Mapping the ends of transmembrane segments in a polytopic membrane protein. Scanning N-glycosylation mutagenesis of extracytosolic loops in the anion exchanger, band 3

Organization and Dynamics of the Red Blood Cell Band 3 Anion Exchanger SLC4A1: Insights From Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have provided new insights into the organization and dynamics of the red blood cell Band 3 anion exchanger (AE1, SLC4A1). Band 3, like many solute carriers, works by an alternating access mode of transport where the protein rapidly (104/s) changes its conformation between outward and inward-facing states via a transient occluded anion-bound intermediate. While structural studies of membrane proteins usually reveal valuable structural information, these studies provide a static view often in the presence of detergents. Membrane transporters are embedded in a lipid bilayer and associated lipids play a role in their folding and function. In this review, we highlight MD simulations of Band 3 in realistic lipid bilayers that revealed specific lipid and protein interactions and were used to re-create a model of the Wright (Wr) blood group antigen complex of Band 3 and Glycophorin A. Current MD studies of Band 3 and related transporters are focused on describing the trajectory of substrate binding and translocation in real time. A structure of the intact Band 3 protein has yet to be achieved experimentally, but cryo-electron microscopy in combination with MD simulations holds promise to capture the conformational changes associated with anion transport in exquisite molecular detail.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Large conformational dynamics in Band 3 protein: Significance for erythrocyte senescence signalling

Band 3 (Anion Exchanger 1, AE1), the predominant protein of erythrocyte membranes, facilitates Cl^-/HCO₃^- exchange and anchors the plasma membrane to the cytoskeleton. The Band 3 crystal structure revealed the amino acid 812-830 region as intracellular, conflicting with protein chemical data that suggested extracellular disposition. Further, circulating senescent cell auto-antibody that cannot enter erythrocytes, binds two regions of Band 3: residues 538-554 and 812-830. To reconcile this discrepancy, we assessed localization of residues 812-830 with Band 3 expressed in HEK293 cells and human erythrocytes, using chemical labeling probes and an antibody against residues 812-830. Antibody and chemical probes revealed reorientation of 812-830 region between extracellular and intracellular. This dramatic conformational change is an intrinsic property of the Band 3 molecule, occurring when expressed in HEK293 cells and without the damage that occurs during erythrocyte circulation. Conditions used to crystallize Band 3 for structural determination did not alter conformational dynamics. Collectively, these data reveal large Band 3 conformational dynamics localized to a region previously identified as an erythrocyte senescence epitope. Surface exposure of the senescence epitope (812-830), limited by conformational dynamics, may act as the "molecular clock" in erythrocyte senescence.

Interaction of the human erythrocyte Band 3 anion exchanger 1 (AE1, SLC4A1) with lipids and glycophorin A: Molecular organization of the Wright (Wr) blood group antigen

The Band 3 (AE1, SLC4A1) membrane protein is found in red blood cells and in kidney where it functions as an electro-neutral chloride/bicarbonate exchanger. In this study, we have used molecular dynamics simulations to provide the first realistic model of the dimeric membrane domain of human Band 3 in an asymmetric lipid bilayer containing a full complement of phospholipids, including phosphatidylinositol 4,5–bisphosphate (PIP₂) and cholesterol, and its partner membrane protein Glycophorin A (GPA). The simulations show that the annular layer in the inner leaflet surrounding Band 3 was enriched in phosphatidylserine and PIP₂ molecules. Cholesterol was also enriched around Band 3 but also at the dimer interface. The interaction of these lipids with specific sites on Band 3 may play a role in the folding and function of this anion transport membrane protein. GPA associates with Band 3 to form the Wright (Wr) blood group antigen, an interaction that involves an ionic bond between Glu658 in Band 3 and Arg61 in GPA. We were able to recreate this complex by performing simulations to allow the dimeric transmembrane portion of GPA to interact with Band 3 in a model membrane. Large-scale simulations showed that the GPA dimer can bridge Band 3 dimers resulting in the dynamic formation of long strands of alternating Band 3 and GPA dimers.

Human Band 3 (AE1, SLC4A1), an abundant 911 amino acid glycoprotein, catalyzes the exchange of bicarbonate and chloride across the red blood cell membrane, a process necessary for efficient respiration. Malfunction of Band 3 leads to inherited diseases such as Southeast Asian Ovalocytosis, hereditary spherocytosis and distal renal tubular acidosis. Despite much available structural and functional data about Band 3, key questions about the conformational changes associated with transport and the molecular details of its interaction with lipids and other proteins remain unanswered. In this study, we have used computer simulations to investigate the dynamics of Band 3 in lipid bilayers that resemble the red blood cell plasma membrane. Our results suggest that negatively charged phospholipids and cholesterol interact strongly with Band 3 forming an annulus around the protein. Glycophorin A (GPA) interacts with Band 3 to form the Wright (Wr) blood group antigen. We were able to recreate this complex and show that GPA promotes the clustering of Band 3 in red blood cell membranes. Understanding the molecular details of the interaction of Band 3 with GPA has provided new insights into the nature of the Wright blood group antigen.

