PROTEIN-KINASES IN ZUCCHINI - CHARACTERIZATION OF CALCIUM-REQUIRING PLASMA-MEMBRANE KINASES

Using an in situ phosphorylation assay with zucchini (Cucurbita pepo L . cv Dark Green) seedling tissue, we have identified numerous polypept ides that are capable of acting as protein kinases. Total protein prep arations from different organs contain different kinase profiles, but all are within the range of 55 to 70 kD. At least four kinases are ass ociated with highly purified plasma membranes from etiolated zucchini hypocotyls. The major phosphorylated polypeptides from plasma membrane s range in apparent molecular mass from 58 to 68 kD. The plasma membra ne kinases are activated by micromolar concentrations of calcium and p hosphorylate serine, and, to a lesser extent, threonine residues. Thes e characteristics are similar to those of a soluble calcium-dependent protein kinase that has been purified to homogeneity from soybean susp ension cultures. Three of the zucchini plasma membrane kinases share a ntigenic epitopes with the soluble soybean kinase. The presence of kin ase activity at different apparent molecular masses may be indicative of separate kinases with similar characteristics. The zucchini hypocot yl protein kinases are not removed from plasma membrane vesicles by 0. 5 m NaCl/5 mm ethylenediaminetetraacetate or by detergent concentratio ns below the critical micelle concentration of two types of detergent. This indicates that the plasma membrane protein kinases are tightly a ssociated with the membrane in zucchini seedlings.