A putative receptor mediating cell-density sensing in Dictyostelium

When Dictyostelium cells starve, they begin secreting a glycoprotein called conditioned medium factor (CMF). When there is a high density of starved cells, as indicated by a high concentration of CMF, the cells begin expressing some genes and aggregate using pulses of cAMP as a chemoattractant. CMF regulates gene expression via a G protein-independent pathway, whereas CMF regulates cAMP signal transduction via a G protein-dependent pathway. To elucidate receptors mediating cell density sensing, we used CMF-Sepharose to isolate membrane proteins that bind CMF. We identified a 50-kDa protein, CMFR1, that is sensitive to trypsin treatment of whole cells. We obtained partial amino acid sequence of CMFR1 and isolated the cDNA encoding it, The derived amino acid sequence has no significant similarity to known proteins and has two or three predicted transmembrane domains. Expression of CMFR1 in insect cells caused an increase in CMF binding. Repression of CMFR1 in Dictyostelium by gene disruption resulted in a similar to 50% decrease of the CMF binding and a loss of CMF-induced G protein-independent gene expression. The G protein-dependent CMF signal transduction pathways appear to be functional in cmfr1 cells, suggesting that cells sense the density-sensing factor CMF using two or more different receptors.