Phosphorylation of endogenous membrane proteins by endogenous protein kinase at the outer surface of Ehrlich cells

Phosphoprotein phosphatase activity at the outer surface of intact normal and transformed 3T3 fibroblasts

Using 32P-labeled phosphocasein or phosphohistones as exogenous substrates it was possible to detect a phosphoprotein phosphatase activity on the outer surface of intact normal and transformed 3T3 fibroblasts. Incubation of monolayers of intact cells in buffered salt solution with the radioactively labeled substrate resulted in the release of alkali-labile 32P counts into the surrounding medium. The reaction was: (a) linear with time (at least up to 20 min); (b) proportional to the cell density; (c) dependent on the temperature and pH of the incubation medium; (d) stimulated by K+; and (e) inhibited by sodium fluoride, inorganic pyrophosphate, zinc chloride and relatively impermeant sulfhydryl reagents. Less than 2% of the externally located phosphoprotein phosphatase activity was detectable in pooled cell-free washings of the intact cell monolayer. Phosphocasein did not cause any detectable leakage of intracellular lactate dehydrogenase or soluble phosphoprotein phosphatase activity into the external medium; incubation of the cells with phosphohistones, on the other hand, resulted in appreciable leakage of both these cytoplasmic activities. Neoplastic transformation was associated with a nearly two-fold decrease in the activity of the surface phosphoprotein phosphatase. Addition of serum to either non-transformed 3T3 or spontaneously transformed 3T6 cells resulted in a rapid and remarkeable drop in the cell surface dephosphorylating activity. Acrylamide gel electrophoresis of the dephosphorylated casein or histone substrate revealed no proteolytic degradation or change in electrophoretic mobility. The intact cells showed no damage upon microscopic examination as a result of exposure to phosphocasein or phosphohistones.

Phosphorylation of endogenous proteins by endogenous protein kinase of density gradient--purified plasma membrane vesicles of Ehrlich cells

A phosphorylation of endogenous acceptor protein(s) has been demonstrated to occur in membraneous vesicles prepared from Ehrlich ascites tumour cells. The reaction was catalyzed by endogenous protein kinase in the presence of exogenous (gamma32P)ATP. A considerable increase of the specific protein kinase activity took place when the plasma membrane preparation was subjected to a further gradient centrifugation in Dextran T 150. This was done in the presence of a slightly alkaline phosphate buffer containing Mg-ions which resulted in the formation of a well defined vesicular preparation at density 1.035 in the gradient. The apparent Km and Vmax for the reaction with vesicles and exogenous (gamma32P)ATP were determined and found to be 0.022 mM and 0.23 nmol x mg-1 x 10 min-1, respectively. Neither cyclic AMP nor cyclic GMP did stimulate the protein kinase-catalyzed reaction. Instead, a clear inhibition of the reaction by the cyclic nucleotides was unexpectedly registered. Adenosine at 0.5 mM also inhibited the reaction. Calcium ions were inhibitory at all concentrations tested in the presence of a fixed (gamma32P)ATP/Mg2+ ratio. When Mg-ions were stoichiometrically replaced by Ca-ions practically no activity was observed.

Phosphoryl group transfer from Ehrlich cells prelabeled in vivo with (32P)-orthophosphate and (14C)-glucose to unlabeled glioma cells

A transfer of (32P)-labeled phosphoryl groups has been demonstrated to occur between prelabeled Ehrlich cells (donor cells) and glioma cells (acceptor cells). The Ehrlich cells were generally prelabeled in vivo by an intraabdominal injection of (32P)-orthophosphate 24 hours prior to scarifice of the tumor-bearing mice. In some experiments two injections of (14C)-glucose were given to the animals in addition to the injection of (32P)-orthophosphate prior to sacrifice. The prelabeled Ehrlich cells were mixed with the glioma cells, that were still attached to the culture plates, and incubation took place in vitro. Labeled (32P)-phosphoryl (14C)-serine was isolated from the glioma cells as well as from the Ehrlich cells. The 32P/14C-ratio of (32P)-phosphoryl (14C)-serine of the glioma cells (acceptor cells) was significantly higher than found for the Ehrlich cells (donor cells). This was indicative of an interactive mechanism between the cells partly due to binding of membrane fragments from the Ehrlich cells on to the surface of the glioma cells and partly due to a phosphoryl group transfer between the two types of cells. Interaction experiments with a plasma membrane fraction prepared from the prelabeled Ehrlich cells vis à vis the glioma cells clearly showed that the phosphoryl group transfer mechanism between the cells was dominating and only a minor part of the interaction was due to adhesion. This transphosphorylation reaction represents a new type of interaction between cells. It was not influenced by either adenosine 3', 5'-monophosphate or adenosine 5'-diphosphate.

Cyclic 3',5'-GMP independent protein kinase at the outer surface of intact Ehrlich cells

The phosphoryl group transfer from (gamma(32)P)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.