A single glutamate residue controls the oligomerization, function, and stability of the aquaglyceroporin GlpF

The Hidden Intricacies of Aquaporins: Remarkable Details in a Common Structural Scaffold

Evolution turned aquaporins (AQPs) into the most efficient facilitators of passive water flow through cell membranes at no expense of solute discrimination. In spite of a plethora of solved AQP structures, many structural details remain hidden. Here, by combining extensive sequence- and structural-based analysis of a unique set of 20 non-redundant high-resolution structures and molecular dynamics simulations of four representatives, key aspects of AQP stability, gating, selectivity, pore geometry, and oligomerization, with a potential impact on channel functionality, are identified. The general view of AQPs possessing a continuous open water pore is challenged and it is depicted that AQPs' selectivity is not exclusively shaped by pore-lining residues but also by the relative arrangement of transmembrane helices. Moreover, this analysis reveals that hydrophobic interactions constitute the main determinant of protein thermal stability. Finally, a numbering scheme of the conserved AQP scaffold is established, facilitating direct comparison of, for example, disease-causing mutations and prediction of potential structural consequences. Additionally, the results pave the way for the design of optimized AQP water channels to be utilized in biotechnological applications.

Membrane Structure of Aquaporin Observed with Combined Experimental and Theoretical Sum Frequency Generation Spectroscopy

High-resolution structural information on membrane proteins is essential for understanding cell biology and for the structure-based design of new medical drugs and drug delivery strategies. X-ray diffraction (XRD) can provide angstrom-level information about the structure of membrane proteins, yet for XRD experiments, proteins are removed from their native membrane environment, chemically stabilized, and crystallized, all of which can compromise the conformation. Here, we describe how a combination of surface-sensitive vibrational spectroscopy and molecular dynamics simulations can account for the native membrane environment. We observe the structure of a glycerol facilitator channel (GlpF), an aquaporin membrane channel finely tuned to selectively transport water and glycerol molecules across the membrane barrier. We find subtle but significant differences between the XRD structure and the inferred in situ structure of GlpF.

A comparison of aquaporin expression in mosquito larvae (Aedes aegypti) that develop in hypo-osmotic freshwater and iso-osmotic brackish water

The mosquito Aedes aegypti vectors the arboviral diseases yellow fever, dengue, Zika and chikungunya. Larvae are usually found developing in freshwater; however, more recently they have been increasingly found in brackish water, potential habitats which are traditionally ignored by mosquito control programs. Aedes aegypti larvae are osmo-regulators maintaining their hemolymph osmolarity in a range of ~ 250 to 300 mOsmol l-1. In freshwater, the larvae must excrete excess water while conserving ions while in brackish water, they must alleviate an accumulation of salts. The compensatory physiological mechanisms must involve the transport of ions and water but little is known about the water transport mechanisms in the osmoregulatory organs of these larvae. Water traverses cellular membranes predominantly through transmembrane proteins named aquaporins (AQPs) and Aedes aegypti possesses 6 AQP homologues (AaAQP1 to 6). The objective of this study was to determine if larvae that develop in freshwater or brackish water have differential aquaporin expression in osmoregulatory organs, which could inform us about the relative importance and function of aquaporins to mosquito survival under these different osmotic conditions. We found that AaAQP transcript abundance was similar in organs of freshwater and brackish water mosquito larvae. Furthermore, in the Malpighian tubules and hindgut AaAQP protein abundance was unaffected by the rearing conditions, but in the gastric caeca the protein level of one aquaporin, AaAQP1 was elevated in brackish water. We found that AaAQP1 was expressed apically while AaAQP4 and AaAQP5 were found to be apical and/or basal in the epithelia of osmoregulatory organs. Overall, the results suggest that aquaporin expression in the osmoregulatory organs is mostly consistent between larvae that are developing in freshwater and brackish water. This suggests that aquaporins may not have major roles in adapting to longterm survival in brackish water or that aquaporin function may be regulated by other mechanisms like post-translational modifications.

