CHARACTERIZATION OF A NEW VACUOLAR MEMBRANE AQUAPORIN SENSITIVE TO MERCURY AT A UNIQUE SITE

The membranes of plant and animal cells contain aquaporins, proteins t hat facilitate the transport of water. In plants, aquaporins are found in the vacuolar membrane (tonoplast) and the plasma membrane. Many aq uaporins are mercury sensitive, and AQP1, a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala moti f. Here, we report the molecular analysis of a new Arabidopsis aquapor in, delta-TIP (for tonoplast intrinsic protein), and show that it is l ocated in the tonoplast. The water channel activity of delta-TIP is se nsitive to mercury. However, the mercury-sensitive cysteine residue fo und in mammalian aquaporins is not present in delta-TIP or in gamma-TI P, a previously characterized mercury-sensitive tonoplast aquaporin. S ite-directed mutagenesis was used to identify the mercury-sensitive si te in these two aquaporins as Cys-116 and Cys-118 for delta-TIP and ga mma-TIP, respectively. These mutations are at a conserved position in a presumed membrane-spanning domain not previously known to have a rol e in aquaporin mercury sensitivity. Comparing the tissue expression pa tterns of delta-TIP with gamma-TIP and alpha-TIP showed that the TIPs are differentially expressed.