The vacuolar membrane (tonoplast) of higher plant cells contains an abundant 27 kDa protein called TIP (tonoplast intrinsic protein) that occurs in different isoforms and belongs to a large family of homologous channel-like proteins found in bacteria, plants and animals. In the present study, we identified and characterized the function of gamma-TIP from Arabidopsis thaliana by expression of the protein in Xenopus oocytes. 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 results were obtained with the homologous human erythrocyte protein CHIP28, recently identified as the erythrocyte water channel. The bacterial homolog 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. Voltage clamp experiments provided evidence showing that gamma-TIP induced no electrogenic ion transport in oocytes, especially during osmotic challenge that resulted in massive transport of water. These results allow us to conclude that the various protein members of the MIP family have unique and specific transport functions and that the plant protein gamma-TIP likely functions as a water specific channel in the vacuolar membrane.