Structural basis for pH gating of plant aquaporins

Intra-helical salt-bridge and helix destabilizing residues within the same helical turn: Role of functionally important loop E half-helix in channel regulation of major intrinsic proteins

The superfamily of major intrinsic proteins (MIPs) includes aquaporin (AQP) and aquaglyceroporin (AQGP) and it is involved in the transport of water and neutral solutes across the membrane. Diverse MIP sequences adopt a unique hour-glass fold with six transmembrane helices (TM1 to TM6) and two half-helices (LB and LE). Loop E contains one of the two conserved NPA motifs and contributes two residues to the aromatic/arginine selectivity filter. Function and regulation of majority of MIP channels are not yet characterized. We have analyzed the loop E region of 1468 MIP sequences and their structural models from six different organism groups. They can be phylogenetically clustered into AQGPs, AQPs, plant MIPs and other MIPs. The LE half-helix in all AQGPs contains an intra-helical salt-bridge and helix-breaking residues Gly/Pro within the same helical turn. All non-AQGPs lack this salt-bridge but have the helix destabilizing Gly and/or Pro in the same positions. However, the segment connecting LE half-helix and TM6 is longer by 10-15 residues in AQGPs compared to all non-AQGPs. We speculate that this longer loop in AQGPs and the LE half-helix of non-AQGPs will be relatively more flexible and this could be functionally important. Molecular dynamics simulations on glycerol-specific GlpF, water-transporting AQP1, its mutant and a fungal AQP channel confirm these predictions. Thus two distinct regions of loop E, one in AQGPs and the other in non-AQGPs, seem to be capable of modulating the transport. These regions can also act in conjunction with other extracellular residues/segments to regulate MIP channel transport.

New subfamilies of major intrinsic proteins in fungi suggest novel transport properties in fungal channels: implications for the host-fungal interactions

Background

Aquaporins (AQPs) and aquaglyceroporins (AQGPs) belong to the superfamily of Major Intrinsic Proteins (MIPs) and are involved in the transport of water and neutral solutes across the membranes. MIP channels play significant role in plant-fungi symbiotic relationship and are believed to be important in host-pathogen interactions in human fungal diseases. In plants, at least five major MIP subfamilies have been identified. Fungal MIP subfamilies include orthodox aquaporins and five subgroups within aquaglyceroporins. XIP subfamily is common to both plants and fungi. In this study, we have investigated the extent of diversity in fungal MIPs and explored further evolutionary relationships with the plant MIP counterparts.

Results

We have extensively analyzed the available fungal genomes and examined nearly 400 fungal MIPs. Phylogenetic analysis and homology modeling exhibit the existence of a new MIP cluster distinct from any of the known fungal MIP subfamilies. All members of this cluster are found in microsporidia which are unicellular fungal parasites. Members of this family are small in size, charged and have hydrophobic residues in the aromatic/arginine selectivity filter and these features are shared by small and basic intrinsic proteins (SIPs), one of the plant MIP subfamilies. We have also found two new subfamilies (δ and γ2) within the AQGP group. Fungal AQGPs are the most diverse and possess the largest number of subgroups. We have also identified distinguishing features in loops E and D in the newly identified subfamilies indicating their possible role in channel transport and gating.

Conclusions

Fungal SIP-like MIP family is distinct from any of the known fungal MIP families including orthodox aquaporins and aquaglyceroporins. After XIPs, this is the second MIP subfamily from fungi that may have possible evolutionary link with a plant MIP subfamily. AQGPs in fungi are more diverse and possess the largest number of subgroups. The aromatic/arginine selectivity filter of SIP-like fungal MIPs and the δ AQGPs are unique, hydrophobic in nature and are likely to transport novel hydrophobic solutes. They can be attractive targets for developing anti-fungal drugs. The evolutionary pattern shared with their plant counterparts indicates possible involvement of new fungal MIPs in plant-fungi symbiosis and host-pathogen interactions.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

Mercury increases water permeability of a plant aquaporin through a non-cysteine-related mechanism

Water transport across cellular membranes is mediated by a family of membrane proteins known as AQPs (aquaporins). AQPs were first discovered on the basis of their ability to be inhibited by mercurial compounds, an experiment which has followed the AQP field ever since. Although mercury inhibition is most common, many AQPs are mercury insensitive. In plants, regulation of AQPs is important in order to cope with environmental changes. Plant plasma membrane AQPs are known to be gated by phosphorylation, pH and Ca²⁺. We have previously solved the structure of the spinach AQP SoPIP2;1 (Spinacia oleracea plasma membrane intrinsic protein 2;1) in closed and open conformations and proposed a mechanism for how this gating can be achieved. To study the effect of mercury on SoPIP2;1 we solved the structure of the SoPIP2;1-mercury complex and characterized the water transport ability using proteoliposomes. The structure revealed mercury binding to three out of four cysteine residues. In contrast to what is normally seen for AQPs, mercury increased the water transport rate of SoPIP2;1, an effect which could not be attributed to any of the cysteine residues. This indicates that other factors might influence the effect of mercury on SoPIP2;1, one of which could be the properties of the lipid bilayer.

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

Oxygenic photosynthetic organisms use sunlight energy to oxidize water to molecular oxygen. This process is mediated by the photosystem II complex at the lumenal side of the thylakoid membrane. Most research efforts have been dedicated to understanding the mechanism behind the unique water oxidation reactions, whereas the delivery pathways for water molecules into the thylakoid lumen have not yet been studied. The most common mechanisms for water transport are simple diffusion and diffusion facilitated by specialized channel proteins named aquaporins. Calculations using published data for plant chloroplasts indicate that aquaporins are not necessary to sustain water supply into the thylakoid lumen at steady state photosynthetic rates. Yet, arguments for their presence in the plant thylakoid membrane and beneficial action are presented.

The physiological response of Populus tremula x alba leaves to the down-regulation of PIP1 aquaporin gene expression under no water stress

In order to study the role of PIP1 aquaporins in leaf water and CO2 transport, several lines of PIP1-deficient transgenic Populus tremula x alba were generated using a reverse genetic approach. These transgenic lines displayed no visible developmental or morphological phenotypes when grown under conditions of no water stress. Major photosynthetic parameters were also not affected by PIP1 down regulation. However, low levels of PIP1 expression resulted in greater leaf hydraulic resistance (an increase of 27%), which effectively implicated PIP1 role in water transport. Additionally, the expression level of PIP1 genes in the various transgenic lines was correlated with reductions in mesophyll conductance to CO2 (gm), suggesting that in poplar, these aquaporins influenced membrane permeability to CO2. Overall, although analysis showed that PIP1 genes contributed to the mass transfer of water and CO2 in poplar leaves, their down-regulation did not dramatically impair the physiological needs of this fast growing tree when cultivated under conditions of no stress.