Expression of water channel proteins in Mesembryanthemum crystallinum

We have characterized transcripts for nine major intrinsic proteins (MIPs), some of which function as water channels (aquaporins), from the ice plant Mesembryanthemum crystallinum. To determine the cellular distribution and e xpression of these MIPs, oligopeptide-based antibodies were generated again st MIP-A, MIP-B, MIP-C, or MIP-F, which, according to sequence and function al characteristics, are located in the plasma membrane (PM) and tonoplast, respectively. MIPs were most abundant in cells involved in bulk water flow and solute flux. The tonoplast MIP-F was found in all cells, while signatur e cell types identified different PM-MIPs: MIP-A predominantly in phloem-as sociated cells, MIP-B in xylem parenchyma, and MIP-C in the epidermis and e ndodermis of immature roots. Membrane protein analysis confirmed MIP-F as t onoplast located. MIP-A and MIP-B were found in tonoplast fractions and als o in fractions distinct from either the tonoplast or PM. MIP-C was most abu ndant but not exclusive to PM fractions, where it is expected based on its sequence signature. We suggest that within the cell, MIPs are mobile, which is similar to aquaporins cycling through animal endosomes. MIP cycling and the differential regulation of these proteins observed under conditions of salt stress may be fundamental for the control of tissue water flux.