CELL-DENSITY SENSING MEDIATED BY A G-PROTEIN-COUPLED RECEPTOR ACTIVATING PHOSPHOLIPASE-C

When the unicellular eukaryote Dictyostelium discoideum starves, it se nses the local density of other starving cells by simultaneously secre ting and sensing a glycoprotein called conditioned medium factor (CMF) . When the density of starving cells is high, the corresponding high d ensity of CMF permits signal transduction through cAR1, the chemoattra ctant cAMP receptor, cAR1 activates a heterotrimeric G protein whose a lpha-subunit is G alpha 2, CMF regulates cAMP signal transduction in p art by regulating the lifetime of the cAMP-stimulated G alpha 2-GTP co nfiguration. We find here that guanosine 5'-3-O-(thio)triphosphate (GT P gamma S) inhibits the binding of CMF to membranes, suggesting that t he putative CMF receptor is coupled to a G protein, Cells lacking G al pha 1 (G alpha 1 null) do not exhibit GTP gamma S inhibition of CMF bi nding and do not exhibit CMF regulation of cAMP signal transduction, s uggesting that the putative CMF receptor interacts with G alpha 1. Wor k by others has suggested that G alpha 1 inhibits phospholipase C (PLC ), yet when cells lacking either G alpha 1 or PLC were starved at high cell densities (and thus in the presence of CMF), they developed norm ally and had normal cAMP signal transduction. We find that CMF activat es PLC. G alpha 1 null cells starved in the absence or presence of CMF behave in a manner similar to control cells starved in the presence o f CMF in that they extend pseudopods, have an activated PLC, have a lo w cAMP-stimulated GTPase, permit cAMP signal transduction, and aggrega te, Cells lacking G beta have a low PLC activity that cannot be stimul ated by CMF. Cells lacking PLC exhibit IP3 levels and cAMP-stimulated GTP hydrolysis rates intermediate to what is observed in wild-type cel ls starved in the absence or in the presence of an optimal amount of C MF. We hypothesize that CMF binds to its receptor, releasing G beta ga mma from G alpha 1. This activates PLC, which causes the G alpha 2 GTP ase to be inhibited, prolonging the lifetime of the cAMP-activated G a lpha 2-GTP configuration. This, in turn, allows cAR1-mediated cAMP sig nal transduction to take place.