THE ROLE OF CA2- AUTOACTIVATION IS MEDIATED BY THE MOBILIZATION OF CA2+ WHILE AMOEBAL EMERGENCE REQUIRES ENTRY OF EXTERNAL CA2+( DURING SPORE GERMINATION IN DICTYOSTELIUM )

One of the developmental pathways used by the social amoeba Dictyostel ium discoideum produces dormant spores. As with any temporary resistan t stage, these spores must be able to germinate rapidly in response to positive environmental stimuli. One such stimulus is the autoactivato r, an endogenous, diffusible molecule that is secreted by spores. Prev ious work has shown that three phases of germination, autoactivation, spore swelling and amoebal emergence, require the activity of the Ca2-dependent, regulatory protein calmodulin, implicating Ca2+ as an esse ntial cation during germination. In this study we used a pharmacologic al approach coupled with the direct measurement of Ca2+ levels in germ inating spore populations by atomic adsorption to examine Ca2+-depende nt signal transduction during spore activation and germination in D. d iscoideum. Inhibitors of both phospholipase C and internal Ca2+ releas e inhibited autoactivation while exogenously added Ins(1,4,5)P-3, acte d synergistically with the autoactivator. The antagonists specifically affected spore activation as mediated by the autoactivator, since nei ther had any effect on heat-activated spores. In contrast, La3+, an in hibitor of Ca2+ uptake, had little or no effect on either autoactivati on or the swelling of autoactivated spores. However, an inhibition of Ca2+ influx by La3+ inhibited both the swelling of heat-activated spor es and amoebal emergence following each period of autoactivation or he at activation. Ca2+ levels change in the spore population during germi nation. During activation and swelling, Ca2+ efflux occurs from the sp ores. Both of the activating stimuli used here, the autoactivator and heat, caused this Ca2+ efflux. The efflux is reversed during emergence when there is a net Ca2+ uptake by the spores and cells from the medi um. Together these data provide the first evidence that autoactivation is mediated by Ca2+-dependent signal transduction, leading to Ca2+ ef flux, and that the late event of germination, amoebal emergence, requi res Ca2+ uptake to proceed. The data also suggest that the responses o f the spore to the each of autoactivator and heat, i.e. Ca2+ movements and germination, are mediated by different mechanisms.