Topological analysis of DPY19L3, a human C-mannosyltransferase

C-mannosylation is a rare type of protein glycosylation, the functions and mechanisms of which remain unclear. Recently, we identified DPY19L3 as a C-mannosyltransferase of R-spondin1 in human cells. DPY19L3 is predicted to be a multipass transmembrane protein that localizes to the endoplasmic reticulum (ER); however, its structure is undetermined. In this study, we propose a topological structure of DPY19L3 by in silico analysis and experimental methods such as redox-sensitive luciferase assay and introduction of N-glycosylation sites, suggesting that DPY19L3 comprises 11 transmembrane regions and two re-entrant loops with the N- and C-terminal ends facing the cytoplasm and ER lumen, respectively. Furthermore, DPY19L3 has four predicted N-glycosylation sites, and we have demonstrated that DPY19L3 is N-glycosylated at Asn¹¹⁸ and Asn⁷⁰⁴ but not Asn³¹⁹ and Asn⁴³⁹ , supporting our topological model. By mass spectrometry, we measured the C-mannosyltransferase activity of N-glycosylation-defective mutants of DPY19L3 and isoform2, a splice variant, which lacks the C-terminal luminal region of DPY19L3. Isoform2 does not possess C-mannosyltransferase activity, indicating the importance of the C-terminal region; however, N-glycosylations of DPY19L3 do not have any roles for its enzymatic activity. These novel findings on DPY19L3 provide important insights into the mechanism of C-mannosylation.

Predicting Alpha Helical Transmembrane Proteins Using HMMs

Alpha helical transmembrane (TM) proteins constitute an important structural class of membrane proteins involved in a wide variety of cellular functions. The prediction of their transmembrane topology, as well as their discrimination in newly sequenced genomes, is of great importance for the elucidation of their structure and function. Several methods have been applied for the prediction of the transmembrane segments and the topology of alpha helical transmembrane proteins utilizing different algorithmic techniques. Hidden Markov Models (HMMs) have been efficiently used in the development of several computational methods used for this task. In this chapter we give a brief review of different available prediction methods for alpha helical transmembrane proteins pointing out sequence and structural features that should be incorporated in a prediction method. We then describe the procedure of the design and development of a Hidden Markov Model capable of predicting the transmembrane alpha helices in proteins and discriminating them from globular proteins.

Molecular mechanism for the red blood cell senescence clock

Lacking protein synthesis machinery and organelles necessary for autophagy or apoptosis, aged red blood cells (RBCs) are marked by circulating auto-antibodies for macrophage-mediated clearance. The antigen recognized by these auto-antibodies is the major protein of the RBC membrane, Band 3. To ensure regulation and specificity in clearance, the molecular "clock" must mark senescent cells in a way that differentiates them from younger cells, to prevent premature clearance. Predominant models of Band 3 senescence signaling are reviewed, and merits are discussed in light of the recently published crystal structure of the Band 3 membrane domain. © 2017 IUBMB Life, 70(1):32-40, 2018.

Dissecting binding of a β-barrel membrane protein by phage display

Membrane proteins (MPs) constitute a third of all proteomes, and contribute to a myriad of cellular functions including intercellular communication, nutrient transport and energy generation. For example, TonB-dependent transporters (TBDTs) in the outer membrane of Gram-negative bacteria play an essential role transporting iron and other nutrients into the bacterial cell. The inherently hydrophobic surfaces of MPs complicates protein expression, purification, and characterization. Thus, dissecting the functional contributions of individual amino acids or structural features through mutagenesis can be a challenging ordeal. Here, we apply a new approach for the expedited protein characterization of the TBDT ShuA from Shigella dysenteriae, and elucidate the protein's initial steps during heme-uptake. ShuA variants were displayed on the surface of an M13 bacteriophage as fusions to the P8 coat protein. Each ShuA variant was analyzed for its ability to display on the bacteriophage surface, and functionally bind to hemoglobin. This technique streamlines isolation of stable MP variants for rapid characterization of binding to various ligands. Site-directed mutagenesis studies targeting each extracellular loop region of ShuA demonstrate no specific extracellular loop is required for hemoglobin binding. Instead two residues, His420 and His86 mediate this interaction. The results identify a loop susceptible to antibody binding, and also a small molecule motif capable of disrupting ShuA from S. dysenteriae. The approach is generalizable to the dissection of other phage-displayed TBDTs and MPs.

