CTD phosphorylation as part of a mechanism for gating water and ion transport
The central pore, formed in the middle of the four monomeric channels, has been proposed to be the exclusive channel for ion transport through the AtPIP2;1 tetramer (Kourghi et al., 2018), however, whether or not water and ion transport events are mutually exclusive are yet to be determined. Phosphorylation at the CTD of AtPIP2;1 has the potential to almost completely block the water transport function of AtPIP2;1 when compared to H2O injected oocytes without abolishing ion transport (Figure 3b,c and Figure S4). In contrast, the double CTD phosphorylation-deficient mimic showed a tendency to completely prevent the ion transport function without affecting the Pos(Figure 3c). These data indicate that manipulation of phosphorylation at the CTD of AtPIP2;1 might provide a key control point for regulating the water and ion gating and net channel selectivity.
Some plant AQPs have been shown to be permeable to multiple types of molecules and PIP2;1 is a special example. Not only does it play roles in maintaining water homeostasis, but it is also involved in multiple signalling processes (Chaumont and Tyerman, 2014; Maurel et al.,2015). AtPIP2;1 showed H2O2 transport capacity and has been proposed to act as a signalling intermediate involved in ROS detoxification and guard cell signalling (Bienert and Chaumont, 2014; Dynowski et al., 2008a; Rodrigues et al.,2017). Phosphorylation at the CTD of AtPIP2;1 and its effect on ion/water selectivity, indicates a capacity to rapidly switch substrates between H2O and ions (Na+ and K+). This could quickly adjust cytosolic osmotic and electrochemical gradients, potentially in the vicinity of the water channel. The phosphorylation status may also change the distribution of ion-specific and H2O-specific channels on the plasma membrane. In such a case, the membrane density of functional cation-selective AtPIP2;1 could be small depending on the single channel conductance and channel open probability. Only a relatively small number of cation conducting AtPIP2;1 may be needed to change the membrane potential while water transport can be easily substituted for by other AtPIP2 and AtPIP1 aquaporins. In this context AtPIP2;1 could be acting as a signalling module of ionic changes to downstream targets to respond to stress.