Common Cell-Surface Receptors for Discoidin I and Discoidin II in Dictyostelium discoideum

Abstract

Following nutrient depletion, cells of the cellular slime mould Dictyostelium discoideum become cohesive and aggregate to form multicellular complexes. Several proteins that accumulate on the cell surface during this period have been implicated in mediating aggregative-phase cell cohesion, namely contact sites A (CsA), gp 150, and two endogenous lectins (discoidin I and discoidin II). The aggregating cells also possess receptors for both discoidin I and discoidin II but these have not yet been isolated and characterised for both lectins. In the present study we investigated the relationship between the receptors for these lectins, in particular to what extent discoidin I and discoidin II receptors are common. Radio-iodinated discoidin I and discoidin II were purified and used in binding assays for lectin receptors on the surface of aggregated (10 h stage of development) D. discoideum NC4 cells. Sugar competition of 125I-labelled discoidin I and 125I-labelled discoidin II binding indicated distinct but overlapping sugar specificities for these lectins when binding to their in vivo receptors. Competition of the binding of radio-iodinated lectin with either unlabelled discoidin I or unlabelled discoidin II showed that at least 50% of the cell-surface binding sites for these lectins are in common and for these receptors the binding affinity of discoidin I is 9–20 times higher than for discoidin II. Approximately 35% of discoidin II binding sites appear to be unavailable for discoidin I binding.