Characterisation of the major intrinsic protein (MIP) from bovine lens fibre membranes by electron microscopy and hydrodynamics

Effects of truncations in the N- and C-terminal domains of filensin on filament formation with phakinin in cell-free conditions and cultured cells

Filensin and phakinin are lens fiber cell-specific proteins that constitute the beaded filaments (BFs) that are critical for maintaining lens transparency. In the Shumiya cataract rat, filensin 94 kDa undergoes N- and C-terminal proteolytic processing to give a transient 50 kDa fragment and a final 38 kDa fragment, just before opacification. To characterize the effects of this processing on filensin function, recombinant proteins representing the two filensin fragments, termed Fil(30-416) and Fil(30-369), respectively, were examined. Fil(30-416) lacks the N-terminal 29 amino acids and the C-terminal 248 amino acids. Fil(30-369) lacks the N-terminal 29 residues and the C-terminal 295 residues. In cell-free assembly characterized by electron microscopy, filensin and Fil(30-416) co-polymerized with phakinin and formed rugged, entangled filaments, whereas Fil(30-369) formed only aggregates. In cultured SW-13 and MCF-7 cells expressing fluorescent fusion proteins, filensin and Fil(30-416) co-polymerized with phakinin and formed cytoplasmic sinuous filaments with different widths, while Fil(30-369) gave aggregates. Therefore, while truncation of the N-terminal 29 amino acids did not affect filament formation, truncation of the C-terminal 295 but not the 248 residues resulted in failure of filament formation. These results indicate that the tail B region (residues 370-416) of rat filensin is essential for filament formation with phakinin. Truncation of the tail B region by proteolytic processing in the cataract rat lens might interfere with BF formation and thereby contribute to opacification.

Rgc2 Regulator of Glycerol Channel Fps1 Functions as a Homo- and Heterodimer with Rgc1

The plasma membrane aquaglyceroporin Fps1 is responsible for glycerol transport in yeast in response to changes in extracellular osmolarity. Fps1 functions as a homotetramer, and control of its channel activity in response to hyperosmotic shock involves a redundant pair of fungus-specific regulators, Rgc1 and Rgc2 (regulators of the glycerol channel), and the mitogen-activatd protein kinase (MAPK) Hog1 (high-osmolarity glycerol response). Rgc1 and Rgc2 maintain Fps1 in an open-channel state by binding to its C-terminal cytoplasmic domain. Phosphorylation of Rgc1 and Rgc2 by Hog1 induces their eviction from Fps1 and consequent channel closure. In the absence of Fps1 channel function, cells experience chronic cell wall stress, which may be exploited for antifungal drug development. We show here that Rgc1 and Rgc2 form homodimers and heterodimers with each other and that dimer formation of Rgc2 is mediated by its N-terminal domain. Mutations that prevent Rgc2 dimerization block its ability to open Fps1. Therefore, the Rgc-Rgc dimer interface might be an attractive drug target.

Functional characterization of an AQP0 missense mutation, R33C, that causes dominant congenital lens cataract, reveals impaired cell-to-cell adhesion

Aquaporin 0 (AQP0) performs dual functions in the lens fiber cells, as a water pore and as a cell-to-cell adhesion molecule. Mutations in AQP0 cause severe lens cataract in both humans and mice. An arginine to cysteine missense mutation at amino acid 33 (R33C) produced congenital autosomal dominant cataract in a Chinese family for five generations. We re-created this mutation in wild type human AQP0 (WT-AQP0) cDNA by site-directed mutagenesis, and cloned and expressed the mutant AQP0 (AQP0-R33C) in heterologous expression systems. Mutant AQP0-R33C showed proper trafficking and membrane localization like WT-AQP0. Functional studies conducted in Xenopus oocytes showed no significant difference (P > 0.05) in water permeability between AQP0-R33C and WT-AQP0. However, the cell-to-cell adhesion property of AQP0-R33C was significantly reduced (P < 0.001) compared to that of WT-AQP0, indicated by cell aggregation and cell-to-cell adhesion assays. Scrape-loading assay using Lucifer Yellow dye showed reduction in cell-to-cell adhesion affecting gap junction coupling (P < 0.001). The data provided suggest that this mutation might not have caused significant alterations in protein folding since there was no obstruction in protein trafficking or water permeation. Reduction in cell-to-cell adhesion and development of cataract suggest that the conserved positive charge of Extracellular Loop A may play an important role in bringing fiber cells closer. The proposed schematic models illustrate that cell-to-cell adhesion elicited by AQP0 is vital for lens transparency and homeostasis.

