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Plant and Cell Physiology, 1985, Vol. 26, No. 6 977-986
© 1985

Article

Calcium- and Calmodulin-Promoted Phosphorylation of Membrane Proteins during Senescence in Apples1,2

Gopinadhan Paliyath and B. W. Poovaiah3

Department of Horticulture and Landscape Architecture, Washington State University Pullman, WA 99164-64l4 U.S.A.

3To whom request for reprints should be addressed

Sodium dodecylsulfate-polyacrylamide gel electrophoresis of microsomal membrane proteins from post-climacteric apples at an early and an advanced stage of senescence showed only slight qualitative changes in the protein pattern. Though there was a 30% reduction in the total microsomal protein content in apples at an advanced stage of senescence, a polypeptide with 18,000 molecular weight increased in quantity during senescence. In vitro phosphorylation of several proteins was promoted by calcium in membranes from apples at an early stage of senescence. Phosphorylation of proteins with molecular weights of 95,000, 91,000, 53,000 and 50,000 was promoted by calcium and calmodulin. Phosphorylation of these proteins increased with increasing calcium concentration. Proteins with molecular weights of 53,000 and 50,000 showed marked promotion of phosphorylation over the calcium-promoted level when the amount of calmodulin in the assay mixture was increased. Calcium- and calmodulin-promoted phosphorylation of membrane proteins showed considerable decrease when the apples were at an advanced stage of senescence. Moreover, increasing the concentrations of calcium and calmodulin in the assay mixture did not have any promoting effect on the phosphorylation of these proteins. Phosphoprotein phosphatase activity as measured by the loss of label from phosphorylated proteins following chase with cold ATP, did not differ to a great extent in membrane preparations from normal and senesced apples. Hydrolysis of ATP by senesced apple membrane preparation, however, was found to be relatively higher. The significance of these observations in relation to senescence is discussed.

1 Scientific Paper No. 7084, College of Agriculture and Home Economics, Washington State University, Pullman, Project 0321.

2 Supported in part by grants from the Washington State Tree Fruit Research Commission, and National Science Foundation Grant PCM-8208408.

(Received October 19, 1984; Accepted April 12, 1985)
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