A membrane glycoprotein of aggregating Dictyostelium cells with the properties of contact sites

Abstract

Aggregating cells of Dictyostelium discoideum form EDTA-stable contacts which are blocked by a Fab (antigen-binding fragment) preparation from antisera raised against membranes. The target site of the blocking Fab fragments has been identified as a specific glycoprotein. In this paper its purification, carbohydrate and amino acid composition are described. Purification was 800-fold, starting with cells lysed by digitonin. The plasma membranes, preserved as ghosts by this treatment, were purified in a two-phase system and extracted with butan-1-ol. The water phase contained predominantly concanavalin-A-binding glycoproteins and was particularly rich in contact sites A. These were further purified on DE-cellulose and sucrose gradients. Sodium dodecylsulphate/polyacrylamide gel electrophoresis of the purified material revealed one major glycoprotein band in the molecular weight region of 80 000 to 90 000, depending on the acrylamide concentration. The sugars found in contact sites A were mannose, N-acetylglucosamine, fucose, and possibly glucose. The protein moeity contained 8% proline and was particularly rich in hydroxy amino acids.