Instability of aquaglyceroporin (AQP) 2 contributes to drug resistance in Trypanosoma brucei

Defining mode of action is vital for both developing new drugs and predicting potential resistance mechanisms. Sensitivity of African trypanosomes to pentamidine and melarsoprol is predominantly mediated by aquaglyceroporin 2 (TbAQP2), a channel associated with water/glycerol transport. TbAQP2 is expressed at the flagellar pocket membrane and chimerisation with TbAQP3 renders parasites resistant to both drugs. Two models for how TbAQP2 mediates pentamidine sensitivity have emerged; that TbAQP2 mediates pentamidine translocation across the plasma membrane or via binding to TbAQP2, with subsequent endocytosis and presumably transport across the endosomal/lysosomal membrane, but as trafficking and regulation of TbAQPs is uncharacterised this remains unresolved. We demonstrate that TbAQP2 is organised as a high order complex, is ubiquitylated and is transported to the lysosome. Unexpectedly, mutation of potential ubiquitin conjugation sites, i.e. cytoplasmic-oriented lysine residues, reduced folding and tetramerization efficiency and triggered ER retention. Moreover, TbAQP2/TbAQP3 chimerisation, as observed in pentamidine-resistant parasites, also leads to impaired oligomerisation, mislocalisation and increased turnover. These data suggest that TbAQP2 stability is highly sensitive to mutation and that instability contributes towards the emergence of drug resistance.

Transmembrane Peptides as Inhibitors of Protein-Protein Interactions: An Efficient Strategy to Target Cancer Cells?

Cellular functions are regulated by extracellular signals such as hormones, neurotransmitters, matrix ligands, and other chemical or physical stimuli. Ligand binding on its transmembrane receptor induced cell signaling and the recruitment of several interacting partners to the plasma membrane. Nowadays, it is well-established that the transmembrane domain is not only an anchor of these receptors to the membrane, but it also plays a key role in receptor dimerization and activation. Indeed, interactions between transmembrane helices are associated with specific biological activity of the proteins as cell migration, proliferation, or differentiation. Overexpression or constitutive dimerization (due notably to mutations) of these transmembrane receptors are involved in several physiopathological contexts as cancers. The transmembrane domain of tyrosine kinase receptors as ErbB family proteins (implicated in several cancers as HER2 in breast cancer) or other receptors as Neuropilins has been described these last years as a target to inhibit their dimerization/activation using several strategies. In this review, we will focus on the strategy which consists in using peptides to disturb in a specific manner the interactions between transmembrane domains and the signaling pathways (induced by ligand binding) of these receptors involved in cancer. This approach can be extended to inhibit other transmembrane protein dimerization as neuraminidase-1 (the catalytic subunit of elastin receptor complex), Discoidin Domain Receptor 1 (a tyrosine kinase receptor activated by type I collagen) or G-protein coupled receptors (GPCRs) which are involved in cancer processes.

Covalently Linking Oligomerization-Impaired GlpF Protomers Does Not Completely Re-establish Wild-Type Channel Activity

Integral membrane proteins of the aquaporin family facilitate rapid water flux across cellular membranes in all domains of life. Although the water-conducting pore is clearly defined in an aquaporin monomer, all aquaporins assemble into stable tetramers. In order to investigate the role of protomer–protomer interactions, we analyzed the activity of heterotetramers containing increasing fractions of mutated monomers, which have an impaired oligomerization propensity and activity. In order to enforce interaction between the protomers, we designed and analyzed a genetically fused homotetramer of GlpF, the aquaglyceroporin of the bacterium Escherichia coli (E. coli). However, increasing fractions of the oligomerization-impaired mutant GlpF E43A affected the activity of the GlpF heterotetramer in a nearly linear manner, indicating that the reduced protein activity, caused by the introduced mutations, cannot be fully compensated by simply covalently linking the monomers. Taken together, the results underline the importance of exactly positioned monomer–monomer contacts in an assembled GlpF tetramer.

Single-Molecule Analyses Reveal Rhomboid Proteins Are Strict and Functional Monomers in the Membrane

Intramembrane proteases hydrolyze peptide bonds within the membrane as a regulatory paradigm that is conserved across all forms of cellular life. Many of these enzymes are thought to be oligomeric, and that their resulting quaternary interactions form the basis of their regulation. However, technical limitations have precluded directly determining the oligomeric state of intramembrane proteases in any membrane. Using single-molecule photobleaching, we determined the quaternary structure of 10 different rhomboid proteins (the largest superfamily of intramembrane proteases) and six unrelated control proteins in parallel detergent micelle, planar supported lipid bilayer, and whole-cell systems. Bacterial, parasitic, insect, and human rhomboid proteases and inactive rhomboid pseudoproteases all proved to be monomeric in all membrane conditions but dimeric in detergent micelles. These analyses establish that rhomboid proteins are, as a strict family rule, structurally and functionally monomeric by nature and that rhomboid dimers are unphysiological.