Effect of the Southeast Asian Ovalocytosis Deletion on the Conformational Dynamics of Signal-Anchor Transmembrane Segment 1 of Red Cell Anion Exchanger 1 (AE1, Band 3, or SLC4A1)

The first transmembrane (TM1) helix in the red cell anion exchanger (AE1, Band 3, or SLC4A1) acts as an internal signal anchor that binds the signal recognition particle and directs the nascent polypeptide chain to the endoplasmic reticulum (ER) membrane where it moves from the translocon laterally into the lipid bilayer. The sequence N-terminal to TM1 forms an amphipathic helix that lies at the membrane interface and is connected to TM1 by a bend at Pro403. Southeast Asian ovalocytosis (SAO) is a red cell abnormality caused by a nine-amino acid deletion (Ala400-Ala408) at the N-terminus of TM1. Here we demonstrate, by extensive (∼4.5 μs) molecular dynamics simulations of TM1 in a model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membrane, that the isolated TM1 peptide is highly dynamic and samples the structure of TM1 seen in the crystal structure of the membrane domain of AE1. The SAO deletion not only removes the proline-induced bend but also causes a "pulling in" of the part of the amphipathic helix into the hydrophobic phase of the bilayer, as well as the C-terminal of the peptide. The dynamics of the SAO peptide very infrequently resembles the structure of TM1 in AE1, demonstrating the disruptive effect the SAO deletion has on AE1 folding. These results provide a precise molecular view of the disposition and dynamics of wild-type and SAO TM1 in a lipid bilayer, an important early biosynthetic intermediate in the insertion of AE1 into the ER membrane, and extend earlier results of cell-free translation experiments.

The E3 Ubiquitin Ligase TMEM129 Is a Tri-Spanning Transmembrane Protein

Misfolded proteins from the endoplasmic reticulum (ER) are transported back into the cytosol for degradation via the ubiquitin-proteasome system. The human cytomegalovirus protein US11 hijacks this ER-associated protein degradation (ERAD) pathway to downregulate human leukocyte antigen (HLA) class I molecules in virus-infected cells, thereby evading elimination by cytotoxic T-lymphocytes. Recently, we identified the E3 ubiquitin ligase transmembrane protein 129 (TMEM129) as a key player in this process, where interference with TMEM129 activity in human cells completely abrogates US11-mediated class I degradation. Here, we set out to further characterize TMEM129. We show that TMEM129 is a non-glycosylated protein containing a non-cleaved signal anchor sequence. By glycosylation scanning mutagenesis, we show that TMEM129 is a tri-spanning ER-membrane protein that adopts an N_exo–C_cyto orientation. This insertion in the ER membrane positions the C-terminal really interesting new gene (RING) domain of TMEM129 in the cytosol, making it available to catalyze ubiquitination reactions that are required for cytosolic degradation of secretory proteins.

The Stalk Domain of NKp30 Contributes to Ligand Binding and Signaling of a Preassembled NKp30-CD3ζ Complex

The natural cytotoxicity receptor (NCR) NKp30 (CD337) is a key player for NK cell immunosurveillance of infections and cancer. The molecular details of ligand recognition and its connection to CD3ζ signaling remain unsolved. Here, we show that the stalk domain (¹²⁹KEHPQLGAGTVLLLR¹⁴³) of NKp30 is very sensitive to sequence alterations, as mutations lead to impaired ligand binding and/or signaling capacity. Surprisingly, the stalk domains of NKp30 and NKp46, another NCR employing CD3ζ for signaling, were not exchangeable without drastic deficiencies in folding, plasma membrane targeting, and/or ligand-induced receptor signaling. Further mutational studies, N-glycosylation mapping, and plasma membrane targeting studies in the absence and presence of CD3ζ suggest two interconvertible types of NCR-CD3ζ assemblies: 1) a signaling incompetent structural NKp30-CD3ζ complex and 2) a ligand-induced signaling competent NKp30-CD3ζ complex. Moreover, we propose that ligand binding triggers translocation of Arg-143 from the membrane interface into the membrane to enable alignment with oppositely charged aspartate residues within CD3ζ and activation of CD3ζ-signaling.

