THE VACUOLAR MEMBRANE-PROTEIN GAMMA-TIP CREATES WATER SPECIFIC CHANNELS IN XENOPUS-OOCYTES

The vacuolar membrane (tonoplast) of higher plant cells contains an ab undant 27 kDa protein called TIP (tonoplast intrinsic protein) that oc curs in different isoforms and belongs to a large family of homologous channel-like proteins found in bacteria, plants and animals. In the p resent study, we identified and characterized the function of gamma-TI P from Arabidopsis thaliana by expression of the protein in Xenopus oo cytes. gamma-TIP increased the osmotic water permeability of oocytes 6 - to 8-fold, to values in the range 1-1.5 x 10(-2) cm/s. Similar resul ts were obtained with the homologous human erythrocyte protein CHIP28, recently identified as the erythrocyte water channel. The bacterial h omolog GlpF did not affect the osmotic water permeability of oocytes, but facilitated glycerol uptake, in accordance with its known function . By contrast, gamma-TIP did not promote glycerol permeability. Voltag e clamp experiments provided evidence showing that gamma-TIP induced n o electrogenic ion transport in oocytes, especially during osmotic cha llenge that resulted in massive transport of water. These results allo w us to conclude that the various protein members of the MIP family ha ve unique and specific transport functions and that the plant protein gamma-TIP likely functions as a water specific channel in the vacuolar membrane.