Dt. Brazill et al., CELL-DENSITY SENSING MEDIATED BY A G-PROTEIN-COUPLED RECEPTOR ACTIVATING PHOSPHOLIPASE-C, The Journal of biological chemistry, 273(14), 1998, pp. 8161-8168
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.