Water channel proteins, AQPs (aquaporins), of the PIP (plasma membrane intrinsic protein) subfamily, provide a means for fine and quick adjustments of the plant water status. A molecular model for gating of PIPs by cytosolic protons (H+) and divalent cations was derived from the atomic structure of spinach SoPIP2;1 (Spinacia oleracea PIP2;1) in an open- and a closed-pore conformation. In the present study, we produced the Arabidopsis AtPIP2;1 (Arabidopsis thaliana PIP2;1) homologue in Pichia pastoris, either WT (wild-type) or mutations at residues supposedly involved in gating. Stopped-flow spectrophotometric measurements showed that, upon reconstitution in proteoliposomes, all forms function as water channels. The first functional evidence for a direct gating of PIPs by divalent (bivalent) cations was obtained. In particular, cadmium and manganese were identified, in addition to calcium (Ca2+) and H+ as potent inhibitors of WT AtPIP2;1. Our results further show that His199, the previously identified site for H+ sensing, but also N-terminal located Glu31, and to a lesser extent Asp28, are involved in both divalent-cation- and H+-mediated gating. In contrast, mutation of Arg124 rendered AtPIP2;1 largely insensitive to Ca2+ while remaining fully sensitive to H+. The role of these residues in binding divalent cations and/or stabilizing the open or closed pore conformations is discussed.

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