A Ca2+ transport system associated with the plasma membrane of Dictyostelium discoideum is activated by different chemoattractant receptors

Abstract

Amebae of Dictyostelium exhibit a transient uptake of extracellular Ca2+ ∼5 s after activation of surface folate or cAMP receptors (Bumann, J., B. Wurster, and D. Malchow. 1984. J. Cell Biol. 98:173-178). To further characterize these Ca2+ entry systems, we analyzed 45Ca2+ uptake by resting and activated amebae. Like the surface chemoreceptors, folate- and cAMP-induced Ca2+ uptake responses were developmentally regulated; the former response was evident in vegetative but not aggregation-competent cells, whereas the latter response displayed the opposite pattern of expression. In contrast, other characteristics of these Ca2+-uptake pathways were remarkably similar. Both systems (a) exhibited comparable kinetic properties, (b) displayed a high specificity for Ca2+, and (c) were inhibited effectively by Ruthenium Red, sodium azide, and carbonylcyanide m-chlorophenylhydrazone. These results, together with the finding that vegetative cells transformed with a plasmid expressing the surface cAMP receptor exhibit a cAMP-induced Ca2+ uptake, suggest that different chemoreceptors activate a single Ca2+ entry pathway. Additional pharmacological and ion competition studies indicated that receptor-mediated Ca2+ entry probably does not involve a Na+/Ca2+ exchanger or voltage-activated channels. Chemoattractant binding appears to generate intracellular signals that induce activation and adaption of the Ca2+-uptake response. Analysis of putative signaling mutants suggests that Ca2+ entry is not regulated by the guanine nucleotide-binding (G) protein subunits Gα1 or Gα2, or by G protein-mediated changes in intracellular cAMP or guanosine 3,′5′-cyclic monophosphate (cGMP).