Yeast Fps1 glycerol facilitator functions as a homotetramer

The Saccharomyces cerevisiae Fps1 glycerol channel is a member of the major intrinsic protein (MIP) family of plasma membrane channel proteins that functions in osmoregulatory pathways to transport glycerol passively out of the cell. The MIP family is subdivided into members that are selectively permeable to water (aquaporins) and those permeated by glycerol (aquaglyceroporins or glycerol facilitators). Although aquaporins function as homo-tetramers with each monomer possessing its own channel, previous studies have suggested that aquaglyceroporins may function as monomers. Here we provide both genetic and biochemical evidence that Fps1 functions as a homotetramer to regulate glycerol transport in yeast.

Structure and stability of the spinach aquaporin SoPIP2;1 in detergent micelles and lipid membranes

Background

SoPIP2;1 constitutes one of the major integral proteins in spinach leaf plasma membranes and belongs to the aquaporin family. SoPIP2;1 is a highly permeable and selective water channel that has been successfully overexpressed and purified with high yields. In order to optimize reconstitution of the purified protein into biomimetic systems, we have here for the first time characterized the structural stability of SoPIP2;1.

Methodology/Principal Finding

We have characterized the protein structural stability after purification and after reconstitution into detergent micelles and proteoliposomes using circular dichroism and fluorescence spectroscopy techniques. The structure of SoPIP2;1 was analyzed either with the protein solubilized with octyl-β-D-glucopyranoside (OG) or reconstituted into lipid membranes formed by E. coli lipids, diphytanoylphosphatidylcholine (DPhPC), or reconstituted into lipid membranes formed from mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE), 1-palmitoyl-2oleoyl-phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-phosphatidylserine (POPS), and ergosterol. Generally, SoPIP2;1 secondary structure was found to be predominantly α-helical in accordance with crystallographic data. The protein has a high thermal structural stability in detergent solutions, with an irreversible thermal unfolding occurring at a melting temperature of 58°C. Incorporation of the protein into lipid membranes increases the structural stability as evidenced by an increased melting temperature of up to 70°C.

Conclusion/Significance

The results of this study provide insights into SoPIP2;1 stability in various host membranes and suggest suitable choices of detergent and lipid composition for reconstitution of SoPIP2;1 into biomimetic membranes for biotechnological applications.

Cell-free synthesis of functional aquaporin Z in synthetic liposomes

The challenges involved in producing sufficient quantities of aquaporins for precise biophysical characterization have limited our knowledge of this important class of molecules. This article describes a cell-free protein synthesis method for producing high concentrations of the E. coli water transporter, aquaporin Z (AqpZ), in synthetic liposomes. To our knowledge, this is the first report of in vitro synthesis of a membrane protein directly into synthetic liposomes with verified function, (i.e., transport activity and selectivity). Titration of DOPC lipid vesicles added to the cell-free reaction show that production yields of active AqpZ are dependent on the concentration of DOPC lipid vesicles added to the cell-free reaction, with 224 +/- 24 lipids required per aquaporin monomer. Supplementation of the signal recognition particle receptor (FtsY) to the cell-free reaction increases production of vesicle-associated AqpZ but not active AqpZ. Cell-free reactions using 7 mg/mL lipids that were not supplemented with FtsY produced 507 +/- 11 microg/mL of vesicle-associated AqpZ that exhibited a specific water transport activity of (2.2 +/- 0.3) x 10(-14) cm(3) s(-1) monomer(-1). Proteinase K protection, activation energy determination, and selectivity against glycerol and urea transport also confirmed the production of correctly folded AqpZ. This technique is capable of producing milligram quantities of aquaporin that can be readily assayed for function, facilitating biophysical characterization and high-throughput analysis.

Determination of membrane protein molecular weight using sedimentation equilibrium analytical ultracentrifugation

Both the stoichiometry and stability of native membrane protein complexes pose challenges to understanding the biology of these proteins. Sedimentation equilibrium analytical ultracentrifugation is well recognized as a thermodynamically rigorous technique for determining these quantities for macromolecules. This unit describes the experimental strategies that can be used to extract this information for membrane proteins reconstituted in vitro in detergent micelle or detergent/lipid mixed micelle solutions.

