Plants counteract fluctuations in water supply by regulating all aquaporins in the cell plasma membrane. Channel closure results either from the dephosphorylation of two conserved serine residues under conditions of drought stress, or from the protonation of a conserved histidine residue following a drop in cytoplasmic pH due to anoxia during flooding. Here we report the X-ray structure of the spinach plasma membrane aquaporin SoPIP2;1 in its closed conformation at 2.1 A resolution and in its open conformation at 3.9 A resolution, and molecular dynamics simulations of the initial events governing gating. In the closed conformation loop D caps the channel from the cytoplasm and thereby occludes the pore. In the open conformation loop D is displaced up to 16 A and this movement opens a hydrophobic gate blocking the channel entrance from the cytoplasm. These results reveal a molecular gating mechanism which appears conserved throughout all plant plasma membrane aquaporins.

Retinoids are micronutrients that are stored as retinyl esters in the retina and hepatic stellate cells (HSCs). HSCs are key players in fibrogenesis in chronic liver diseases. The enzyme responsible for hydrolysis and release of retinyl esters from HSCs is unknown and the relationship between retinoid metabolism and liver disease remains unclear. We hypothesize that the patatin-like phospholipase domain-containing 3 (PNPLA3) protein is involved in retinol metabolism in HSCs. We tested our hypothesis both in primary human HSCs and in a human cohort of subjects with non-alcoholic fatty liver disease (N = 146). Here we show that PNPLA3 is highly expressed in human HSCs. Its expression is regulated by retinol availability and insulin, and increased PNPLA3 expression results in reduced lipid droplet content. PNPLA3 promotes extracellular release of retinol from HSCs in response to insulin. We also show that purified wild-type PNPLA3 hydrolyzes retinyl palmitate into retinol and palmitic acid. Conversely, this enzymatic activity is markedly reduced with purified PNPLA3 148M, a common mutation robustly associated with liver fibrosis and hepatocellular carcinoma development. We also find the PNPLA3 I148M genotype to be an independent (P = 0.009 in a multivariate analysis) determinant of circulating retinol-binding protein 4, a reliable proxy for retinol levels in humans. This study identifies PNPLA3 as a lipase responsible for retinyl-palmitate hydrolysis in HSCs in humans. Importantly, this indicates a potential novel link between HSCs, retinoid metabolism and PNPLA3 in determining the susceptibility to chronic liver disease.

Recent observations that some membrane proteins act as gas channels seem surprising in view of the classical concept that membranes generally are highly permeable to gases. Here, we study the gas permeability of membranes for the case of CO(2), using a previously established mass spectrometric technique. We first show that biological membranes lacking protein gas channels but containing normal amounts of cholesterol (30-50 mol% of total lipid), e.g., MDCK and tsA201 cells, in fact possess an unexpectedly low CO(2) permeability (P(CO2)) of ∼0.01 cm/s, which is 2 orders of magnitude lower than the P(CO2) of pure planar phospholipid bilayers (∼1 cm/s). Phospholipid vesicles enriched with similar amounts of cholesterol also exhibit P(CO2) ≈ 0.01 cm/s, identifying cholesterol as the major determinant of membrane P(CO2). This is confirmed by the demonstration that MDCK cells depleted of or enriched with membrane cholesterol show dramatic increases or decreases in P(CO2), respectively. We demonstrate, furthermore, that reconstitution of human AQP-1 into cholesterol-containing vesicles, as well as expression of human AQP-1 in MDCK cells, leads to drastic increases in P(CO2), indicating that gas channels are of high functional significance for gas transfer across membranes of low intrinsic gas permeability.

Atomic-resolution X-ray crystallography, functional analyses, and molecular dynamics simulations suggest a novel mechanism for the regulation of water flux through the yeast Aqy1 water channel.

The patatin-like phospholipase domain containing 3 (PNPLA3, also called adiponutrin, ADPN) is a membrane-bound protein highly expressed in the liver. The genetic variant I148M (rs738409) was found to be associated with progression of chronic liver disease. We aimed to establish a protein purification protocol in a yeast system (Pichia pastoris) and to examine the human PNPLA3 enzymatic activity, substrate specificity and the I148M mutation effect. hPNPLA3 148I wild type and 148M mutant cDNA were cloned into P. pastoris expression vectors. Yeast cells were grown in 3L fermentors. PNPLA3 protein was purified from membrane fractions by Ni-affinity chromatography. Enzymatic activity was assessed using radiolabeled substrates. Both 148I wild type and 148M mutant proteins are localized to the membrane. The wild type protein shows a predominant lipase activity with mild lysophosphatidic acid acyl transferase activity (LPAAT) and the I148M mutation results in a loss of function of both these activities. Our data show that PNPLA3 has a predominant lipase activity and I148M mutation results in a loss of function.

