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.