Congenital cataracts and their molecular genetics

Cataract can be defined as any opacity of the crystalline lens. Congenital cataract is particularly serious because it has the potential for inhibiting visual development, resulting in permanent blindness. Inherited cataracts represent a major contribution to congenital cataracts, especially in developed countries. While cataract represents a common end stage of mutations in a potentially large number of genes acting through varied mechanisms in practice most inherited cataracts have been associated with a subgroup of genes encoding proteins of particular importance for the maintenance of lens transparency and homeostasis. The increasing availability of more detailed information about these proteins and their functions and is making it possible to understand the pathophysiology of cataracts and the biology of the lens in general.

Determination of membrane protein molecular weights and association equilibrium constants using sedimentation equilibrium and sedimentation velocity

Regulated molecular interactions are essential for cellular function and viability, and both homo- and hetero-interactions between all types of biomolecules play important cellular roles. This chapter focuses on interactions between membrane proteins. Knowing both the stoichiometries and stabilities of these interactions in hydrophobic environments is a prerequisite for understanding how this class of proteins regulates cellular activities in membranes. Using examples from the authors' work, this chapter highlights the application of analytical ultracentrifugation methods in the determination of these parameters for integral membrane proteins. Both theoretical and practical aspects of carrying out these experiments are discussed.

Co-axial Association of Recombinant Eye Lens Aquaporin-0 Observed in Loosely Packed 3D Crystals

Aquaporin-0 (AQP0) is the major membrane protein in vertebrate eye lenses. It has been proposed that AQP0 tetramers mediate contact between membranes of adjacent lens fiber cells, which would be consistent with the extraordinarily narrow inter-cellular spacing. We have obtained 3D crystals of recombinant bovine AQP0 that diffract to 7.0 A resolution. The crystal packing was determined by molecular replacement and shows that, within the cubic lattice, AQP0 tetramers are associated head-to-head along their 4-fold axes. Oligomeric states larger than the tetramer were also observed in solution by native gel electrophoresis and analytical ultracentrifugation methods. In the crystals, there are no direct contacts between octamers, and it can thus be inferred that crystalline order is mediated solely by the detergent belts surrounding the membrane protein. Across the tetramer-tetramer interface, extracellular loops A and C interdigitate at the center and the perimeter of the octamer, respectively. The octamer structure is compared with that of the recently determined structure of truncated ovine AQP0 derived from electron diffraction of 2D crystals. Intriguingly, also in these crystals, octamers are observed, but with significantly different relative tetramer-tetramer orientations. The interactions observed in the loosely packed 3D crystals reported here may in fact represent an in vivo association mode between AQP0 tetramers from juxtaposed membranes in the eye lens.

Conical tomography of freeze-fracture replicas: a method for the study of integral membrane proteins inserted in phospholipid bilayers

We have used conical tomography to study the structure of integral proteins in their phospholipid bilayer environments. Complete conical series were collected from replicas of the water channel aquaporin-0 (AQP0), a 6.6 nm side tetramer with a molecular weight of approximately 120 kDa that was purified and reconstituted in liposomes. The replicas were tilted at 38 degrees , 50 degrees or 55 degrees and rotated by 2.5 degrees , 4 degrees , or 5 degrees increments until completing 360 degrees turns. The elliptical paths of between 6 and 12 freeze-fracture particles aligned the images to a common coordinate system. Using the weighted back projection algorithm, small volumes of the replicas were independently reconstructed to reconstitute the field. Using the Fourier Shell Correlation computed from reconstructions of even and odd projections of the series, we estimated a resolution of 2-3 nm, a value that was close to the thickness of the replica (approximately 1.5 nm). The 3D reconstructions exhibited isotropic resolution along the x-y plane, which simplified the analysis of particles oriented randomly in the membrane plane. In contrast to reconstructions from single particles imaged using random conical tilt [J. Mol. Biol. 325 (2003) 210], the reconstructions using conical tomography allowed the size and shape of individual particles representing the AQP0 channel to be identified without averaging or imposing symmetry. In conclusion, the reconstruction of freeze-fracture replicas with electron tomography has provided a novel experimental approach for the study of integral proteins inserted in phospholipid bilayers.

