Erythroid expression and oligomeric state of the AQP3 protein

Anomalous Oligomerization Behavior of E. coli Aquaporin Z in Detergent and in Nanodiscs

Aquaporins are tetrameric integral membrane proteins that act as water channels, and can also permeabilize membranes to other solutes. The monomer appears to be the functional form despite all aquaporins being organized as tetramers, which therefore must provide a clear functional advantage. In addition to this quaternary organization, some aquaporins can act as adhesion molecules in membrane junctions, when tetramers located in opposing membranes interact via their extracellular domains. These stacked forms have been observed in a range of aquaporins, whether using lipidic membrane environments, in electron crystallography, or using detergent micelles, in single-particle cryo-electron microscopy (cryo-EM). In the latter technique, structural studies can be performed when the aquaporin is reconstituted into nanodiscs of lipids that are surrounded by a protein scaffold. During attempts to study E. coli Aquaporin Z (AqpZ), we have found that in some conditions these nanodiscs tend to form filaments that appear to be either thicker head-to-tail or thinner side-to-side stacks of nanodiscs. Nanodisc oligomerization was observed using orthogonal analytical techniques analytical ultra-centrifugation and mass photometry, although the nature of the oligomers (head-to-tail or side-to-side) could not be determined. Using the latter technique, the AqpZ tetramer itself formed oligomers of increasing size when solubilized only in detergent, which is consistent with multiple stacking of AqpZ tetramers. We observed images consistent with both of these filaments in negative staining EM conditions, but only thicker filaments in cryo-EM conditions. We hypothesize that the apparent nanodisc side-to-side arrangement that can only be visualized in negative staining conditions is related to artifacts due to the sample preparation. Filaments of any kind were not observed in EM when nanodiscs did not contain AqpZ, or after addition of detergent into the nanodisc cryo-EM preparation, at concentrations that did not disrupt nanodisc formation. To our knowledge, these filaments have not been observed in nanodiscs preparations of other membrane proteins. AqpZ, like other aquaporins has a charge asymmetry between the cytoplasmic (more positive) and the extracellular sides, which may explain the likely head-to-tail stacking observed, both in nanodisc preparations and also in detergent micelles.

Localization of aquaglyceroporins in human and murine white adipose tissue

The glycerol channel AQP7 facilitates glycerol efflux from adipose tissue (AT), and AQP7 deficiency has been suggested to promote obesity. However, the release of glycerol from AT is not fully blocked in AQP7-deficient mice, which suggests that either alternative glycerol channels are present in AT or significant simple diffusion of glycerol occurs. Previous investigations of the expression of other aquaglyceroporins (AQP3, AQP9, AQP10) than AQP7 in AT are contradictory. Therefore, we here aim at determining the cellular localization of AQP3 and AQP9 in addition to AQP7 in human and mouse AT using well-characterized antibodies for immunohistochemistry (IHC) and immunoblotting as well as available single-cell transcriptomic data from human and mouse AT. We confirm that AQP7 is expressed in endothelial cells and adipocytes in human AT and find ex vivo evidence for interaction between AQP7 and perilipin-1 in adipocytes. In addition, labeling for AQP7 in human AT also includes CD68-positive cells. No labeling for AQP3 or AQP9 was identified in endothelial cells or adipocytes in human or mouse AT using IHC. Instead, in human AT, AQP3 was predominantly found in erythrocytes, whereas AQP9 expression was observed in a small number of CD15-positive cells. The transcriptomic data revealed that AQP3 mRNA was found in a low number of cells in most of the identified cell clusters, whereas AQP9 mRNA was found in myeloid cell clusters as well as in clusters likely representing mesothelial progenitor cells. No AQP10 mRNA was identified in human AT. In conclusion, the presented results do not suggest a functional overlap between AQP3/AQP9/AQP10 and AQP7 in human or mouse white AT.

