Post-translational modification acts as a digital like switch influencing AtPIP2;1 water and cation permeability

Plant aquaporins (AQPs) were initially described as a family of membrane-localized proteins exclusively facilitating water transport. Subsequently, sub-sets of plant AQPs have exhibited diverse functionalities beyond water transport. The aquaporin AtPIP2;1, an abundant Plasma membrane Intrinsic Protein in Arabidopsis thaliana, can transport water but also CO₂, H₂O₂ and monovalent cations under certain conditions. However, the mechanisms regulating the selectivity of AtPIP2;1, particularly for cations and water, remain to be fully explored. Here we report the outcome of mutating four AtPIP2;1 serine phosphorylation sites to mimic states of phosphorylation and dephosphorylation in loops B and D, and the C-terminal domain. Expression of the mutated proteins in Xenopus laevis oocytes allowed analysis of both water and ion conduction. Concurrent modifications at the four phosphorylation sites may collectively act as a 'selectivity switch,' modulating the permeability between cations and water for the homotetramer of AtPIP2;1, allowing for the possibility of simultaneous transport, with one substrate remaining dominant. The reciprocal relationship between cation conductance and water transport fits with the model of a gated ion-permeable pore of the tetramer being dependent on the four individual monomer water conductance states. Notably, in several instances, cation conductance can be turned off, reaching levels comparable to those of the H₂O-injected control, and these instances corresponded with maximal water transport. In contrast, when cation conductance was significantly increased, water transport was reduced but not completely silenced. AtPIP2;1 triple mutant S194A/S280DS283D (A/DD, Loop D and C-terminal regions respectively) displayed very high cation conductance with a selectivity sequence for univalent cations of K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺ > TEA⁺ (tetraethylammonium⁺) > choline⁺ > NMDG⁺ (N-methyl-d-glucamine). In conclusion, our results suggest that post-translational regulations may provide AtPIP2;1 with the flexibility to switch between predominantly cation transport or predominantly water transport. This dynamic 'switch' likely contributes to maintaining water and ion homeostasis under diverse environmental conditions.