PLANT PHYSIOLOGY , Vol 103, Issue 2 413-419, Copyright © 1993 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
Protein Kinases in Zucchini (Characterization of Calcium-Requiring Plasma Membrane Kinases)
S. D. Verhey, J. C. Gaiser and T. L. Lomax
Department of Botany and Plant Pathology and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331-2902
Using an in situ phosphorylation assay with zucchini (Cucurbita pepo L. cv
Dark Green) seedling tissue, we have identified numerous polypeptides that
are capable of acting as protein kinases. Total protein preparations from
different organs contain different kinase profiles, but all are within the
range of 55 to 70 kD. At least four kinases are associated with highly
purified plasma membranes from etiolated zucchini hypocotyls. The major
phosphorylated polypeptides from plasma membranes range in apparent
molecular mass from 58 to 68 kD. The plasma membrane kinases are activated
by micromolar concentrations of calcium and phosphorylate serine, and, to a
lesser extent, threonine residues. These characteristics are similar to
those of a soluble calcium-dependent protein kinase that has been purified
to homogeneity from soybean suspension cultures. Three of the zucchini
plasma membrane kinases share antigenic epitopes with the soluble soybean
kinase. The presence of kinase activity at different apparent molecular
masses may be indicative of separate kinases with similar characteristics.
The zucchini hypocotyl protein kinases are not removed from plasma membrane
vesicles by 0.5 M NaCl/5 mM ethylenediaminetetraacetate or by detergent
concentrations below the critical micelle concentration of two types of
detergent. This indicates that the plasma membrane protein kinases are
tightly associated with the membrane in zucchini seedlings.