The GlpF residue Trp219 is part of an amino-acid cluster crucial for aquaglyceroporin oligomerization and function

The vestibule loop regions of aquaglyceroporins are involved in accumulation of glycerol inside the channel pore. Even though most loop regions do not show high sequence similarity among aquaglyceroporins, loop E is highly conserved in aquaglyceroporins, but not in members of the homologous aquaporins. Specifically, a tryptophan residue is extremely conserved within this loop. We have investigated the role of this residue (Trp219) that deeply protrudes into the protein and potentially interacts with adjacent loops, using the E. coli aqualgyeroporin GlpF as a model. Replacement of Trp219 affects the activity of GlpF and impairs the stability of the tetrameric protein. Furthermore, we have identified an amino acid cluster involving Trp219 that stabilizes the GlpF tetramer. Based on our results we propose that Trp219 is key for formation of a defined vestibule structure, which is crucial for glycerol accumulation as well as for the stability of the active GlpF tetramer.

Making Monomeric Aquaporin Z by Disrupting the Hydrophobic Tetramer Interface

The assembly of integral membrane proteins depends on the packing of hydrophobic interfaces. The forces driving this packing remain unclear. In this study, we have investigated the effect of mutations in these hydrophobic interfaces on the structure and function of the tetrameric Escherichia coli water channel aquaporin Z (AqpZ). Among the variants, we have constructed several fail to form tetramers and are monomeric. In particular, both of the mutants which are expected to create interfacial cavities become monomeric. Furthermore, one of the mutations can be compensated by a second-site mutation. We suggest that the constraints imposed by the nature of the lipid solvent result in interfaces that respond differently to modifications of residues. Specifically, the large size and complex conformations of lipid molecules are unable to fill small interfacial holes. Further, we observe in AqpZ that there is a link between the oligomeric state and the water channel activity. This despite the robustness of both protein folding and topology, both of which remain unchanged by the mutations we introduce. We propose that this linkage may result from the specific modes of structural flexibility in the monomeric protein.

A mosquito entomoglyceroporin, Aedes aegypti AQP5 participates in water transport across the Malpighian tubules of larvae

The mosquito, Aedes aegypti, is the primary vector for arboviral diseases such as Zika fever, dengue fever, chikungunya, and yellow fever. The larvae reside in hypo-osmotic freshwater habitats, where they face dilution of their body fluids from osmotic influx of water. The Malpighian tubules help maintain ionic and osmotic homeostasis by removing excess water from the hemolymph, but the transcellular pathway for this movement remains unresolved. Aquaporins are transmembrane channels thought to permit transcellular transport of water from the hemolymph into the Malpighian tubule lumen. Immunolocalization of Aedes aegypti aquaporin 5 (AaAQP5) revealed expression by Malpighian tubule principal cells of the larvae, with localization to both the apical and basolateral membranes. Knockdown of AaAQP5 with double stranded RNA decreased larval survival, reduced rates of fluid, K+, and Na+ secretion by the Malpighian tubules and reduced Cl− concentrations in the hemolymph. These findings indicate that AaAQP5 participates in transcellular water transport across the Malpighian tubules of larval Aedes aegypti where global AaAQP5 expression is important for larval survival.

Anionic Lipids Modulate the Activity of the Aquaglyceroporin GlpF

The structure and composition of a biological membrane can severely influence the activity of membrane-embedded proteins. Here, we show that the E. coli aquaglyceroporin GlpF has only little activity in lipid bilayers formed from native E. coli lipids. Thus, at first glance, GlpF appears to not be optimized for its natural membrane environment. In fact, we found that GlpF activity was severely affected by negatively charged lipids regardless of the exact chemical nature of the lipid headgroup, whereas GlpF was not sensitive to changes in the lateral membrane pressure. These observations illustrate a potential mechanism by which the activity of an α-helical membrane protein is modulated by the negative charge density around the protein.

The AQP2 mutation V71M causes nephrogenic diabetes insipidus in humans but does not impair the function of a bacterial homolog

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The aquaporin 2 mutation V71M causes nephrogenic diabetes insipidus in humans.

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Val71 is highly conserved in aqua(glycero)porins and points into the translocation pore.

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The V71M mutation does not impair the activity and oligomerization of a bacterial homolog.

