Plasma membrane aquaporins from different subtypes (PIP1 and PIP2) interact modifying their individual response to pH

MARQUEZ M, ALLEVA K, MUT P, SUTKA MR AND G AMODEO

Montevideo, Uruguay

Congreso; 6th Internacional Conference of Biological Physics, ICBP and 5th Southern Cone Biophysics Congress.; 2007

Comite organizador ad hoc

Water channels are integral membrane proteins that enhance water and/or small solute transport in living cells. Those expressed at the level of plant plasma membrane (PIPs) are divided in two main groups that are generally characterized by showing differential behavior in terms of water permeability, high (mainly PIP2 subtype) or low (mainly PIP1 subtype). However, recent studies have demonstrated that co-expression of these isoforms can enhance water permeability that is not necessarily the sum of their individual contribution. We therefore decided to test if the coexpression also affects aquaporin regulatory mechanisms, among them those triggered by abiotic stress signals (pH, Ca2+, etc.) . In particular, it is well known that cytoplasmic pH modulates PIP activity by blocking its water permeability at low pH values. The pH effect can be monitored when aquaporins are expressed in Xenopus oocytes and medium bath is alternatively perfused with AcNa (at pH 6 or 7.2) or NaCl (at pH 6 or 7.2). This experimental design allowed us to discriminate pHe from pHi as the intracellular pH of the oocyte can be modified only in the presence of the permeable buffer AcNa. Oocyte volume changes after three days of RNA injection are tracked by videomicroscopy. As expected, when PIP2 subtype (40 ng) was injected alone its initial water permeability (75±19 um.s-1, n=7) only decreases when pHi drops to pH 6 (44 ± 2 um.s-1, n=7). Plasma membrane water channels subtype PIP1 have per se low permeability (22 ± 6 um.s-1, n=11) but surprisingly, when they are coinjected in Xenopus oocytes with PIP2 subtype, the enhanced water permeability due to this coinjection (54 ± 9 um.s-1, n=11) is not only sensitive to pHi (23 ± 3 um.s-1, n=11) but it is enhanced by lowering external pH (117 ± 20 um.s-1, n=13), an effect that is not observed when these aquaporins are individually expressed. In conclusion, the coexpression of different aquaporin subtypes not only contributes to enhance water permeability but opens a new frame of regulatory mechanisms.