Phosphorylation influences water and ion channel function of AtPIP2;1

The phosphorylation state of two serine residues within the C-terminal domain of AtPIP2;1 (S280, S283) regulates its plasma membrane localization in response to salt and osmotic stress. Here, we investigated whether the phosphorylation state of S280 and S283 also influence AtPIP2;1 facilitated water and cation transport. A series of single and double S280 and S283 phosphomimic and phosphonull AtPIP2;1 mutants were tested in heterologous systems. In Xenopus laevis oocytes, phosphomimic mutants AtPIP2;1 S280D, S283D, and S280D/S283D had significantly greater ion conductance for Na⁺ and K⁺, whereas the S280A single phosphonull mutant had greater water permeability. We observed a phosphorylation-dependent inverse relationship between AtPIP2;1 water and ion transport with a 10-fold change in both. The results revealed that phosphorylation of S280 and S283 influences the preferential facilitation of ion or water transport by AtPIP2;1. The results also hint that other regulatory sites play roles that are yet to be elucidated. Expression of the AtPIP2;1 phosphorylation mutants in Saccharomyces cerevisiae confirmed that phosphorylation influences plasma membrane localization, and revealed higher Na⁺ accumulation for S280A and S283D mutants. Collectively, the results show that phosphorylation in the C-terminal domain of AtPIP2;1 influences its subcellular localization and cation transport capacity.