CHANGES IN H-PUMPS AND A TONOPLAST INTRINSIC PROTEIN OF VACUOLAR MEMBRANES DURING THE DEVELOPMENT OF PEAR FRUIT()

Vacuolar H+-ATPase (V-ATPase) was purified from pear fruit and antibod ies were raised against the subunits of 55 and 33 kDa. Antibodies agai nst mung bean H+-pyrophosphatase (V-PPase) and radish VM23, which is a tonoplast intrinsic protein (TIP) and a water channel, crossreacted w ith the vacuolar membrane proteins of pear fruit. To clarify the roles of these proteins in development of pear fruit, we determined their l evels relative to the total amount of protein by immunoblot analysis. The levels of subunits of the V-ATPase increased with fruit developmen t. By contrast, the level of V-PPase was particularly high at the cell -division stage and remained almost the same at other stages. The chan ges in the activities of V-ATPase and V-PPase corresponded to those in their protein levels. The ratio of V-PPase activity to V-ATPase activ ity indicated that V-PPase is a major H+-pump of the vacuolar membrane s of young fruit and that the contribution of V-ATPase increases with fruit development, finally, V-ATPase becomes the major H+-pump during the later stages of fruit development. The level of a protein analogou s to VM23 (VM23P) was especially high during the active cell-expansion stage in young fruit, and VM23P might, therefore, play an important r ole in the rapid expansion of cells as a vacuolar water channel. Our r esults show that the levels of V-ATPase, V-PPase and VM23P change diff erently and reflect the roles of the respective proteins in the develo pment of pear fruit.