Aquaporin Gating: A New Twist to Unravel Permeation through Water Channels

Functional regulation of aquaporin dynamics by lipid bilayer composition

With the diversity of lipid-protein interactions, any observed membrane protein dynamics or functions directly depend on the lipid bilayer selection. However, the implications of lipid bilayer choice are seldom considered unless characteristic lipid-protein interactions have been previously reported. Using molecular dynamics simulation, we characterize the effects of membrane embedding on plant aquaporin SoPIP2;1, which has no reported high-affinity lipid interactions. The regulatory impacts of a realistic lipid bilayer, and nine different homogeneous bilayers, on varying SoPIP2;1 dynamics are examined. We demonstrate that SoPIP2;1's structure, thermodynamics, kinetics, and water transport are altered as a function of each membrane construct's ensemble properties. Notably, the realistic bilayer provides stabilization of non-functional SoPIP2;1 metastable states. Hydrophobic mismatch and lipid order parameter calculations further explain how lipid ensemble properties manipulate SoPIP2;1 behavior. Our results illustrate the importance of careful bilayer selection when studying membrane proteins. To this end, we advise cautionary measures when performing membrane protein molecular dynamics simulations.

Real-Time Dynamics of Water Transport in the Roots of Intact Maize Plants in Response to Water Stress: The Role of Aquaporins and the Contribution of Different Water Transport Pathways

Using an original methodological and technical approach, we studied the real-time dynamics of radial water transfer in roots and transpiration rate in intact maize plants in response to water stress. It was shown that the response of maize plants to water stress, induced by 10% PEG 6000, was accompanied by changes in the intensity and redistribution of water transfer along different pathways of radial water transport in the roots. It was shown that during the first minutes of water stress impact, the intensity of transcellular and symplastic water transport in the roots decreased with a parallel short-term increase in the transpiration rate in leaves and, presumably, in apoplastic transport in roots. Further, after a decrease in transpiration rate, the intensity of transcellular and symplastic water transport was restored to approximately the initial values and was accompanied by parallel upregulation of some PIP aquaporin genes in roots and leaves, changes in aquaporin localization in root tissues, and changes in xylem sap pH. Under water stress conditions, cell-to-cell water transport in roots becomes dominant, and aquaporins contribute to the simultaneous regulation of water transport in roots and shoots under water stress.

Mechanosensitive aquaporins

Cellular systems must deal with mechanical forces to satisfy their physiological functions. In this context, proteins with mechanosensitive properties play a crucial role in sensing and responding to environmental changes. The discovery of aquaporins (AQPs) marked a significant breakthrough in the study of water transport. Their transport capacity and regulation features make them key players in cellular processes. To date, few AQPs have been reported to be mechanosensitive. Like mechanosensitive ion channels, AQPs respond to tension changes in the same range. However, unlike ion channels, the aquaporin's transport rate decreases as tension increases, and the molecular features of the mechanism are unknown. Nevertheless, some clues from mechanosensitive ion channels shed light on the AQP-membrane interaction. The GxxxG motif may play a critical role in the water permeation process associated with structural features in AQPs. Consequently, a possible gating mechanism triggered by membrane tension changes would involve a conformational change in the cytoplasmic extreme of the single file region of the water pathway, where glycine and histidine residues from loop B play a key role. In view of their transport capacity and their involvement in relevant processes related to mechanical forces, mechanosensitive AQPs are a fundamental piece of the puzzle for understanding cellular responses.

An Aquaporin Gene (KoPIP2;1) Isolated from Mangrove Plant Kandelia obovata Had Enhanced Cold Tolerance of Transgenic Arabidopsis thaliana

Aquaporins (AQPs) are essential channel proteins that play central roles in maintaining water homeostasis. Here, a novel aquaporin gene, named KoPIP2;1, was cloned from the mangrove plant Kandelia obovata by RACE technology. The KoPIP2;1 gene was 1404 bp in length with an open reading frame (ORF) of 852 bp, encoded with 283 amino acids. Database comparisons revealed that KoPIP2;1 protein shared the highest identity (91.26%) with the aquaporin HbPIP2;2, which was isolated from Hevea brasiliensis. Gene expression analysis revealed that the KoPIP2;1 gene was induced higher in leaves than in stems and roots of K. obovata under cold stress. Transient expression of KoPIP2;1 in Nicotiana benthamiana epidermal cells revealed that the KoPIP2;1 protein was localized to the plasma membrane. Overexpressing KoPIP2;1 in Arabidopsis significantly enhanced the lateral root number of the transgenic lines. KoPIP2;1 transgenic Arabidopsis demonstrated better growth, elevated proline content, increased superoxide dismutase (SOD) and peroxidase (POD) activities, and reduced malondialdehyde (MDA) content compared with the wild-type Arabidopsis when exposed to cold stress. The findings suggest that overexpression of KoPIP2;1 probably conferred cold tolerance of transgenic Arabidopsis by enhancing osmoregulation and antioxidant capacity. This present data presents a valuable gene resource that contributes to the advancement of our understanding of aquaporins and their potential application in enhancing plant stress tolerance.