Altered cyclic-AMP receptor activity and morphogenesis in a chemosensory mutant of Dictyostelium discoideum

Abstract

The functional properties of the cell-surface cyclic-AMP receptor that controls chemotaxis were found to be altered in an aggregation mutant of Dictyostelium discoideum. The mutant aggregated without stream formation and had a tenfold increased cell-density requirement for the initiation of aggregation. After aggregation, mounds formed multiple tips and subsequently subdivided to give multiple fruits that were small and abnormally proportioned. Cyclic-AMP-induced light-scattering changes in cell suspensions indicated that the mutant had a diminished response to external cyclic-AMP signals. Associated with these altered functional responses was a physical change in the cyclic-AMP sensory system. Cyclic-AMP-binding studies showed that the parent had two classes of cyclic-AMP binding sites, i.e., Kd = 32 and 110 nM. In contrast, the mutant had two- to threefold or more high-affinity sites (Kd = 25n M) and altered low-affinity sites (Kd < 3μ M). These results indicate that both affinity classes of binding site are independently mutable. This observation suggests that the two affinity classes can be interconverted by mutation, or the mutation alters a single molecular species and its equilibrium between binding sites with different affinities for cyclic AMP, as postulated in receptor cycling models.