The channel architecture of aquaporin 0 at a 2.2-A resolution

We determined the x-ray structure of bovine aquaporin 0 (AQP0) to a resolution of 2.2 A. The structure of this eukaryotic, integral membrane protein suggests that the selectivity of AQP0 for water transport is based on the identity and location of signature amino acid residues that are hallmarks of the water-selective arm of the AQP family of proteins. Furthermore, the channel lumen is narrowed only by two, quasi-2-fold related tyrosine side chains that might account for reduced water conductance relative to other AQPs. The channel is functionally open to the passage of water because there are eight discreet water molecules within the channel. Comparison of this structure with the recent electron-diffraction structure of the junctional form of sheep AQP0 at pH 6.0 that was interpreted as closed shows no global change in the structure of AQP0 and only small changes in side-chain positions. We observed no structural change to the channel or the molecule as a whole at pH 10, which could be interpreted as the postulated pH-gating mechanism of AQP0-mediated water transport at pH >6.5. Contrary to the electron-diffraction structure, the comparison shows no evidence of channel gating induced by association of the extracellular domains of AQP0 at pH 6.0. Our structure aids the analysis of the interaction of the extracellular domains and the possibility of a cell-cell adhesion role for AQP0. In addition, our structure illustrates the basis for formation of certain types of cataracts that are the result of mutations.

The Kohonen self-organizing map: a tool for the clustering and alignment of single particles imaged using random conical tilt

An important step in determining the three-dimensional structure of single macromolecules is to bring common features in the images into register through alignment and classification. Here, we took advantage of the striking computational properties of the Kohonen self-organizing map (SOM) to align and classify images of channels obtained by random conical geometry into more homogeneous subsets. First, we used simulations with artificially created images to deduce simple geometrical rules governing the mapping of bounded (differing in size and shape) and unbounded (differing in in-plane orientation) variations in the output plane. Second, we measured the effect of noise on the accuracy of the algorithm to separate homogeneous subsets. Finally, we applied the rules ascertained in the previous steps to separate freeze-fracture images of the cytoplasmic and external domains of the small (approximately 118 kDa) aquaporin-0 water channel. Comparison with the results obtained from a similar input set using alignment-through-classification showed that both methods converged to stable classes exhibiting the same overall shapes (tetragonal and octagonal) for the cytoplasmic and external views of the channel. Processing with the SOM, however, was simplified by the utilization of the geometric rules governing the mapping of bounded and unbounded variations as well as the lack of subjectivity in selecting the reference images during alignment.

Differential protein expression in lens epithelial whole-mounts and lens epithelial cell cultures

PURPOSE

Lens fibergenesis is a problem in several types of cataract and in the posterior capsular opacification following cataract surgery. To correct improper fiber differentiation or to prevent unwanted growth on the posterior capsule following cataract surgery requires a thorough understanding of normal and abnormal fiber formation. To this end, studies were initiated to characterize fiber differentiation in the bovine lens and in lens epithelial cell cultures.

METHODS

Indirect immunofluorescence and immunoblot analysis were employed to study the expression of vimentin, beta-crystallin, gamma-crystallin, filensin, aquaporin 0 and the Na, K-ATPase catalytic subunit isoforms (alpha1, alpha2, alpha3) in bovine lens epithelium whole-mounts as well as lens epithelial cell cultures propagated in medium containing 10% bovine serum or in medium supplemented with bovine serum concentrations < or =4%.

RESULTS

Three distinct cell types were observed in the bovine lens epithelium. The cells of the central zone were identified by a polarized distribution of two distinct Na, K-ATPase catalytic subunit isoforms, alpha1 to the apical (fiber side) and alpha3 to the basal (aqueous humor side) membranes. Lateral to the polarized central zone, was the germinative zone of cells, best characterized by perinuclear vimentin basket-like structures and the loss of polarized Na, K-ATPase catalytic subunit isoforms. Lateral to the germinative zone were the cells of the transition zone (meridinal rows) where expression of the lens specific proteins beta-crystallin, gamma-crystallin, filensin and aquaporin 0 as well as the lens fiber-, adipocyte- and brain glia-specific Na, K-ATPase catalytic subunit, alpha2 are expressed. The cultured cells propagated in medium supplemented with 10% serum bore no resemblance to any of the cells of the bovine lens epithelium whole-mounts. The cells propagated in the medium supplemented with the lower bovine serum levels resembled the differentiating fibers of the transition zone of the bovine lens epithelium whole-mounts as well as superficial cortical fibers.

CONCLUSIONS

Since the low-serum lens epithelial cell cultures bear a remarkable resemblance to early differentiating fibers, they are reasonable models for the study of early fiber differentiation or prevention of differentiation. The culture conditions employed do not yield the polarized cells of the central zone. Nor has the function of these polarized cells in lens fluid, nutrient and ion homeostasis been determined.

