Physiological and biochemical characterization of aggregation-deficient mutants of Dictyostelium discoideum: detection and response to exogenous cyclic AMP

Abstract

Forty independently isolated aggregation-deficient (Agg⁻) mutants of Dictyostelium discoideum, which have been partially characterized genetically, were examined for the ability to respond chemotactically to exogenous cAMP and for certain cellular and extracellular activities thought to be involved in this process. Only five of the mutants failed to produce a statistically significant chemotactic response to cAMP. When shaken in buffer, most of the Agg⁻ mutants produced reduced levels of the cAMP-binding sites and the cell-bound cAMP phosphodiesterase reported to be components of a cell-surface cAMP receptor involved in the chemotactic response. In addition, most of the mutants failed to form contact sites A and were unable to develop on water agar plates. When the mutants were pulsed with 100 nM cAMP, 36 of 40 mutants produced detectable contact sites A and three-quarters of these strains underwent further development when placed on a solid substratum. Treatment with cAMP pulses also stimulated certain mutants to form higher levels of cAMP receptors. These observations suggest that the impaired differentiation of the plasma membrane of many Agg⁻ mutants is due, at least in part, to the abnormal synthesis and (or) secretion of cAMP by these strains. When grown in bacterial suspension, all 40 of the mutants produced extracellular phosphodiesterase (ePD) activity, and all but 8 of the mutants secreted detectable amounts of the ePD inhibitor. In general, Agg⁻ strains carrying mutations at the same aggregation locus (aggF–J) exhibited very similar physiological and biochemical properties.