PROTON TRANSPORT AND PHOSPHORYLATION OF TONOPLAST POLYPEPTIDES FROM ZUCCHINI ARE STIMULATED BY THE PHOSPHOLIPID PLATELET-ACTIVATING-FACTOR

The ether phospholipid platelet-activating factor and certain similar phospholipids, including lysophosphatidylcholine, are known to stimulate both H+ transport and protein phosphorylation in plant microsomal membranes. In the present work, several polypeptides in highly purified tonoplast membranes from zucchini (Cucurbita pepo L) showed platelet-activating factor-dependent phosphorylation. Comparison of protein phosphorylation in different membrane fractions separated by sucrose step density gradient centrifugation indicated that some of the phosphoproteins were contaminants or were common to several membrane fractions, but platelet-activating factor-dependent phosphorylation of peptides at 30, 53, and perhaps 100 kilodaltons was tonoplast specific. The phosphoprotein of 53 kilodaltons was shown by three different approaches (one- and two-dimensional polyacrylamide gel electrophoresis, western blots, and immunoprecipitation) to cross-read with antibody raised against the B subunit of the tonoplast ATPase from red beet (Beta vulgaris L.).