Anti-Inflammatory and Anti-Allergic Effects of Saponarin and Its Impact on Signaling Pathways of RAW 264.7, RBL-2H3, and HaCaT Cells

Saponarin{5-hydroxy-2-(4-hydroxyphenyl)-6-[3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]-7-[3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]oxychromen-4-one}, a flavone found in young green barley leaves, is known to possess antioxidant, antidiabetic, and hepatoprotective effects. In the present study, the anti-inflammatory, anti-allergic, and skin-protective effects of saponarin were investigated to evaluate its usefulness as a functional ingredient in cosmetics. In lipopolysaccharide-induced RAW264.7 (murine macrophage) cells, saponarin (80 μM) significantly inhibited cytokine expression, including tumor necrosis factor (TNF)-α, interleukin (IL)-1β, inducible nitric oxide synthase, and cyclooxygenase (COX)-2. Saponarin (80 μM) also inhibited the phosphorylation of extracellular signal-regulated kinase (ERK) and p38 involved in the mitogen-activated protein kinase signaling pathway in RAW264.7 cells. Saponarin (40 μM) significantly inhibited β-hexosaminidase degranulation as well as the phosphorylation of signaling effectors (Syk, phospholipase Cγ1, ERK, JNK, and p38) and the expression of inflammatory mediators (tumor necrosis factor [TNF]-α, IL-4, IL-5, IL-6, IL-13, COX-2, and FcεRIα/γ) in DNP-IgE- and DNP-BSA-stimulated RBL-2H3 (rat basophilic leukemia) cells. In addition, saponarin (100 μM) significantly inhibited the expression of macrophage-derived chemokine, thymus and activation-regulated chemokine, IL-33, thymic stromal lymphopoietin, and the phosphorylation of signaling molecules (ERK, p38 and signal transducer and activator of transcription 1 [STAT1]) in TNF-α- and interferon (IFN)-γ-stimulated HaCaT (human immortalized keratinocyte) cells. Saponarin (100 μM) also significantly induced the expression of hyaluronan synthase-3, aquaporin 3, and cathelicidin antimicrobial peptide (LL-37) in HaCaT cells, which play an important role as skin barriers. Saponarin remarkably inhibited the essential factors involved in the inflammatory and allergic responses of RAW264.7, RBL-2H3, and HaCaT cells, and induced the expression of factors that function as physical and chemical skin barriers in HaCaT cells. Therefore, saponarin could potentially be used to prevent and relieve immune-related skin diseases, including atopic dermatitis.

Dioleoylphosphatidylglycerol Accelerates Corneal Epithelial Wound Healing

Purpose

In contact with the external environment, the cornea can easily be injured. Although corneal wounds generally heal rapidly, the pain and increased risk of infection associated with a damaged cornea, as well as the impaired healing observed in some individuals, emphasize the need for novel treatments to accelerate corneal healing. We previously demonstrated in epidermal keratinocytes that the glycerol channel aquaporin-3 (AQP3) interacts with phospholipase D2 (PLD2) to produce the signaling phospholipid phosphatidylglycerol (PG), which has been shown to accelerate skin wound healing in vivo. We hypothesized that the same signaling pathway might be operational in corneal epithelial cells.

Methods

We used co-immunoprecipitation, immunohistochemistry, scratch wound healing assays in vitro, and corneal epithelial wound healing assays in vivo to determine the role of the AQP3/PLD2/PG signaling pathway in corneal epithelium.

Results

AQP3 was present in human corneas in situ, and AQP3 and PLD2 were co-immunoprecipitated from corneal epithelial cell lysates. The two proteins could also be co-immunoprecipitated from insect cells simultaneously infected with AQP3- and PLD2-expressing baculoviruses, suggesting a likely direct interaction. A particular PG, dioleoylphosphatidylglycerol (DOPG), enhanced scratch wound healing of a corneal epithelial monolayer in vitro. DOPG also accelerated corneal epithelial wound healing in vivo, both in wild-type mice and in a mouse model exhibiting impaired corneal wound healing (AQP3 knockout mice).