Bilateral congenital cataracts result from a gain-of-function mutation in the gene for aquaporin-0 in mice

Cataract Tohoku (Cat(Tohm)) is a dominant cataract mutation that leads to severe degeneration of lens fiber cells. Linkage analysis showed that the Cat(Tohm) mutation is located on mouse chromosome 10, close to the gene for aquaporin-0 (Aqp0), which encodes a membrane protein that is expressed specifically in lens fiber cells. Sequence analysis of Aqp0 revealed a 12-bp deletion without any change in the reading frame, which resulted in a deletion of four amino acids within the second transmembrane region of the AQP0 protein. Targeted expression of the mutated Aqp0 caused lens opacity in transgenic mice, the pathological severity of which depended on the expression level of the transgene. The mutated AQP0 protein was localized to the intracellular and perinuclear spaces rather than to the plasma membranes of the lens fiber cells. The cataract phenotype of Cat(Tohm) is caused by a gain-of-function mutation in the mutated AQP0 protein and not by a loss-of-function mutation.

Structure of functional single AQP0 channels in phospholipid membranes

Aquaporin-0 (AQP0) is the most prevalent intrinsic protein in the plasma membrane of lens fiber cells where it functions as a water selective channel and also participates in fiber-fiber adhesion. We report the 3D envelope of purified AQP0 reconstituted with random orientation in phospholipid bilayers as single particles. The envelope was obtained by combining freeze-fracture, shadowing and random conical tilt electron microscopy followed by single particle image processing. Two-dimensional analysis of 2547 untilted images produced eight class averages exhibiting "square" and "octagonal" shapes with a continuum of variation. We reconstructed in 3D five class averages that best described the data set. The reconstructions ("molds") appeared as metal cups exhibiting external and internal surfaces. We used the internal surface of the mold to calculate the "imprints" that represent the AQP0 particles protruding from the hydrophobic core of the phospholipid bilayer. The complete envelope of the channel, formed by joining the square and octagonal imprints, described accurately the size, shape, oligomeric state, orientation, and molecular weight of the AQP0 channel inserted in the phospholipid bilayer. Rigid body docking of the atomic model of the aquaporin-1 (AQP1) tetramer showed that the freeze-fracture envelope accounted for the conserved transmembrane domain (approximately 73% similarity between AQP0 and AQP1) but not for the amino and carboxyl termini. We suggest that the discrepancy might reflect differences in the location of the amino and carboxyl termini in the crystal and in the phospholipid bilayer.

Aquaglyceroporins, one channel for two molecules

In the light of the recently published structure of GlpF and AQP1, we have analysed the nature of the residues which could be involved in the formation of the selectivity filter of aquaporins, glycerol facilitators and aquaglyceroporins. We demonstrate that the functional specificity for major intrinsic protein (MIP) channels can be explained on one side by analysing the polar environment of the residues that form the selective filter. On the other side, we show that the channel selectivity could be associated with the oligomeric state of the membrane protein. We conclude that a non-polar environment in the vicinity of the top of helix 5 could allow aquaglyceroporins and GlpF to exist as monomers within the hydrophobic environment of the membrane.

Aquaporin-2: COOH terminus is necessary but not sufficient for routing to the apical membrane

Renal regulation of mammalian water homeostasis is mediated by the aquaporin-1 (AQP1) water channel, which is expressed in the apical and basolateral membranes of proximal tubules and descending limbs of Henle, and aquaporin-2 (AQP2), which is redistributed from intracellular vesicles to the apical membrane (AM) of collecting duct cells with vasopressin. In transfected Madin-Darby canine kidney cells, AQP1 and AQP2 are regulated similarly, which indicates that routing elements reside in their primary sequences. We studied the role of the AQP2 COOH terminus in apical routing and AQP2 shuttling. An AQP1 chimera (AQP1 with an AQP2 tail: AQP1/2-N220) was located only in the AM independent of forskolin treatment. Forskolin increased the apical expression of AQP1 and AQP1/2-N220 less than twofold; that of AQP2 increased more than fourfold with concomitant changes in osmotic water permeabilities. The dimeric AQP2 tail coupled to placental alkaline phosphatase (AQP2-Plap) was retained in intracellular vesicles different from those of homotetrameric wild-type AQP2; the same protein without the AQP2 tail (TMR-Plap) was only expressed in the AM. The study shows that the AQP2 COOH tail is necessary but not sufficient for routing to the AM and suggests that other parts of AQP2 are needed for AQP2 accumulation in intracellular vesicles.

