Characterization of the Major Integral Protein of Vacuolar Membrane

The vacuolar membrane of radish (Raphanus sativus) taproot contained a large quantity of a protein of 23 kilodaltons that accounted for more than 25% of the total membrane proteins. The protein, tentatively named VM 23, was purified and characterized. VM 23 tends to aggregate at high temperature even in the presence of 1% sodium dodecyl sulfate. The apparent molecular size of VM 23 was estimated to be about 400 kilodaltons by polyacrylamide gel electrophoresis in the presence of 0.1% Triton X-100. VM 23 was partially extracted from the vacuolar membranes with chloroform:methanol, indicating its high hydrophobicity. The hydrophobic carboxyl modifier N,N'-dicyclohexylcarbodiimide bound covalently to VM 23. The results suggest that VM 23 may act as a secondary transport system coupled with the proton transport. The antibody against radish VM 23 reacted with the major proteins in the vacuolar membranes of mung bean (Vigna radiata) and castor bean (Ricinus communis) hypocotyls and pumpkin (Cucurbita moschata) epicotyl, but not with that of sugar beet (Beta vulgaris) taproot. VM 23 comigrated with vacuolar H(+)-pyrophosphatase on sucrose density gradient centrifugation after sonication of membranes, indicating that it is associated with the vacuolar membrane.