Conclusions

These results indicate the importance of the AQP3/PLD2/PG signaling pathway in corneal epithelial cells and suggest the possibility of developing DOPG as a pharmacologic therapy to enhance corneal wound healing in patients.

Effects of Apigenin on RBL-2H3, RAW264.7, and HaCaT Cells: Anti-Allergic, Anti-Inflammatory, and Skin-Protective Activities

Apigenin (4',5,7-trihydroxyflavone, flavonoid) is a phenolic compound that is known to reduce the risk of chronic disease owing to its low toxicity. The first study on apigenin analyzed its effect on histamine release in the 1950s. Since then, anti-mutation and antitumor properties of apigenin have been widely reported. In the present study, we evaluated the apigenin-mediated amelioration of skin disease and investigated its applicability as a functional ingredient, especially in cosmetics. The effect of apigenin on RAW264.7 (murine macrophage), RBL-2H3 (rat basophilic leukemia), and HaCaT (human immortalized keratinocyte) cells were analyzed. Apigenin (100 μM) significantly inhibited nitric oxide (NO) production, cytokine expression (interleukin (IL)-1β, IL6, cyclooxygenase (COX)-2, and inducible nitric oxide synthase [iNOS]), and phosphorylation of mitogen-activated protein kinase (MAPK) signal molecules, including extracellular signal-regulated kinase (ERK) and c-Jun N-terminal protein kinase (JNK) in RAW264.7 cells. Apigenin (30 M) also inhibited the phosphorylation of signaling molecules (Lyn, Syk, phospholipase Cγ1, ERK, and JNK) and the expression of high-affinity IgE receptor FcεRIα and cytokines (tumor necrosis factor (TNF)-α, IL-4, IL-5, IL-6, IL-13, and COX-2) that are known to induce inflammation and allergic responses in RBL-2H3 cells. Further, apigenin (20 μM) significantly induced the expression of filaggrin, loricrin, aquaporin-3, hyaluronic acid, hyaluronic acid synthase (HAS)-1, HAS-2, and HAS-3 in HaCaT cells that are the main components of the physical barrier of the skin. Moreover, it promoted the expression of human β-defensin (HBD)-1, HBD-2, HBD-3, and cathelicidin (LL-37) in HaCaT cells. These antimicrobial peptides are known to play an important role in the skin as chemical barriers. Apigenin significantly suppressed the inflammatory and allergic responses of RAW264.7 and RBL cells, respectively, and would, therefore, serve as a potential prophylactic and therapeutic agent for immune-related diseases. Apigenin could also be used to improve the functions of the physical and chemical skin barriers and to alleviate psoriasis, acne, and atopic dermatitis.

Structural Determinants of Oligomerization of the Aquaporin-4 Channel

The aquaporin (AQP) family of integral membrane protein channels mediate cellular water and solute flow. Although qualitative and quantitative differences in channel permeability, selectivity, subcellular localization, and trafficking responses have been observed for different members of the AQP family, the signature homotetrameric quaternary structure is conserved. Using a variety of biophysical techniques, we show that mutations to an intracellular loop (loop D) of human AQP4 reduce oligomerization. Non-tetrameric AQP4 mutants are unable to relocalize to the plasma membrane in response to changes in extracellular tonicity, despite equivalent constitutive surface expression levels and water permeability to wild-type AQP4. A network of AQP4 loop D hydrogen bonding interactions, identified using molecular dynamics simulations and based on a comparative mutagenic analysis of AQPs 1, 3, and 4, suggest that loop D interactions may provide a general structural framework for tetrameric assembly within the AQP family.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Folding and stability of the aquaglyceroporin GlpF: Implications for human aqua(glycero)porin diseases

Aquaporins are highly selective polytopic transmembrane channel proteins that facilitate the permeation of water across cellular membranes in a large diversity of organisms. Defects in aquaporin function are associated with common diseases, such as nephrogenic diabetes insipidus, congenital cataract and certain types of cancer. In general, aquaporins have a highly conserved structure; from prokaryotes to humans. The conserved structure, together with structural dynamics and the structural framework for substrate selectivity is discussed. The folding pathway of aquaporins has been a topic of several studies in recent years. These studies revealed that a conserved protein structure can be reached by following different folding pathways. Based on the available data, we suggest a complex folding pathway for aquaporins, starting from the insertion of individual helices up to the formation of the tetrameric aquaporin structure. The consequences of some known mutations in human aquaporin-encoding genes, which most likely affect the folding and stability of human aquaporins, are discussed.

