Water-specific aquaporins (AQP), such as the prototypical mammalian AQP1, stringently exclude the passage of solutes, ions, and even protons. Supposedly, this is accomplished by two conserved regions within the pore, a pair of canonical asparagine-prolinealanine (NPA) motifs, the central constriction, and an aromatic͞ arginine (ar͞R) constriction, the outer constriction. Here, we analyzed the function of three residues in the ar͞R constriction (Phe-56, His-180, and Arg-195) in rat AQP1. Individual or joint replacement of His-180 and Arg-195 by alanine and valine residues, respectively (AQP1-H180A, AQP1-R195V, and AQP1-H180A͞ R195V), did not affect water permeability. The double mutant AQP1-H180A͞R195V allowed urea to pass. In line with the predicted solute discrimination by size, replacement of both Phe-56 and His-180 (AQP1-F56A͞H180A) enlarged the maximal diameter of the ar͞R constriction 3-fold and enabled glycerol and urea to pass. We further show that ammonia passes through all four AQP1 mutants, as determined (i) by growth complementation of yeast deletion strains with ammonia, (ii) by ammonia uptake from the external solution into oocytes, and (iii) by direct recordings of ammonia induced proton currents in oocytes. Unexpectedly, removal of the positive charge in the ar͞R constriction in AQP1-R195V and AQP1-H180A͞R195V appeared to allow the passage of protons through AQP1. The data indicate that the ar͞R constriction is a major checkpoint for solute permeability, and that the exquisite electrostatic proton barrier in AQPs comprises both the NPA constriction as well as the ar͞R constriction. mutational analysis ͉ proton filter ͉ solute selectivity O rthodox aquaporins (AQPs) constitute one branch of water-conducting channels within the superfamily of major intrinsic proteins (1). In recent years, the protein structure of the prototypical mammalian AQP1 has been refined to 2.2-Å resolution (2-5). Two highly conserved structural features within the channel were proposed as filters that exclude the passage of solutes larger than water and of charged molecules, including protons. A central constriction is formed by the capping amino acids Asn-Pro-Ala (NPA constriction) at each positive end of two short ␣-helices, such that the two NPA motifs are pinched in the center of the pore. Proline and alanine are exchangeable to some extent, whereas asparagine is invariable (1). Extensive molecular dynamics͞quantum mechanical simulations suggest that the free energy barrier located at the NPA constriction predominates in the exclusion of protons (reviewed in ref. 6). Depending on the computational approach, a second significant energy barrier, termed aromatic͞arginine (ar͞R) constriction, exists. It is located below the channel mouth and is even narrower than the central NPA constriction (6, 7). It is formed by four amino acids (Phe-56, His-180, Cys-189, and Arg-195 in rat AQP1). The ensemble of Phe, His, and Arg is highly conserved in orthodox AQPs. Their side chains directly flank the pore, whereas the less-conserved C...

In this study the plane elasticity problem for a nonhomogeneous layer containing a crack perpendicular to the boundaries is considered. It is assumed that the Young’s modulus of the medium varies continuously in the thickness direction. The problem is solved under three different loading conditions, namely fixed grip, membrane loading, and bending applied to the layer away from the crack region. Mode I stress intensity factors are presented for embedded as well as edge cracks for various values of dimensionless parameters representing the size and the location of the crack and the material nonhomogeneity. Some sample results are also given for the crack-opening displacement and the stress distribution.

The aquaporin protein family generally seems to be designed for the selective passage of water or glycerol. Charged molecules, metal ions and even protons are strictly excluded. Recently, particular aquaporin isoforms were reported to conduct unconventional permeants, i.e., the unpolar gases carbon dioxide and nitric oxide, the polar gas ammonia, the oxidative oxygen species hydrogen peroxide, and the metalloids antimonite, arsenite and silicic acid. Here, we summarize the available data on permeability properties and physiological settings of these aquaporins and we analyze which structural features might be connected to permeability for non-water, non-glycerol solutes.

In most organisms, high affinity ammonium uptake is catalyzed by members of the ammonium transporter family (AMT/MEP/Rh). A single point mutation (G458D) in the cytosolic C terminus of the plasma membrane transporter LeAMT1;1 from tomato leads to loss of function, although mutant and wild type proteins show similar localization when expressed in yeast or plant protoplasts. Co-expression of LeAMT1;1 and mutant in Xenopus oocytes inhibited ammonium transport in a dominant negative manner, suggesting homo-oligomerization. In vivo interaction between LeAMT1;1 proteins was confirmed by the split ubiquitin yeast two-hybrid system. LeAMT1;1 is isolated from root membranes as a high molecular mass oligomer, converted to a ϳ35-kDa polypeptide by denaturation. To investigate interactions with the LeAMT1;2 paralog, co-localizing with LeAMT1;1 in root hairs, LeAMT1;2 was characterized as a lower affinity NH 4 ؉ uniporter. Co-expression of wild types with the respective G458D/G465D mutants inhibited ammonium transport in a dominant negative manner, supporting the formation of heteromeric complexes in oocytes. Thus, in yeast, oocytes, and plants, ammonium transporters are able to oligomerize, which may be relevant for regulation of ammonium uptake. Ammonium transporters (AMTs)1 of the AMT/MEP/Rh protein family have been identified in all domains of life, including plants, bacteria, archea, yeast, and animals (1, 2). AMT/ MEP/Rh proteins are highly hydrophobic membrane proteins with a predicted molecular mass of ϳ45-55 kDa and 11 or 12 putative transmembrane spans. Initially AMT/MEP/Rh ammonium transporters from yeast and plants were identified molecularly by functional complementation of a yeast mutant defective in ammonium uptake (3-5). Later, homologs were isolated from bacteria (6) and animals (Caenorhabditis elegans), and phylogenetic analysis showed that mammalian Rh (rhesus) blood group polypeptides belong to the same superfamily (7). Heterologously expressed RhAG and a homolog from kidney (RhGK ϭ RhCG) were also shown to function as ammonium transporters (8, 9).Plants require transporters for NH 4 ϩ acquisition from a wide range of external concentrations and are able to concentrate and transiently accumulate NH 4 ϩ in the cytosol before being metabolized or further compartmentalized (10). Ammonium transport across root plasma membranes is biphasic, consisting of a high-affinity and a low-affinity nonsaturating component (11,12). The high-affinity transport system, which operates predominantly at low external ammonium concentrations, is energized by the membrane potential. In tomato, the NH 4 ϩ -uniporter LeAMT1;1 encodes a component of the high-affinity transport system that depends on the membrane potential (13). The molecular identity of the low-affinity transport system, however, is less clear. It contributes significantly to overall NH 4 ϩ uptake at higher external ammonium concentrations (Ͼ1 mM) and may have a distinct transport mechanism, because uncharged NH 3 or charged NH 4 ϩ may be the substrate (11, 12, 14,...