MP20, the second most abundant lens membrane protein and member of the tetraspanin superfamily, joins the list of ligands of galectin-3

BACKGROUND

Although MP20 is the second most highly expressed membrane protein in the lens its function remains an enigma. Putative functions for MP20 have recently been inferred from its assignment to the tetraspanin superfamily of integral membrane proteins. Members of this family have been shown to be involved in cellular proliferation, differentiation, migration, and adhesion. In this study, we show that MP20 associates with galectin-3, a known adhesion modulator.

RESULTS

MP20 and galectin-3 co-localized in selected areas of the lens fiber cell plasma membrane. Individually, these proteins purified with apparent molecular masses of 60 kDa and 22 kDa, respectively. A 104 kDa complex was formed in vitro upon mixing the purified proteins. A 102 kDa complex of MP20 and galectin-3 could also be isolated from detergent-solubilized native fiber cell membranes. Binding between MP20 and galectin-3 was disrupted by lactose suggesting the lectin site was involved in the interaction.

CONCLUSIONS

MP20 adds to a growing list of ligands of galectin-3 and appears to be the first representative of the tetraspanin superfamily identified to possess this specificity.

Molecular characterization of Vibrio parahaemolyticus vSGLT: a model for sodium-coupled sugar cotransporters

The Na(+)/galactose cotransporter (vSGLT) of Vibrio parahaemolyticus, tagged with C-terminal hexahistidine, has been purified to apparent homogeneity by Ni(2+) affinity chromatography and gel filtration. Resequencing the vSGLT gene identified an important correction: the N terminus constitutes an additional 13 functionally essential residues. The mass of His-tagged vSGLT expressed under its native promoter, as determined by electrospray ionization-mass spectrometry (ESI-MS), verifies these 13 residues in wild-type vSGLT. A fusion protein of vSGLT and green fluorescent protein, comprising a mass of over 90 kDa, was also successfully analyzed by ESI-MS. Reconstitution of purified vSGLT yields proteoliposomes active in Na(+)-dependent galactose uptake, with sugar preferences (galactose > glucose > fucose) reflecting those of wild-type vSGLT in vivo. Substrates are transported with apparent 1:1 stoichiometry and apparent K(m) values of 129 mm (Na(+)) and 158 microm (galactose). Freeze-fracture electron microscopy of functional proteoliposomes shows intramembrane particles of a size consistent with vSGLT existing as a monomer. We conclude that vSGLT is a suitable model for the study of sugar cotransporter mechanisms and structure, with potential applicability to the larger SGLT family of important sodium:solute cotransporters. It is further demonstrated that ESI-MS is a powerful tool for the study of proteomics of membrane transporters.

The role of aquaporins in cellular and whole plant water balance

Aquaporins are water channel proteins belonging to the major intrinsic protein (MIP) superfamily of membrane proteins. More than 150 MIPs have been identified in organisms ranging from bacteria to animals and plants. In plants, aquaporins are present in the plasma membrane and in the vacuolar membrane where they are abundant constituents. Functional studies of aquaporins have hitherto mainly been performed by heterologous expression in Xenopus oocytes. A main issue is now to understand their role in the plant, where they are likely to be important both at the cellular and at the whole plant level. Plants contain a large number of aquaporin isoforms with distinct cell type- and tissue-specific expression patterns. Some of these are constitutively expressed, whereas the expression of others is regulated in response to environmental factors, such as drought and salinity. At the protein level, regulation of water transport activity by phosphorylation has been reported for some aquaporins.

Structural clues in the sequences of the aquaporins

The large number of sequences available for the aquaporin family represents a valuable source of information to incorporate into three-dimensional structure determination. Phylogenetic analysis was used to define type sequences to avoid extreme over-representation of some subfamilies, and as a measure of the quality of multiple sequence alignment. Inspection of the sequence alignment suggested eight conserved segments that define the core architecture of six transmembrane helices and two functional loops, B and E, projecting into the plane of the membrane. The sum of the core segments and the minimum lengths of the interlinking loops constitute the 208 residues necessary to satisfy the aquaporin architecture. Analysis of hydrophobic and conservation periodicity and of correlated mutations across the alignment indicated the likely assignment and orientation of the helices in the bilayer. This assignment is examined with respect to the structure of the erythrocyte aquaporin 1 determined by electron crystallography. The aquaporin 1 tetramer is described as three rings of helices, each ring with a different exposure to the lipid environment. The sequence analysis clearly suggests that two helices are exposed along their whole lengths, two helices are exposed only at their N termini, and two helices are not exposed to lipid. It is further proposed that, besides loops B and E, the highly conserved motifs on helices 1 and 4, ExxxTxxF/L, could line the water channel.

