The plasma membrane of Arabidopsis thaliana contains a mercury-insensitive aquaporin that is a homolog of the tonoplast water channel protein TIP.

Plant cells contain proteins that are members of the major intrinsic protein (MIP) family, an ancient family of membrane channel proteins characterized by six membrane-spanning domains and two asparagine-proline-alanine (NPA) amino acid motifs in the two halves of the protein. We recently demonstrated that gamma-TIP, one of the MIP homologs found in the vacuolar membrane of plant cells, is an aquaporin or water channel protein (C. Maurel, J. Reizer, J.I. Schroeder, M.J. Chrispeels [1993] EMBO J 12: 2241-2247). RD28, another MIP homolog in Arabidopsis thaliana, was first identified as being encoded by a turgor-responsive transcript. To find out if RD28 is a water channel protein, rd28 cRNA was injected into Xenopus laevis oocytes. Expression of RD28 caused a 10- to 15-fold increase in the osmotic water permeability of the oocytes, indicating that the protein creates water channels in the plasma membrane of the oocytes and is an aquaporin just like its homology gamma-TIP. Although RD28 has several cysteine residues, its activity is not inhibited by mercury, and in this respect it differs from gamma-TIP and all but one of the mammalian water channels that have been described. Introduction of a cysteine residue next to the second conserved NPA motif creates a mercury-sensitive water channel, suggesting that this conserved loop is critical to the activity of the protein. Antibodies directed at the C terminus of RD28 were used in combination with a two-phase partitioning method to demonstrate that RD28 is located in the plasma membrane. The protein is present in leaves and roots of well-watered plants, suggesting that its presence in plants does not require a specific desiccation regime. These results demonstrate that plant cells contain constitutively expressed aquaporins in their plasma membranes (RD28), as well as in their tonoplasts (gamma-TIP).