Apparent Positive Cooperativity at a Surface cAMP Receptor in Dictyostelium

In the large species of the cellular slime mold Dictyostelium , cell aggregation is regulated by extracellular cAMP. During aggregation, cAMP is released in pulses from cells in the aggregation centers and these rhythmic signals are propagated through the population by a signal relay system. In addition to triggering the relay response, the pulsatile signals also regulate the chemotactic movement of the cells and early cell differentiation. These different cellular responses to exogenous cAMP are thought to be mediated via cAMP receptors, which appear on the cell surface shortly after starvation.
Using a sensitive assay, the equilibrium binding properties of these receptors were analyzed at low cAMP concentrations. As reported earlier, Scatchard plots of cAMP binding to preaggregative amoebae of D. discoideum strain NP187 in the concentration range 2–500 nM were curvilinear suggesting either receptor heterogeneity or negative cooperative interactions. However, at cAMP concentrations below approximately 1.5 nM, the affinity of the receptors was found to decline as a function of decreasing receptor occupancy. This apparent positive cooperativity was observed with binding sites on crude plasma membranes as well as on intact cells, and it occurred at both 0°C and 22°C. Moreover, apparent positive cooperativity was a property of the receptors on all strains of D. discoideum examined and on one strain of D. purpureum . Unlike preaggregative cells, receptors on postaggregative cells often lacked this property.
The lowest concentration of cAMP pulses that can appreciably stimulate membrane differentiation in strain NP187 was found to be 0.15–1.5 nM. Since similar concentrations of exogenous cAMP have been reported to trigger minimal chemotactic and relay responses in D. discoideum , the apparent positive cooperative behavior of the cAMP receptor might function to generate a steep cellular response threshold.