Like discoidin, pallidin contains no detectable hexosamine or neutral sugar. These results establish that agglutinins from two species of cellular slime molds are distinct. The different properties of the cell-surface agglutinins, pallidin and discoidin, are consistent with their suggested role in species-specific cellular recognition and adhesion in the species of slime mold from which they are derived.