Maintenance of a high glycolytic flow rate is critical for the rapid growth and virulence of malarial parasites. The parasites release two moles of lactic acid per mole of glucose as the anaerobic end product. However, the molecular identity of the Plasmodium lactate transporter is unknown. Here we show that a member of the microbial formate-nitrite transporter family, PfFNT, acts as a lactate/proton symporter in Plasmodium falciparum. Besides L-lactate, PfFNT transports physiologically relevant D-lactate, as well as pyruvate, acetate and formate, and is inhibited by the antiplasmodial compounds phloretin, furosemide and cinnamate derivatives, but not by p-chloromercuribenzene sulfonate (pCMBS). Our data on PfFNT monocarboxylate transport are consistent with those obtained with living parasites. Moreover, PfFNT is the only transporter of the plasmodial glycolytic pathway for which structure information is available from crystals of homologous proteins, rendering it amenable to further evaluation as a novel antimalarial drug target.

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Aquaporins (AQP) conduct small, uncharged molecules, such as water (orthodox AQPs), ammonia (aquaammoniaporins) or glycerol (aquaglyceroporins). The physiological functions of AQPs are involved in osmotic volume regulation or the transport of biochemical precursors and metabolic waste products. The recent identification of hydrogen peroxide (H 2 O 2 ) as a permeant of certain AQPs suggests additional roles in mitigating oxidative stress or enabling paracrine H 2 O 2 signalling. Yet, an analysis of the structural requirements of the H 2 O 2 permeability of AQPs is missing. We subjected a representative set of wild-type and mutant AQPs to a newly established quantitative phenotypic assay. We confirmed high H 2 O 2 permeability of the human aquaammoniaporin AQP8 and found intermediate H 2 O 2 permeability of the prototypical orthodox water channel AQP1 from the rat. Differences from an earlier report showing an absence of H 2 O 2 permeability of human AQP1 can be explained by expression levels. By generating point mutations in the selectivity filter of rat orthodox aquaporin AQP1, we established a correlation of H 2 O 2 permeability primarily with water permeability and secondarily with the pore diameter. Even the narrowest pore of the test set (i.e. rat orthodox aquaporin AQP1 H180F with a pore diameter smaller than that of natural orthodox AQPs) conducted water and H 2 O 2 . We further found that H 2 O 2 permeability of the aquaglyceroporin from the malaria parasite Plasmodium falciparum was lower despite its wider pore diameter. The data suggest that all water-permeable AQPs are H 2 O 2 channels, yet H 2 O 2 permeability varies with the isoform. Thus, generally, AQPs must be considered as putative players in situations of oxidative stress (e.g. in Plasmodium-infected red blood cells, immune cells, the cardiovascular system or cells expressing AQP8 in their mitochondria).

Plastid-to-nucleus retrograde signaling is critical for normal growth and development in plants. The dual-function and dual-located ssDNA binding protein WHIRLY1 (WHY1) has been proposed to coordinate the retrograde signaling from plastids to the nucleus. However, the regulatory mechanism governing the functional switch of WHY1 for mediating plastid-to-nucleus retrograde signaling remains unknown. Here, we report that the Calcineurin B-Like-Interacting Protein Kinase14 (CIPK14) interacts with and phosphorylates WHY1 in Arabidopsis. Phosphorylation of WHY1 results in increased accumulation in the nucleus and enhanced binding with the promoter of WRKY53, which encodes a key transcription factor regulating leaf senescence in Arabidopsis. Transgenic plants overexpressing CIPK14 showed an increased nuclear isoform but decreased plastid isoform of WHY1, among which 95% of transgenic lines showed the stay-green phenotype and 5% of lines showed the variegated pale-green phenotype. Interestingly, the phenotypes of both types of transgenic plants could be recovered by overexpression of plastid-form WHY1. In contrast, knockdown of CIPK14 caused early senescence and even seedling-lethal phenotypes along with elevated expression of senescence-related genes such as WRKY53, SAG12, and NDHF but decreased expression of MER11, RAD50, and POR genes, which could be rescued by overexpression of CIPK14 but not by overexpressing plastid-form or nuclear-form WHY1; the stay-green plants overexpressing CIPK14 showed reduced expression of WRKY53, SAG12, NDHF, and large plastid rRNA. Consistently, the accumulation of nuclear-form WHY1 was significantly reduced in the CIPK14 knockdown lines, resulting in a low ratio of nuclear-/plastid-form WHY1. Taken together, our results demonstrate that CIPK14 regulates the phosphorylation and organellar distributions of WHY1 and pinpoint that CIPK14 may function as a cellular switch between leaf senescence and plastid development for coordinating the intercellular signaling in Arabidopsis.