Submitted to: PH96 International Postharvest Science Conference New Zealand
Publication Type: Proceedings
Publication Acceptance Date: September 12, 1996
Publication Date: N/A
Hypodermal-mesocarp plasma membrane (PM) vesicles isolated from mature-prehavest, postharvest and stored muskmelon (Cucumis melo L. var. reticulatus Naud), showed vanadate-sensitive ATPase activity in PM declines following fruit maturation. Protein- kinase activity is markedly stimulated by Ca2+, 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 conspic- uously phosphorylated in membranes from mature-preharvest 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. Washing the PM with buffer containing EGTA to remove Ca2+ and Ca2+-binding proteins like calmodulin, decreases the kinase activity of the membranes from mature preharvest and postharvest melons. When brain calmodulin is added to these membranes, the kinase activity is restored to levels similar to those in non- EGTA washed membranes. The kinase activity of membranes from postharvest stored fruits, in contrast, increases after an EGTA wash, and does not increase further when calmodulin is added. This indicates, that although a protein kinase inhibitor eliminated by EGTA washing, may be present and develop with muskmelon fruit postharvest aging, the changes in PM H+-ATPase activity during aging is better attributed to a decline in protein levels of the H+-ATPase than to phosphorylation status.