The topogenic function of S4 promotes membrane insertion of the voltage-sensor domain in the KvAP channel

Voltage-dependent K⁺ (K_V) channels control K⁺ permeability in response to shifts in the membrane potential. Voltage sensing in K_V channels is mediated by the positively charged transmembrane domain S4. The best-characterized K_V channel, KvAP, lacks the distinct hydrophilic region corresponding to the S3-S4 extracellular loop that is found in other K⁺ channels. In the present study, we evaluated the topogenic properties of the transmembrane regions within the voltage-sensing domain in KvAP. S3 had low membrane insertion activity, whereas S4 possessed a unique type-I signal anchor (SA-I) function, which enabled it to insert into the membrane by itself. S4 was also found to function as a stop-transfer signal for retention in the membrane. The length and structural nature of the extracellular S3-S4 loop affected the membrane insertion of S3 and S4, suggesting that S3 membrane insertion was dependent on S4. Replacement of charged residues within the transmembrane regions with residues of opposite charge revealed that Asp⁷² in S2 and Glu⁹³ in S3 contributed to membrane insertion of S3 and S4, and increased the stability of S4 in the membrane. These results indicate that the SA-I function of S4, unique among K⁺ channels studied to date, promotes the insertion of S3 into the membrane, and that the charged residues essential for voltage sensing contribute to the membrane-insertion of the voltage sensor domain in KvAP.

Validation of Protein Knockout in Mutant Zebrafish Lines Using In Vitro Translation Assays

Advances in genome-editing technology have made creation of zebrafish mutant lines accessible to the community. Experimental validation of protein knockout is a critical step in verifying null mutants, but this can be a difficult task. Absence of protein can be confirmed by Western blotting; however, this approach requires target-specific antibodies that are generally not available for zebrafish proteins. We address this issue using in vitro translation assays, a fast and standard procedure that can be easily implemented.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Structure and Function of SLC4 Family [Formula: see text] Transporters

The solute carrier SLC4 family consists of 10 members, nine of which are [Formula: see text] transporters, including three Na(+)-independent Cl(-)/[Formula: see text] exchangers AE1, AE2, and AE3, five Na(+)-coupled [Formula: see text] transporters NBCe1, NBCe2, NBCn1, NBCn2, and NDCBE, as well as "AE4" whose Na(+)-dependence remains controversial. The SLC4 [Formula: see text] transporters play critical roles in pH regulation and transepithelial movement of electrolytes with a broad range of demonstrated physiological relevances. Dysfunctions of these transporters are associated with a series of human diseases. During the past decades, tremendous amount of effort has been undertaken to investigate the topological organization of the SLC4 transporters in the plasma membrane. Based upon the proposed topology models, mutational and functional studies have identified important structural elements likely involved in the ion translocation by the SLC4 transporters. In the present article, we review the advances during the past decades in understanding the structure and function of the SLC4 transporters.

Glycosylation of solute carriers: mechanisms and functional consequences

Solute carriers (SLCs) are one of the largest groups of multi-spanning membrane proteins in mammals and include ubiquitously expressed proteins as well as proteins with highly restricted tissue expression. A vast number of studies have addressed the function and organization of SLCs as well as their posttranslational regulation, but only relatively little is known about the role of SLC glycosylation. Glycosylation is one of the most abundant posttranslational modifications of animal proteins and through recent advances in our understanding of protein-glycan interactions, the functional roles of SLC glycosylation are slowly emerging. The purpose of this review is to provide a concise overview of the aspects of glycobiology most relevant to SLCs, to discuss the roles of glycosylation in the regulation and function of SLCs, and to outline the major open questions in this field, which can now be addressed given major technical advances in this and related fields of study in recent years.

Cell Surface and Membrane Engineering: Emerging Technologies and Applications

Membranes constitute the interface between the basic unit of life—a single cell—and the outside environment and thus in many ways comprise the ultimate “functional biomaterial”. To perform the many and often conflicting functions required in this role, for example to partition intracellular contents from the outside environment while maintaining rapid intake of nutrients and efflux of waste products, biological membranes have evolved tremendous complexity and versatility. This article describes how membranes, mainly in the context of living cells, are increasingly being manipulated for practical purposes with drug discovery, biofuels, and biosensors providing specific, illustrative examples. Attention is also given to biology-inspired, but completely synthetic, membrane-based technologies that are being enabled by emerging methods such as bio-3D printers. The diverse set of applications covered in this article are intended to illustrate how these versatile technologies—as they rapidly mature—hold tremendous promise to benefit human health in numerous ways ranging from the development of new medicines to sensitive and cost-effective environmental monitoring for pathogens and pollutants to replacing hydrocarbon-based fossil fuels.

