GERMINATION OF DICTYOSTELIUM-DISCOIDEUM SPORES - INHIBITION OF THE AUTOACTIVATION PATHWAY BY VANADATE

Addition of vanadate to spores of Dictyostelium discoideum engaged in the process of autoactivation resulted in an inhibition of germination which was dependent on the time of addition of the compound. If vanadate was added directly after the washing of dormant spores, then few spores swelled or released myxamoebae. Autoactivator was not secreted from vanadate-inhibited spores. Addition of vanadate to spores that had begun to autoactivate in moist fruiting bodies did not completely inhibit germination. Spores blocked from autoactivation by vanadate responded to the presence of exogenous autoactivator by germinating, but the resulting myxamoebae contained prominent vesicles. Heat shock also overcame vanadate-induced inhibition. This inhibition of autoactivation by vanadate is similar to that imposed on spores by the natural autoinhibitor: thus (1) both compounds block the initiation of the autoactivation cascade and spores do not swell; (2) autoactivator does not transiently accumulate in the culture broths under these conditions; and (3) addition of exogenous autoactivator overrides the inhibition caused by these two compounds. It is hypothesized that vanadate blocks the synthesis/secretion of autoactivator by interfering with an early step in the membrane traffic involved in spore autophagy. Exogenous autoactivator appeared in the supernatants when autoactivating spores swelled but decreased just after the appearance of nascent myxamoebae. It was shown that the autoactivator is not cAMP for cGMP and that it is stable to treatment with beef heart cyclic nucleotide phosphodiesterase. Nevertheless, myxamoebae from autoactivating spores precociously began aggregation within 30 min of plating on non-nutrient agar.