Oligomerization state of MIP proteins expressed in Xenopus oocytes as revealed by freeze-fracture electron-microscopy analysis

The MIP (major intrinsic protein) family is a widespread family of membrane proteins exhibiting two major types of channel properties: aquaporins and solute facilitators. In the present study, freeze-fracture electron microscopy was used to investigate the oligomerization state of two MIP proteins heterologously expressed in the plasma membrane of Xenopus laevis oocytes: AQPcic, an aquaporin from the insect Cicadella viridis, and GlpF, a glycerol facilitator from Escherichia coli. Swelling assays performed on oocytes 48 and 72 h following cRNA microinjections showed that these proteins were functionally expressed. Particle density determinations indicated that expression of proteins is related to an increase in particle density on the P fracture face of oocyte plasma membranes. Statistical analysis of particle sizes was performed on protoplasmic fracture faces of the plasma membrane of oocytes expressing AQPcic and GlpF 72 h after cRNA microinjections. Compared to control oocytes, AQPcic-expressing oocytes exhibited a specific population of particles with a mean diameter of 8.7 +/- 0.1 nm. This value is consistent with the previously reported tetrameric organization of AQPcic. In addition, AQPcic particles aggregate and form orthogonal arrays similar to those observed in native membranes of C. viridis, consisting of homotetramers of AQPcic. On the protoplasmic fracture face of oocytes expressing GlpF, the particle density is increased by 4.1-fold and the mean diameter of specifically added particles is 5.8 +/- 0.1 nm. This value fits with a monomer of the 28-kDa GlpF protein plus the platinum-carbon layer. These results clearly demonstrate that GlpF is a monomer when functionally expressed in plasma membranes of Xenopus oocytes and therefore emphasize the key role of the oligomerization state of MIP proteins with respect to their function.

An impaired routing of wild-type aquaporin-2 after tetramerization with an aquaporin-2 mutant explains dominant nephrogenic diabetes insipidus

Autosomal recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, are caused by mutations in the aquaporin-2 (AQP2) gene. Missense AQP2 proteins in recessive NDI have been shown to be retarded in the endoplasmic reticulum, whereas AQP2-E258K, an AQP2 mutant in dominant NDI, was retained in the Golgi complex. In this study, we identified the molecular mechanisms underlying recessive and dominant NDI. Sucrose gradient centrifugation of rat and human kidney proteins and subsequent immunoblotting revealed that AQP2 forms homotetramers. When expressed in oocytes, wild-type AQP2 and AQP2-E258K also formed homotetramers, whereas AQP2-R187C, a mutant in recessive NDI, was expressed as a monomer. Upon co-injection, AQP2-E258K, but not AQP2-R187C, was able to heterotetramerize with wild-type AQP2. Since an AQP monomer is the functional unit and AQP2-E258K is a functional but misrouted water channel, heterotetramerization of AQP2-E258K with wild-type AQP2 and inhibition of further routing of this complex to the plasma membrane is the cause of dominant NDI. This case of NDI is the first example of a dominant disease in which the 'loss-of-function' phenotype is caused by an impaired routing rather than impaired function of the wild-type protein.

Oligomerization state of water channels and glycerol facilitators. Involvement of loop E

The major intrinsic protein (MIP) family includes water channels aquaporins (AQPs) and facilitators for small solutes such as glycerol (GlpFs). Velocity sedimentation on sucrose gradients demonstrates that heterologous AQPcic expressed in yeast or Xenopus oocytes behaves as an homotetramer when extracted by n-octyl beta-D-glucopyranoside (OG) and as a monomer when extracted by SDS. We performed an analysis of GlpF solubilized from membranes of Escherichia coli or of mRNA-injected Xenopus oocytes. The GlpF protein extracted either by SDS or by nondenaturing detergents, OG and Triton X-100, exhibits sedimentation coefficients only compatible with a monomeric form of the protein in micelles. We then substituted in loop E of AQPcic two amino acids predicted to play a role in the functional/structural properties of the MIPs. In two expression systems, yeast and oocytes, the mutant AQPcic-S205D is monomeric in OG and in SDS. The A209K mutation does not modify the tetrameric form of the heterologous protein in OG. This study shows that the serine residue at position 205 is essential for AQPcic tetramerization. Because the serine in this position is highly conserved among aquaporins and systematically replaced by an acid aspartic in GlpFs, we postulate that glycerol facilitators are monomers whereas aquaporins are organized in tetramers. Our data suggest that the role of loop E in MIP properties partly occurs through its ability to allow oligomerization of the proteins.

