Cyclic 3′,5′-GMP Independent Protein Kinase at the Outer Surface of Intact Ehrlich Cells

Abstract

The phosphoryl group transfer from (γ³²P)ATP into acceptor proteins by an endogenous protein kinase at the surface of Ehrlich cells has been further studied as regards possible stimulation by different concentrations of dibuturyl cyclic guanosine monophosphate (3′,5′-GMP). Using the endogenous acceptor protein of the surface of Ehrlich cells the cyclic nucleotide had no stimulatory effect on the protein kinase of the plasma membrane. The lack of stimulatory action of the cyclic nucleotide was also observed when an exogenous acceptor protein was present. Instead, a slight inhibitory effect was usually seen in both types of experiments. Labeled phosphorylserine was always in excess of labeled phosphoryl threonine. Both were isolated from hydrolyzed acceptor proteins. The lack of stimulation by a cyclic nucleotide on the phosphorylation of acceptor protein(s) on the cell surface does not rule out a regulatory function by the protein kinase of the plasma membrane. Instead, we propose an autoregulatory mechanism for the phosphorylation at the cell surface. This mechanism is based upon the high sensitivity of the enzyme to Ca-ions.