IMMUNOLOGICAL DETECTION OF TONOPLAST POLYPEPTIDES IN THE PLASMA-MEMBRANE OF PEA COTYLEDONS

The tonoplast is usually characterized by the presence of two electrog enic proton pumps: a vacuolar-type H+-ATPase and a pyrophosphatase, as well as a putative water-channel-forming protein (gamma-TIP). Using a post-embedding immunogold labelling technique, we have detected the p resence of these transport-protein complexes not only in the tonoplast , but also in the plasma membrane and trans Golgi elements of maturing pea (Pisum sativum L.) cotyledons. These ultrastructural observations are supported by Western blotting with highly purified plasma-membran e fractions. In contrast to the vacuolar-type H+-ATPase, whose activit y was not measurable, considerable pyrophosphatase activity was detect ed in the plasma-membrane fraction. These results are discussed in ter ms of a possible temporary repository for tonoplast proteins en route to the vacuole.