Localization of functional domains of the cAMP chemotactic receptor of Dictyostelium discoideum

The topography and functional domains of the cAMP chemotactic receptor of Dictyostelium discoideum were investigated by protease sensitivity to chymotrypsin. Proteolytic digestion of intact cells produced a 23-kDa fragment of the receptor that retained the photoaffinity label used to identify the receptor. Additionally, this fragment contained the sites phosphorylated by CAR-kinase, the enzyme that phosphorylates the ligand-occupied form of the receptor. The fragment was also found to be phosphorylated in response to cAMP stimulation of cells. Proteolytic digestion of either intact cells or membrane preparations did not appreciably alter the binding properties of the receptor, indicating that the domains which determine the cAMP binding pocket are likely to be transmembrane regions of the protein. Additionally, the sensitivity of down-regulated receptors to chymotrypsin digestion suggests that the initial loss of cAMP binding activity upon incubation of cells with high concentrations of ligand does not require receptor internalization.