Dynamic regulation of aquaglyceroporin expression in erythrocyte cultures from cold- and warm-acclimated Cope's gray treefrog, Hyla chrysoscelis

Cope's gray treefrog, Hyla chrysoscelis,is a freeze-tolerant anuran which accumulates and distributes glycerol as a cryoprotectant before freezing. We hypothesize that HC-3, an aquaglyceroporin member of the MIP family of water pores, may play an important role in the process of freeze tolerance by mediating transmembrane passage of glycerol and water during cold-acclimation. The objectives of this study were two-fold: to examine HC-3 protein abundance and cellular localization in erythrocytes from cold- and warm-acclimated frogs and to develop and characterize an erythrocyte cell culture system for examining HC-3 gene regulation. Compared with warm-acclimated frogs, erythrocytes from cold-acclimated frogs had higher HC-3 protein expression and enhanced plasma membrane localization. Furthermore, erythrocytes from cold- and warm-acclimated frogs maintained in culture at 4 and 20°C exhibited time- and temperature-dependent regulation of HC-3 expression and an increase in the abundance of high molecular weight immunoreactive species within 24 hr of culture at 20°C. Deglycosylation of erythrocyte proteins resulted in the disappearance of the high molecular weight species, indicating that HC-3 is post-translationally modified by N-linked glycosylation. Erythrocytes cultured in media containing glycerol also showed an increased abundance of the high molecular weight bands and enhanced plasma membrane localization of HC-3, suggesting a role for glycerol in regulating HC-3 subcellular trafficking. Thus, the development of this erythrocyte cell culture system from H. chrysoscelis opened an opportunity to study the properties of cells with changing expression of an aquaglyceroporin, HC-3, and to explore the factors regulating that expression.

Aquaporins in sperm osmoadaptation: an emerging role for volume regulation

Upon ejaculation, mammalian sperm experience a natural osmotic decrease during male to female reproductive tract transition. This hypo-osmotic exposure not only activates sperm motility, but also poses potential harm to sperm structure and function by inducing unwanted cell swelling. In this physiological context, regulatory volume decrease (RVD) is the major mechanism that protects cells from detrimental swelling, and is essential to sperm survival and normal function. Aquaporins are selective water channels that enable rapid water transport across cell membranes. Aquaporins have been implicated in sperm osmoregulation. Recent discoveries show that Aquaporin-3 (AQP3), a water channel protein, is localized in sperm tail membranes and that AQP3 mutant sperm show defects in volume regulation and excessive cell swelling upon physiological hypotonic stress in the female reproductive tract, thereby highlighting the importance of AQP3 in the postcopulatory sperm RVD process. In this paper, we discuss current knowledge, remaining questions and hypotheses about the function and mechanismic basis of aquaporins for volume regulation in sperm and other cell types.

The evolutionary aspects of aquaporin family

Aquaporins (AQPs) were originally identified as channels facilitating water transport across the plasma membrane. They have a pair of highly conserved signature sequences, asparagine-proline-alanine (NPA) boxes, to form a pore. However, some have little conserved amino acid sequences around the NPA boxes unclassifiable to two previous AQP subfamilies, classical AQPs and aquaglyceroporins. These will be called unorthodox AQPs in this review. Interestingly, these unorthodox AQPs have a highly conserved cysteine residue downstream of the second NPA box. AQPs also have a diversity of functions: some related to water transport such as fluid secretion, fluid absorption, and cell volume regulation, and the others not directly related to water transport such as cell adhesion, cell migration, cell proliferation, and cell differentiation. Some AQPs even permeate nonionic small molecules, ions, metals, and possibly gasses. AQP gene disruption studies have revealed their physiological roles: water transport in the kidney and exocrine glands, glycerol transport in fat metabolism and in skin moisture, and nutrient uptakes in plants. Furthermore, AQPs are also present at intracellular organelles, including tonoplasts, mitochondria, and the endoplasmic reticulum. This review focuses on the evolutionary aspects of AQPs from bacteria to humans in view of the structural and functional diversities of AQPs.