Several point mutations have been identified in human aquaporins, but their effects on the function of the respective aquaporins are mostly enigmatic. We analyzed the impact of the aquaporin 2 mutation V71M, which causes nephrogenic diabetes insipidus in humans, on aquaporin structure and activity, using the bacterial aquaglyceroporin GlpF as a model. Importantly, the sequence and structure around the V71M mutation is highly conserved between aquaporin 2 and GlpF. The V71M mutation neither impairs substrate flux nor oligomerization of the aquaglyceroporin. Therefore, the human aquaporin 2 mutant V71M is most likely active, but cellular trafficking is probably impaired.

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.

A designed buried salt bridge modulates heterodimerization of a membrane peptide

Specific helix-helix interactions underpin the correct assembly of multipass membrane proteins. Here, we show that a designed buried salt bridge mediates heterodimer formation of model transmembrane helical peptides in a pH-dependent manner. The model peptides bear side chains functionalized with either a carboxylic acid or a primary amine within a hydrophobic segment. The association behavior was monitored by Förster resonance energy transfer, revealing that heterodimer formation is maximized at a pH close to neutrality (pH 6.5), at which each peptide is found in a charged state. In contrast, heterodimerization is disfavored at low and high values of pH, because either the carboxylic acid or the primary amine is present in its neutral state, thus preventing the formation of a salt bridge. These findings provide a blueprint for the design and modulation of protein-protein interactions in membrane proteins.

Sequential steps in the assembly of the multimeric outer membrane secretin PulD

Investigations into protein folding are largely dominated by studies on monomeric proteins. However, the transmembrane domain of an important group of membrane proteins is only formed upon multimerization. Here, we use in vitro translation-coupled folding and insertion into artificial liposomes to investigate kinetic steps in the assembly of one such protein, the outer membrane secretin PulD of the bacterial type II secretion system. Analysis of the folding kinetics, measured by the acquisition of distinct determinants of the native state, provides unprecedented evidence for a sequential multistep process initiated by membrane-driven oligomerization. The effects of varying the lipid composition of the liposomes indicate that PulD first forms a "prepore" structure that attains the native state via a conformational switch.

Ion selectivity and gating mechanisms of FNT channels

The phospholipid bilayer has evolved to be a protective and selective barrier by which the cell maintains high concentrations of life sustaining organic and inorganic material. As gatekeepers responsible for an immense amount of bidirectional chemical traffic between the cytoplasm and extracellular milieu, ion channels have been studied in detail since their postulated existence nearly three-quarters of a century ago. Over the past fifteen years, we have begun to understand how selective permeability can be achieved for both cationic and anionic ions. Our mechanistic knowledge has expanded recently with studies of a large family of anion channels, the Formate Nitrite Transport (FNT) family. This family has proven amenable to structural studies at a resolution high enough to reveal intimate details of ion selectivity and gating. With five representative members having yielded a total of 15 crystal structures, this family represents one of the richest sources of structural information for anion channels.

Analyzing oligomerization of individual transmembrane helices and of entire membrane proteins in E. coli: A hitchhiker's guide to GALLEX

Genetic systems, which allow monitoring interactions of individual transmembrane α-helices within the cytoplasmic membrane of the bacterium Escherichia coli, are now widely used to probe the structural biology and energetics of helix-helix interactions and the consequences of mutations. In contrast to other systems, the GALLEX system allows studying homo- as well as heterooligomerization of individual transmembrane α-helices, and even enables estimation of the energetics of helix-helix interactions within a biological membrane. Given that many polytopic membrane proteins form oligomers within membranes, the GALLEX system represents a unique and powerful approach to monitor formation and stability of oligomeric complexes of polytopic membrane proteins in vivo.

Genetic systems for monitoring interactions of transmembrane domains in bacterial membranes

In recent years several systems have been developed to study interactions of TM domains within the inner membrane of the Gram-negative bacterium Escherichia coli. Mostly, a transmembrane domain of interest is fused to a soluble DNA-binding domain, which dimerizes in E. coli cytoplasm after interactions of the transmembrane domains. The dimeric DNA-binding domain subsequently binds to a promoter/operator region and thereby activates or represses a reporter gene. In 1996 the first bacterial system has been introduced to measure interactions of TM helices within a bacterial membrane, which is based on fusion of a transmembrane helix of interest to the DNA-binding domain of the Vibrio cholerae ToxR protein. Interaction of a transmembrane helix of interest within the membrane environment results in dimerization of the DNA-binding domain in the bacterial cytoplasm, and the dimeric DNA-binding domain then binds to the DNA and activates a reporter gene. Subsequently, systems with improved features, such as the TOXCAT- or POSSYCCAT system, which allow screening of TM domain libraries, or the GALLEX system, which allows measuring heterotypic interactions of TM helices, have been developed and successfully applied. Here we briefly introduce the currently most applied systems and discuss their advantages together with their limitations.

