Submitted to: Acta Horticulture Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 25, 1996
Publication Date: N/A
Interpretive Summary: The plasma membrane, from under the epidermis, is directly associated with regulating the shelf-life of muskmelon fruits. A marker enzyme for this membrane is H-ATPase. This enzyme is responsible for the energy needed for the selective transfer of compounds across the membrane which are used for cellular function and shelf-life. In order for the H-ATPase to transfer energy, it releases a phosphate, the ATP therefore, needs to be re-phosphorylated. The compound responsible for rephosphoryl- ating the ATP is calcium-dependent protein kinase (CDPK). This study determined that CDPK occurs in muskmelon plasma membranes and they rephosphorylate the H-ATPase. However, the loss in H-ATPase associated with muskmelon plasma membrane senescence is not due to reduced rephosphorylation, but due an actual loss in the ATPase protein.
Hypodermal-mesocarp plasma membrane (PM) vesicles isolated from mature-preharvest, mature-postharvest and stored muskmelon (Cucumis melo L. var. reticulatus Naud.), showed vanadate- sensitive ATPase activity that decreases following fruit maturation. Coincidently, protein-kinase activity in PM declines following fruit maturation. Protein-kinase activity is markedly simulated by Ca(2+), and is responsible for the phosphorylation of many melon PM proteins. Among these, two protein bands are important, a 98 kDa protein corresponding with the weight of H+-ATPase, which is most conspicuously phosphorylated in membranes from mature fruits, and a 70 kDa protein, which appears phosphorylated at all fruit membrane ages. Protein-kinase(s) in melon PM can also phosphorylate Histone-IIIS, a protein used as a kinase substrate. In the presence of Histone-IIIS, Ca(2+)- dependent kinase activity decreases with melon membrane post- harvest aging, in contrast with the Ca(2+)-independent activity that shows a small increase during maturation and postharvest aging. Washing the PM with buffer containing EGTA to remove Ca(2+) and Ca(2+)-binding proteins like calmodulin, decreases the kinase activity of the membranes from mature and postharvest melon. Although a protein kinase inhibitor, eliminated by EGTA washing, may be present and develop with muskmelon fruit postharvest aging, the change in PM H+-ATPase activity during aging is better attributed to a decline in protein levels of the H+-ATPase rather than its phosphorylation status.