Osmosensitivity of prolactin cells is enhanced by the water channel aquaporin-3 in a euryhaline Mozambique tilapia (Oreochromis mossambicus)

In teleost fish, prolactin (PRL) has important actions in the regulation of salt and water balances in freshwater (FW) fish. Consistent with this role, the release of PRL from the pituitary of the Mozambique tilapia is stimulated as extracellular osmolality is reduced. Stretch-activated calcium-permeant ion channels appear to be responsible for the initiation of the signal transduction that leads to increased PRL release when PRL cells are exposed to reductions in extracellular osmolality. In this study, we examined a possible involvement of the aquaporin-3 (AQP3) water channel in this osmoreceptive mechanism in PRL cells of the tilapia. AQP3 expression levels in the rostral pars distalis of the pituitary, consisting predominantly of PRL cells, were higher in fish adapted to FW than in seawater (SW)-adapted fish. Immunohistochemical studies revealed that AQP3 is located in the cell membrane and perinuclear region of PRL cells, with more intense immunosignals in PRL cells of FW-adapted fish than in those of SW fish. In FW PRL cells, the magnitude of hyposmoticity-induced cell volume increase was greater than that seen in SW PRL cells. Mercury, a potent inhibitor of AQP3, inhibited hyposmoticity-induced cell volume increase and PRL release from FW PRL cells. The inhibitory effect of mercury was partially restored by β-mercaptoethanol, whereas no effect of mercury was observed on PRL release stimulated by a depolarizing concentration of KCl, which induces Ca2+influx and stimulates the subsequent Ca2+-signaling pathway. These results indicate significant contribution of AQP3 to osmoreception in PRL cells in FW-adapted tilapia.

Widespread balancing selection and pathogen-driven selection at blood group antigen genes

Historically, allelic variations in blood group antigen (BGA) genes have been regarded as possible susceptibility factors for infectious diseases. Since host-pathogen interactions are major determinants in evolution, BGAs can be thought of as selection targets. In order to verify this hypothesis, we obtained an estimate of pathogen richness for geographic locations corresponding to 52 populations distributed worldwide; after correction for multiple tests and for variables different from selective forces, significant correlations with pathogen richness were obtained for multiple variants at 11 BGA loci out of 26. In line with this finding, we demonstrate that three BGA genes, namely CD55, CD151, and SLC14A1, have been subjected to balancing selection, a process, rare outside MHC genes, which maintains variability at a locus. Moreover, we identified a gene region immediately upstream the transcription start site of FUT2 which has undergone non-neutral evolution independently from the coding region. Finally, in the case of BSG, we describe the presence of a highly divergent haplotype clade and the possible reasons for its maintenance, including frequency-dependent balancing selection, are discussed. These data indicate that BGAs have been playing a central role in the host-pathogen arms race during human evolutionary history and no other gene category shows similar levels of widespread selection, with the only exception of loci involved in antigen recognition.