The controlled export of solutes is crucial for cellular adaptation to hypotonic conditions. In the yeast Saccharomyces cerevisiae glycerol export is mediated by Fps1p, a member of the major intrinsic protein (MIP) family of channel proteins. Here we describe a short regulatory domain that restricts glycerol transport through Fps1p. This domain is required for retention of cellular glycerol under hypertonic stress and hence acquisition of osmotolerance. It is located in the N-terminal cytoplasmic extension close to the first transmembrane domain. Several residues within that domain and its precise position are critical for channel control while the proximal residues 13-215 of the N-terminal extension are not required. The sequence of the regulatory domain and its position are perfectly conserved in orthologs from other yeast species. The regulatory domain has an amphiphilic character, and structural predictions indicate that it could fold back into the membrane bilayer. Remarkably, this domain has structural similarity to the channel forming loops B and E of Fps1p and other glycerol facilitators. Intragenic second-site suppressor mutations of the sensitivity to high osmolarity conferred by truncation of the regulatory domain caused diminished glycerol transport, confirming that elevated channel activity is the cause of the osmosensitive phenotype.

a b s t r a c tAquaporin-mediated water transport across cellular membranes is an ancient, ubiquitous mechanism within cell biology. This family of integral membrane proteins includes both water selective pores (aquaporins) and transport facilitators of other small molecules such as glycerol and urea (aquaglyceroporins). Eukaryotic aquaporins are frequently regulated post-translationally by gating, whereby the rate of flux through the channel is controlled, or by trafficking, whereby aquaporins are shuttled from intracellular storage sites to the plasma membrane. A number of high-resolution X-ray structures of eukaryotic aquaporins have recently been reported and the new structural insights into gating and trafficking that emerged from these studies are described. Basic structural themes reoccur, illustrating how the problem of regulation in diverse biological contexts builds upon a limited set of possible solutions.

Human aquaporin 2 (AQP2) is a water channel found in the kidney collecting duct, where it plays a key role in concentrating urine. Water reabsorption is regulated by AQP2 trafficking between intracellular storage vesicles and the apical membrane. This process is tightly controlled by the pituitary hormone arginine vasopressin and defective trafficking results in nephrogenic diabetes insipidus (NDI). Here we present the X-ray structure of human AQP2 at 2.75 Å resolution. The C terminus of AQP2 displays multiple conformations with the C-terminal α-helix of one protomer interacting with the cytoplasmic surface of a symmetry-related AQP2 molecule, suggesting potential protein-protein interactions involved in cellular sorting of AQP2. Two Cd 2+ -ion binding sites are observed within the AQP2 tetramer, inducing a rearrangement of loop D, which facilitates this interaction. The locations of several NDI-causing mutations can be observed in the AQP2 structure, primarily situated within transmembrane domains and the majority of which cause misfolding and ER retention. These observations provide a framework for understanding why mutations in AQP2 cause NDI as well as structural insights into AQP2 interactions that may govern its trafficking.membrane protein | X-ray crystallography | water channel protein W ater is the major ingredient of the human body, constituting 55-65% of our total body weight (1). Water homeostasis is maintained by the kidneys, which filter ∼180 L of primary urine every day. Although most water is constitutively reabsorbed in the proximal tubules and descending limbs of Henle (2), the body's water balance is fine-tuned by regulated water reabsorption, which takes place in the kidney collecting duct. Water reabsorption is mediated by aquaporins, membranebound water channels, of which seven of the 13 human isoforms have been located in the human kidney (3). Of these, human aquaporin 2 (AQP2) is present in the principal cells of the collecting duct and is responsible for regulated water reabsorption.AQP2 is stored in intracellular vesicles under water-saturating conditions. When the levels of the pituitary antidiuretic hormone arginine vasopressin (AVP) are elevated in response to dehydration or hypernatremia, AVP binding to the vasopressin 2 receptor (V2R) in the basolateral membrane stimulates an increase in intracellular cAMP. This triggers the phosphorylation of Ser256 in the AQP2 C terminus by protein kinase A (PKA) and flags the protein for trafficking from storage vesicles to the apical membrane (4-6). AVP also triggers additional phosphorylation at Ser264 and Ser269 (7,8), with all three sites being phosphorylated in AQP2s targeted to the plasma membrane (9). The resulting redistribution of AQP2 increases transcellular water permeability and concentrates urine (Fig. S1). Once correct water balance is restored, AQP2 is internalized through ubiquitinmediated endocytosis and redirected to storage vesicles or targeted for degradation (10-12).Because of its central role in water homeostasis, dysregula...