Conformational dynamics of a membrane transport protein probed by H/D exchange and covalent labeling: the glycerol facilitator

Glycerol facilitator (GF) is a tetrameric membrane protein responsible for the selective permeation of glycerol and water. Each of the four GF subunits forms a transmembrane channel. Every subunit consists of six helices that completely span the lipid bilayer, as well as two half-helices (TM7 and TM3). X-ray crystallography has revealed that the selectivity of GF is due to its unique amphipathic channel interior. To explore the structural dynamics of GF, we employ hydrogen/deuterium exchange (HDX) and oxidative labeling with mass spectrometry (MS). HDX-MS reveals that transmembrane helices are generally more protected than extramembrane segments, consistent with data previously obtained for other membrane proteins. Interestingly, TM7 does not follow this trend. Instead, this half-helix undergoes rapid deuteration, indicative of a highly dynamic local structure. The oxidative labeling behavior of most GF residues is consistent with the static crystal structure. However, the side chains of C134 and M237 undergo labeling although they should be inaccessible according to the X-ray structure. In agreement with our HDX-MS data, this observation attests to the fact that TM7 is only marginally stable. We propose that the highly mobile nature of TM7 aids in the efficient diffusion of guest molecules through the channel ("molecular lubrication"). In the absence of such dynamics, host-guest molecular recognition would favor semipermanent binding of molecules inside the channel, thereby impeding transport. The current work highlights the complementary nature of HDX, covalent labeling, and X-ray crystallography for the characterization of membrane proteins.

Thermostability of two cyanobacterial GrpE thermosensors

GrpE proteins act as co-chaperones for DnaK heat-shock proteins. The dimeric protein unfolds under heat stress conditions, which results in impaired interaction with a DnaK protein. Since interaction of GrpE with DnaK is crucial for the DnaK chaperone activity, GrpE proteins act as a thermosensor in bacteria. Here we have analyzed the thermostability and function of two GrpE homologs of the mesophilic cyanobacterium Synechocystis sp. PCC 6803 and of the thermophilic cyanobacterium Thermosynechococcus elongatus BP1. While in Synechocystis an N-terminal helix pair of the GrpE dimer appears to be the thermosensing domain and mainly mediates GrpE dimerization, the C-terminal four-helix bundle is involved in additional stabilization of the dimeric structure. The four-helix bundle domain has a key role in the thermophilic cyanobacterium, since dimerization of the Thermosynechococcus protein appears to be mediated by the four-helix bundle domain, and melting of this domain is linked to monomerization of the GrpE protein. Thus, in two related cyanobacteria the GrpE thermosensing function might be mediated by different protein domains.

Functional characterization and hyperosmotic regulation of aquaporin in Synechocystis sp. PCC 6803

The genome of the cyanobacterium Synechocystis sp. PCC 6803 (hereafter, Synechocystis) contains an aqpZ gene (slr2057) which encodes an aquaporin (SsAqpZ), a membrane channel protein that might play a role in osmotic water transport and therefore the growth of Synechocystis. Structural characterization of SsAqpZ by protein sequence analysis and homology modelling revealed that it was more similar to bacterial aquaporin Z than the glycerol facilitator. To understand the functional role of SsAqpZ, the aqpZ knockout (KO) and myc-tagged aqpZ knockin (KI) Synechocystis were constructed. Water channel activity assays showed that SsAqpZ facilitated water transportation. SsAqpZ-mediated changes in cell volume were observed in wild-type (WT) and KI Synechocystis. Expression of SsAqpZ in KI Synechocystis was induced by extracellular hyperosmolarity. In the absence of hyperosmolarity, WT, KO and KI Synechocystis showed the same pattern of growth and no morphological or phenotypical perturbations. Under hyperosmotic condition, while the WT and also KI cells maintained a similar growth rate throughout the entire exponential phase, KO cells grew significantly slower. These results indicate that SsAqpZ has water channel activity and is involved in the adaptation and maintenance of growth of Synechocystis in a hyperosmotic environment.