A DENSITY-SENSING FACTOR REGULATES SIGNAL-TRANSDUCTION IN DICTYOSTELIUM

Dictyostelium discoideum initiates development when cells overgrow the ir bacterial food source and starve, To coordinate development, the ce lls monitor the extracellular level of a protein, conditioned medium f actor (CMF), secreted by starved cells. When a majority of the cells i n a given area have starved, as signaled by CMF secretion, the extrace llular level of CMF rises above a threshold value and permits aggregat ion of the starved cells. The cells aggregate using relayed pulses of cAMP as the chemoattractant. Cells in which CMF accumulation has been blocked by antisense do not aggregate except in the presence of exogen ous CMF. We find that these cells are viable but do not chemotax towar ds cAMP. Videomicroscopy indicates that the inability of CMF antisense cells to chemotax is not due to a gross defect in motility, although both video and scanning electron microscopy indicate that CMF increase s the frequency of pseudopod formation. The activations of Ca2+ influx , adenylyl cyclase, and guanylyl cyclase in response to a pulse of cAM P are strongly inhibited in cells lacking CMF, but are rescued by as l ittle as 10 s exposure of cells to CMF. The activation of phospholipas e C by cAMP is not affected by CMF. Northern blots indicate normal lev els of the cAMP receptor mRNA in CMF antisense cells during developmen t, while cAMP binding assays and Scatchard plots indicate that CMF ant isense cells contain normal levels of the cAMP receptor. In Dictyostel ium, both adenylyl and guanylyl cyclases are activated via G proteins. We find that the interaction of the cAMP receptor with G proteins in vitro is not measurably affected by CMF, whereas the activation of ade nylyl cyclase by G proteins requires cells to have been exposed to CMF . CMF thus appears to regulate aggregation by regulating an early step of cAMP signal transduction.