Structural analysis of cloned plasma membrane proteins by freeze-fracture electron microscopy

We have used freeze-fracture electron microscopy to examine the oligomeric structure and molecular asymmetry of integral plasma membrane proteins. Recombinant plasma membrane proteins were functionally expressed in Xenopus laevis oocytes, and the dimensions of their freeze-fracture particles were analyzed. To characterize the freeze-fracture particles, we compared the particle cross-sectional area of proteins with alpha-helical transmembrane domains (opsin, aquaporin 1, and a connexin) with their area obtained from existing maps calculated from two-dimensional crystals. We show that the cross-sectional area of the freeze-fracture particles corresponds to the area of the transmembrane domain of the protein, and that the protein cross-sectional area varies linearly with the number membrane-spanning helices. On average, each helix occupies 1.40 +/- 0.03 nm2. By using this information, we examined members from three classes of plasma membrane proteins: two ion channels, the cystic fibrosis transmembrane conductance regulator and connexin 50 hemi-channel; a water channel, the major intrinsic protein (the aquaporin 0); and a cotransporter, the Na+/glucose cotransporter. Our results suggest that the cystic fibrosis transmembrane conductance regulator is a dimer containing 25 +/- 2 transmembrane helices, connexin 50 is a hexamer containing 24 +/- 3 helices, the major intrinsic protein is a tetramer containing 24 +/- 3 helices, and the Na+/glucose cotransporter is an asymmetrical monomer containing 15 +/- 2 helices.

Purified lens major intrinsic protein (MIP) forms highly ordered tetragonal two-dimensional arrays by reconstitution

Lens major intrinsic protein (MIP) is the founding member of the MIP family of membrane channel proteins. Its isolation from ovine lens fibre cell membranes and its two-dimensional crystallization are described. Membranes were solubilized with N-octyl-beta-D-glucoside and proteins fractionated by sucrose gradient centrifugation containing decyl-beta-D-maltoside. MIP was purified by cation exchange chromatography, and homogeneity was assessed by mass analysis in the scanning transmission electron microscope. Purified MIP reconstituted into a lipid bilayer at a low lipid-to-protein ratio formed highly ordered tetragonal two-dimensional crystals. The square unit cell had a side length of 6.4 nm, and exhibited in negative stain four stain-excluding elongated domains surrounding a central stain-filled depression. Projection maps of freeze-dried crystals exhibited a resolution of 9 A, and revealed a monomer structure of MIP consisting of distinct densities. Despite significant differences in the packing of tetramers in the crystals, the projection map of the MIP monomer was similar to that of aquaporin-1 (AQP1), the first member of the MIP family which had its structure resolved to 6 A. Our protocols for the purification and reconstitution of MIP establish the feasibility for future work to visualize structure elements which determine the diverse functional properties of the MIP family members.

Progress on the structure and function of aquaporin 1

Life exists in water as universal solvent, and cells need to deal with its influx and efflux. Nature has accomplished the almost impossible, creating membrane channels with both a high flux and a high specificity for water. The first water channel was discovered in red blood cell membranes. Today known as aquaporin-1, this channel was found to be closely related to the major integral protein (MIP)1 of the eye lens. Cloning and sequencing of numerous related proteins of the MIP family revealed the widespread occurrence of such channels, suggesting an essential physiological function. Their structures hold the clues to the remarkable water channel activity, as well as to the arrangement of transmembrane segments in general. Recent medium-resolution three-dimensional electron microscopic studies determined a tetrameric complex with six tilted transmembrane helices per monomer. The helices within each monomer surround a central density formed by two interhelical loops implicated by mutagenesis in the water channel function. A combination of sequence analysis and assignment of the observed densities to predicted helices provides a basis for speculation on the nature of the water course through the protein. In particular, four highly conserved polar residues, E142-N192-N76-E17, are proposed to form a chain of key groups involved in the pathway of water flow through the channel.