Functional characterization of a human aquaporin 0 mutation that leads to a congenital dominant lens cataract

The aquaporin (AQP) transmembrane proteins facilitate the movement of water across the plasma membrane. In the lens, AQP0 is expressed in fiber cells and AQP1 in the epithelium. Recently, two individuals were identified with congenital polymorphic autosomal dominant cataract, due to a single nucleotide base deletion mutation in the lens AQP0. The deletion modified the reading frame resulting in the addition of a premature stop codon. In the present study, we examined the water permeability properties, trafficking and dominant negative effects as well as cytotoxicity due to the mutant AQP0 (Delta213-AQP0) protein. The membrane water permeability (P(w)) of Delta213-AQP0 expressing oocytes (14+/-1 microm/s) was significantly lower than those expressing WT-AQP0 (25+/-3 microm/s). P(w) of water injected control oocytes was 13+/-2 microm/s. Co-expression of WT-AQP0 with Delta213-AQP0 significantly lowered the P(w) (18+/-3 microm/s) compared to WT-AQP0. With or without the EGFP tag, WT-AQP0 protein localized in the plasma membranes of oocytes and cultured cells whereas Delta213-AQP0 was retained in the ER. Forster Resonance Energy Transfer (FRET) showed that WT-AQP0 partly localized with the co-expressed Delta213-AQP0. Co-localization studies suggest that the mutant AQP0 gained its dominant function by trapping the WT-AQP0 in the ER through hetero-oligomerization. Incubating the cells with chemical chaperones, namely, TMAO and DMSO, did not correct the folding/trafficking defects. Cell death in the Delta213-AQP0 expressing cells was due to necrosis caused by the accumulation of Delta213-AQP0 protein in the ER in cytotoxic proportions. The data show that replacement of the distal end of the 6th TM domain and the C-terminal domain of AQP0 due to the deletion mutation resulted in the impairment of cell membrane P(w), localization of the mutant protein in the ER without trafficking to the plasma membrane, and cytotoxicity due to the accumulation of the mutant protein. Cataracts in patients with this mutation might have resulted from the above mentioned consequences.

Expression and localization of aquaporins in the kidney of the musk shrew (Suncus murinus)

Expression and localization of members of the aquaporin (AQP) family (AQP1, 2, 3, 4, and 5) in the kidney of the musk shrew (Suncus murinus) was examined by immunohistochemistry. AQP1 was expressed in the proximal tubules and in the thin limb of the loops of Henle. AQP1 was the only water channel expressed in the proximal nephron examined, indicating that AQP1 may be an independent water transporter in the proximal nephron. AQP2 and AQP5 were localized to the apical cytoplasm of the cortical to medullary collecting duct (CD) cells and AQP3 and AQP4 were localized to the basal aspect of the cortical to medullary CD cells. AQP3 expression was weaker in the cortical cells compared with the medullary cells, whereas AQP4 was strongly positive throughout the CD. These indicate that the CD is the main water reabsorption segment of the nephron and is regulated by AQPs. Indeed, apical water transport of CD cells of the musk shrew may be controlled by both AQP2 and AQP5. The characteristic expression pattern of the AQPs in this animal provides a novel animal model for elucidating the regulation of water reabsorption by AQPs in the mammalian kidney.

Aquaporin 9 is the major pathway for glycerol uptake by mouse erythrocytes, with implications for malarial virulence

Human and rodent erythrocytes are known to be highly permeable to glycerol. Aquaglyceroporin aquaporin (AQP)3 is the major glycerol channel in human and rat erythrocytes. However, AQP3 expression has not been observed in mouse erythrocytes. Here we report the presence of an aquaglyceroporin, AQP9, in mouse erythrocytes. AQP9 levels rise as reticulocytes mature into erythrocytes and as neonatal pups develop into adult mice. Mice bearing targeted disruption of both alleles encoding AQP9 have erythrocytes that appear morphologically normal. Compared with WT cells, erythrocytes from AQP9-null mice are defective in rapid glycerol transport across the cell membrane when measured by osmotic lysis, [(14)C]glycerol uptake, or stopped-flow light scattering. In contrast, the water and urea permeabilities are intact. Although the physiological role of glycerol in the normal function of erythrocytes is not clear, plasma glycerol is an important substrate for lipid biosynthesis of intraerythrocytic malarial parasites. AQP9-null mice at the age of 4 months infected with Plasmodium berghei survive longer during the initial phase of infection compared with WT mice. We conclude that AQP9 is the major glycerol channel in mouse erythrocytes and suggest that this transport pathway may contribute to the virulence of intraerythrocytic stages of malarial infection.

