Properties of CAR-kinase: The enzyme that phosphorylates the cAMP chemotactic receptor ofD. discoideum

The cell surface cAMP chemotactic receptor ofD. discoideum can be phosphorylated in partially purified plasma membrane preparations in a ligand-dependent manner. CAR-kinase, the enzyme responsible for receptor phosphorylation, was shown to be an integral membrane protein. It could utilize either ATP or GTP to phosphorylate the receptor, although ATP was much more efficient. The apparent affinity constant for ATP was approximately 20–25 µM. Maximum CAR-kinase activity was observed betweenpH 6.5 andpH 7, and required the presence of Mg²⁺. Neither Mn²⁺ nor Ca²⁺ could substitute for that divalent cation. The enzyme was found to be sensitive to the ionic strength and temperature of the incubation reaction. Dephosphorylation of the receptor was not observed in the membrane preparations, indicating that the enhanced level of receptor phosphorylation that occurred upon ligand binding was not an indirect reflection of receptor dephosphorylation and subsequent incorporation of radiolabeled phosphate.