PIP aquaporin pH sensing is regulated by the length and charge of the C-terminal region

Membrane tension-dependent conformational change of Isoleucine 106 of loop B diminishes water permeability in FaPIP2;1

Aquaporins (AQPs) are membrane proteins specialized in facilitating water transport across membranes. Mechanical stress is one of the various stimuli that regulate AQPs. Briefly, there are several studies that report a decrease in permeability upon an increase in membrane tension. However, the molecular details of this mechanosensitive (MS) response are still a matter of debate. Our work attempts to close that gap in knowledge by providing evidence of a conformational change that occurs inside the pore of the strawberry aquaporin FaPIP2;1. Via osmotic shock experiments and molecular dynamics (MD) simulations, we found that a residue of loop B, I106, is key to the blocking of the permeation pathway and such a change is almost exclusively found under membrane tensile stress. In detail, osmotic shock experiments exhibited a nonlinear increment in water fluxes for increasing osmolarities, evidencing a decrease in the FaPIP2;1 permeability. MD simulations under membrane tension showed the same trend, with a significant increase in states with a low water permeability. The latter was correlated with a conformational change in I106 that generates a permeation barrier of around 18 kJ mol-1, effectively closing the pore. This work constitutes the first report of a PIP type aquaporin reacting to tensile stress in the membrane. Our findings could pave the way to test whether this conformational change is also responsible for mechanical gating in the other MS aquaporins, both those already reported and those still waiting to be found.

Permeation mechanisms of hydrogen peroxide and water through Plasma Membrane Intrinsic Protein aquaporins

Hydrogen peroxide (H2O2) transport by aquaporins (AQP) is a critical feature for cellular redox signaling. However, the H2O2 permeation mechanism through these channels remains poorly understood. Through functional assays, two Plasma membrane Intrinsic Protein (PIP) AQP from Medicago truncatula, MtPIP2;2 and MtPIP2;3 have been identified as pH-gated channels capable of facilitating the permeation of both water (H2O) and H2O2. Employing a combination of unbiased and enhanced sampling molecular dynamics simulations, we investigated the key barriers and translocation mechanisms governing H2O2 permeation through these AQP in both open and closed conformational states. Our findings reveal that both H2O and H2O2 encounter their primary permeation barrier within the selectivity filter (SF) region of MtPIP2;3. In addition to the SF barrier, a second energetic barrier at the NPA (asparagine-proline-alanine) region that is more restrictive for the passage of H2O2 than for H2O, was found. This behavior can be attributed to a dissimilar geometric arrangement and hydrogen bonding profile between both molecules in this area. Collectively, these findings suggest mechanistic heterogeneity in H2O and H2O2 permeation through PIPs.

Cooperation of an external carbonic anhydrase and HCO3- transporter supports underwater photosynthesis in submerged leaves of the amphibious plant Hygrophila difformis

BACKGROUND AND AIMS

HCO3- can be a major carbon resource for photosynthesis in underwater environments. Here we investigate the underlying mechanism of uptake and membrane transport of HCO3- in submerged leaves of Hygrophila difformis, a heterophyllous amphibious plant. To characterize these mechanisms, we evaluated the sensitivity of underwater photosynthesis to an external carbonic anhydrase (CA) inhibitor and an anion exchanger protein inhibitor, and we attempted to identify components of the mechanism of HCO3- utilization.

METHODS

We evaluated the effects of the external CA inhibitor and anion exchanger protein inhibitor on the NaHCO3 response of photosynthetic O2 evolution in submerged leaves of H. difformis. Furthermore, we performed a comparative transcriptomic analysis between terrestrial and submerged leaves.

KEY RESULTS

Photosynthesis in the submerged leaves was decreased by both the external CA inhibitor and anion exchanger protein inhibitor, but no additive effect was observed. Among upregulated genes in submerged leaves, two α-CAs, Hdα-CA1 and Hdα-CA2, and one β-carbonic anhydrase, Hdβ-CA1, were detected. Based on their putative amino acid sequences, the α-CAs are predicted to be localized in the apoplastic region. Recombinant Hdα-CA1 and Hdβ-CA1 showed dominant CO2 hydration activity over HCO3- dehydration activity.

CONCLUSIONS

We propose that the use of HCO3- for photosynthesis in submerged leaves of H. difformis is driven by the cooperation between an external CA, Hdα-CA1, and an unidentified HCO3- transporter.

Cooperativity in regulation of membrane protein function: phenomenological analysis of the effects of pH and phospholipids

Interaction between membrane proteins and ligands plays a key role in governing a wide spectrum of cellular processes. These interactions can provide a cooperative-type regulation of protein function. A wide variety of proteins, including enzymes, channels, transporters, and receptors, displays cooperative behavior in their interactions with ligands. Moreover, the ligands involved encompass a vast diversity and include specific molecules or ions that bind to specific binding sites. In this review, our particular focus is on the interaction between integral membrane proteins and ligands that can present multiple "binding sites", such as protons or membrane phospholipids. The study of the interaction that protons or lipids have with membrane proteins often presents challenges for classical mechanistic modeling approaches. In this regard, we show that, like Hill's pioneering work on hemoglobin regulation, phenomenological modeling constitutes a powerful tool for capturing essential features of these systems.

Ammonium Uptake, Mediated by Ammonium Transporters, Mitigates Manganese Toxicity in Duckweed, Spirodela polyrhiza

Nitrogen is an essential nutrient that affects all aspects of the growth, development and metabolic responses of plants. Here we investigated the influence of the two major sources of inorganic nitrogen, nitrate and ammonium, on the toxicity caused by excess of Mn in great duckweed, Spirodela polyrhiza. The revealed alleviating effect of ammonium on Mn-mediated toxicity, was complemented by detailed molecular, biochemical and evolutionary characterization of the species ammonium transporters (AMTs). Four genes encoding AMTs in S. polyrhiza, were classified as SpAMT1;1, SpAMT1;2, SpAMT1;3 and SpAMT2. Functional testing of the expressed proteins in yeast and Xenopus oocytes clearly demonstrated activity of SpAMT1;1 and SpAMT1;3 in transporting ammonium. Transcripts of all SpAMT genes were detected in duckweed fronds grown in cultivation medium, containing a physiological or 50-fold elevated concentration of Mn at the background of nitrogen or a mixture of nitrate and ammonium. Each gene demonstrated an individual expression pattern, revealed by RT-qPCR. Revealing the mitigating effect of ammonium uptake on manganese toxicity in aquatic duckweed S. polyrhiza, the study presents a comprehensive analysis of the transporters involved in the uptake of ammonium, shedding a new light on the interactions between the mechanisms of heavy metal toxicity and the regulation of the plant nitrogen metabolism.