A novel tripartite motif involved in aquaporin topogenesis, monomer folding and tetramerization

Aquaporin (AQP) folding in the endoplasmic reticulum is characterized by two distinct pathways of membrane insertion that arise from divergent residues within the second transmembrane segment. We now show that in AQP1 these residues (Asn49 and Lys51) interact with Asp185 at the C terminus of TM5 to form a polar, quaternary structural motif that influences multiple stages of folding. Asn49 and Asp185 form an intramolecular hydrogen bond needed for proper helical packing, monomer formation and function. In contrast, Lys51 interacts with Asp185 on an adjacent monomer to stabilize the AQP1 tetramer. Although these residues are unique to AQP1, they share a highly conserved architecture whose functional properties can be transferred to other family members. These findings suggest a general mechanism by which evolutionary divergence of membrane proteins can confer new functional properties via alternative folding pathways that give rise to a common final structure.

Red blood cell blood group antigens: structure and function

Red blood cell (RBC) blood group antigens are polymorphic, inherited, carbohydrate or protein structures located on the extracellular surface of the RBC membrane. They contribute to the architecture of the RBC membrane, and their individual function(s) are being slowly revealed. The biological qualities assigned to these RBC membrane structures are based on observed physiological alteration in RBCs that lack the component, by documenting similarities in its protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function and by extrapolation to identified functional homologues in other cells. The varied roles of RBC antigens include membrane structural integrity, the transport of molecules through the membrane, as receptors for extracellular ligands, adhesion molecules, enzymes, complement components and regulators, and in glycocalyx formation.

Mosquito (Aedes aegypti ) aquaporin, present in tracheolar cells, transports water, not glycerol, and forms orthogonal arrays in Xenopus oocyte membranes

Previous results showed that mRNA encoding a putative aquaporin (AQP) (GenBank accession number AF218314) is present in the tracheolar cells associated with female Aedes aegypti Malpighian tubules. In this study, immunohistochemistry detected the protein, AeaAQP, also in tracheolar cells, suggesting its involvement in water movement in the respiratory system. When expressed in Xenopus oocytes, AeaAQP increased the osmotic water permeability from 15 x 10(-6) to 150 x 10(-6) m x s-1, which was inhibited by mercury ions. No permeability to glycerol or other solute was observed. AeaAQP expressed in oocytes was solubilized as a homotetramer in nondenaturing detergent as deduced from velocity centrifugation on density gradients. Phylogenetic analysis of MIP (major intrinsic protein) family sequences shows that AeaAQP clusters with other native orthogonal array forming proteins. Specific orthogonal arrays were detected by freeze-fracture analysis of AeaAQP oocyte membranes. We conclude that, in tracheolar cells of A. aegypti, AeaAQP is probably a highly water-permeable homotetrameric MIP which natively can form 2D crystals.

AQP3 deficiency in humans and the molecular basis of a novel blood group system, GIL

AQP3 is a water and glycerol channel present on human erythrocytes and in various tissues. By protein and molecular biology analysis, two unrelated probands who developed alloantibodies to the high frequency antigen GIL were found to be AQP3-deficient. The defect is caused by homozygous mutation affecting the 5' donor splice site of intron 5 of the AQP3 gene. This mutation causes the skipping of exon 5 and generates a frameshift and premature stop codon. Functional studies by 90 degrees light scattering using a stopped-flow spectrometer revealed the absence of facilitated glycerol transport across red cell membranes from the probands, but the water and urea transports were normal. Expression studies into COS-7 cells followed by flow cytometry analysis showed that only cells transfected with AQP3 cDNA strongly reacted with anti-GIL antibodies. These findings represent the first reported cases of AQP3 deficiency in humans and provide the molecular basis of a new blood group system, GIL, encoded by the AQP3 protein.