THE CELL-DENSITY FACTOR CMF REGULATES THE CHEMOATTRACTANT RECEPTOR CAR1 IN DICTYOSTELIUM

Starving Dictyostelium cells aggregate by chemotaxis to cAMP when a se creted protein called conditioned medium factor (CMF) reaches a thresh old concentration. Cells expressing CMF antisense mRNA fail to aggrega te and do not transduce signals from the cAMP receptor. Signal transdu ction and aggregation are restored by adding recombinant CMF. We show here that two other cAMP-induced events, the formation of a slow disso ciating form of the cAMP receptor and the loss of ligand binding, whic h is the first step of ligand-induced receptor sequestration, also req uire CMF. Vegetative cells have very few CMF and cAMP receptors, while starved cells possess similar to 40,000 receptors for CMF and cAMP. T ransformants overexpressing the cAMP receptor gene cAR1 show a 10-fold increase of [H-3]cAMP binding and a similar increase of [I-125]CMF bi nding; disruption of the cAR1 gene abolishes both cAMP and CMF binding , In wild-type cells, downregulation of cAR1 with high levels of cAMP also downregulates CMF binding, and CMF similarly downregulates cAMP a nd CMF binding. This suggests that the cAMP binding and CMF binding ar e closely linked. Binding of similar to 200 molecules of CMF to starve d cells affects the affinity of the majority of the cAR1 cAMP receptor s within min, indicating that an amplifying mechanism allows one activ ated CMF receptor to regulate many cARs, In cells lacking the G-protei n beta subunit, cAMP induces a loss of cAMP binding, but not CMF bindi ng, while CMF induces a reduction of CMF binding without affecting cAM P binding, suggesting that the linkage of the cell density-sensing CMF receptor and the chemoattractant cAMP receptor is through a G-protein .