The plant plasma membrane barrier can express aquaporins (PIP1 and PIP2) that show two intriguing aspects: (1) the potential of modulating whole membrane water permeability by co-expression of both types, which have recently been distinguished for showing a different capacity to reach the plasma membrane; and (2) the faculty to reduce water permeation through the pore after cytosolic acidification, as a consequence of a gating process. Our working hypothesis is that these two key features might enhance plasticity of the membrane water transport capacity if they jointly trigger any cooperative interaction. In previous work, we proved by biophysical approaches that the plasma membrane of the halophyte Beta vulgaris storage root presents highly permeable aquaporins that can be shut down by acidic pH. Root Beta vulgaris PIPs were therefore subcloned and expressed in Xenopus oocytes. Co-expression of BvPIP1;1 and BvPIP2;2 not only enhances oocyte plasma membrane water permeability synergistically but also reinforces pH inhibitory response from partial to complete shut down after cytosolic pH acidification. This pH dependent behavior shows that PIP1-PIP2 co-expression accounts for a different pH sensitivity in comparison with PIP2 expression. These results prove for the first time that PIP co-expression modulates the membrane water permeability through a pH regulatory response, enhancing in this way membrane versatility to adjust its water transfer capacity.

a b s t r a c tThe highly conserved plant aquaporins, known as Plasma membrane Intrinsic Proteins (PIPs), are the main gateways for cell membrane water exchange. Years of research have described in detail the properties of the PIP2 subfamily. However, characterizing the PIP1 subfamily has been difficult due to the failure to localize to the plasma membrane. In addition, the discovery of the PIP1-PIP2 interaction suggested that PIP1 aquaporins could be regulated by a complex posttranslational mechanism that involves trafficking, heteromerization and fine-tuning of channel activity. This review not only considers the evidence and findings but also discusses the complexity of PIP aquaporins. To establish a new benchmark in PIP regulation, we propose to consider PIP1-PIP2 pairs as functional units for the purpose of future research into their physiological roles.

One of the most intriguing properties of plasma membrane intrinsic protein (PIP) aquaporins (AQPs) is their ability to modulate water transport by sensing different levels of intracellular pH through the assembly of homo‐ and heterotetrameric molecular species in the plasma membrane. In this work, using a phenomenological modeling approach, we demonstrate that cooperativity in PIP biological response cannot be directly attributed to a cooperative proton binding, as it is usually considered, since it could also be the consequence of a cooperative conformation transition between open and closed states of the channel. Moreover, our results show that, when mixed populations of homo‐ and heterotetrameric PIP channels are coexpressed in the plasma membrane of the same cell, the observed decrease in the degree of positive cooperativity would result from the simultaneous presence of molecular species with different levels of proton sensing. Indeed, the random mixing between different PIP paralogues as subunits in a single tetramer, plus the possibility of mixed populations of homo‐ and heterotetrameric PIP channels widen the spectrum of cooperative responses of a cell membrane. Our approach offers a deep understanding of cooperative transport of AQP channels, as members of a multiprotein family where the relevant proton binding sites of each member have not been clearly elucidated yet.