Mutations across murine hepatitis virus nsp4 alter virus fitness and membrane modifications

A common feature of infection by positive-sense RNA virus is the modification of host cell cytoplasmic membranes that serve as sites of viral RNA synthesis. Coronaviruses induce double-membrane vesicles (DMVs), but the role of DMVs in replication and virus fitness remains unclear. Coronaviruses encode 16 nonstructural proteins (nsps), three of which, nsp3, nsp4, and nsp6, are necessary and sufficient for DMV formation. It has been shown previously that mutations in murine hepatitis virus (MHV) nsp4 loop 1 that alter nsp4 glycosylation are associated with disrupted DMV formation and result in changes in virus replication and RNA synthesis. However, it is not known whether DMV morphology or another function of nsp4 glycosylation is responsible for effects on virus replication. In this study, we tested whether mutations across nsp4, both alone and in combination with mutations that abolish nsp4 glycosylation, affected DMV formation, replication, and fitness. Residues in nsp4 distinct from glycosylation sites, particularly in the endoplasmic reticulum (ER) luminal loop 1, independently disrupted both the number and morphology of DMVs and exacerbated DMV changes associated with loss of glycosylation. Mutations that altered DMV morphology but not glycosylation did not affect virus fitness while viruses lacking nsp4 glycosylation exhibited a loss in fitness. The results support the hypothesis that DMV morphology and numbers are not key determinants of virus fitness. The results also suggest that nsp4 glycosylation serves roles in replication in addition to the organization and stability of MHV-induced double-membrane vesicles.

IMPORTANCE

All positive-sense RNA viruses modify host cytoplasmic membranes for viral replication complex formation. Thus, defining the mechanisms of virus-induced membrane modifications is essential for both understanding virus replication and development of novel approaches to virus inhibition. Coronavirus-induced membrane changes include double-membrane vesicles (DMVs) and convoluted membranes. Three viral nonstructural proteins (nsps), nsp3, nsp4, and nsp6, are known to be required for DMV formation. It is unknown how these proteins induce membrane modification or which regions of the proteins are involved in DMV formation and stability. In this study, we show that mutations across nsp4 delay virus replication and disrupt DMV formation and that loss of nsp4 glycosylation is associated with a substantial fitness cost. These results support a critical role for nsp4 in DMV formation and virus fitness.

Differential roles of tryptophan residues in the functional expression of human anion exchanger 1 (AE1, Band 3, SLC4A1)

Anion exchanger 1 (AE1) is a 95 kDa glycoprotein that facilitates Cl(-)=HCO(-)(3) exchange across the erythrocyte plasma membrane. This transport activity resides in the 52 kDa C-terminal membrane domain (Gly(361)-Val(911)) predicted to span the membrane 14 times. To explore the role of tryptophan (Trp) residues in AE1 function, the seven endogenous Trp residues in the membrane domain were mutated individually to alanine (Ala) and phenylalanine (Phe). Expression levels, cell surface abundance, inhibitor binding and transport activities of the mutants were measured upon expression in HEK-293 cells. The seven Trp residues divided into three classes according the impact of mutations on the functional expression of AE1: Class 1, dramatically decreased expression (Trp(492) and Trp(496)); Class 2, decreased expression by Ala substitution but not Phe (Trp(648), Trp(662) and Trp(723)); and Class 3, normal expression (Trp(831) and Trp(848)). The results indicate that Trp residues play differential roles in AE1 expression and function depending on their location in the protein and that Trp mutants with low expression are misfolded and retained in the endoplasmic reticulum.

Role of the breast cancer resistance protein (BCRP/ABCG2) in drug transport--an update

The human breast cancer resistance protein (BCRP, gene symbol ABCG2) is an ATP-binding cassette (ABC) efflux transporter. It was so named because it was initially cloned from a multidrug-resistant breast cancer cell line where it was found to confer resistance to chemotherapeutic agents such as mitoxantrone and topotecan. Since its discovery in 1998, the substrates of BCRP have been rapidly expanding to include not only therapeutic agents but also physiological substances such as estrone-3-sulfate, 17β-estradiol 17-(β-D-glucuronide) and uric acid. Likewise, at least hundreds of BCRP inhibitors have been identified. Among normal human tissues, BCRP is highly expressed on the apical membranes of the placental syncytiotrophoblasts, the intestinal epithelium, the liver hepatocytes, the endothelial cells of brain microvessels, and the renal proximal tubular cells, contributing to the absorption, distribution, and elimination of drugs and endogenous compounds as well as tissue protection against xenobiotic exposure. As a result, BCRP has now been recognized by the FDA to be one of the key drug transporters involved in clinically relevant drug disposition. We published a highly-accessed review article on BCRP in 2005, and much progress has been made since then. In this review, we provide an update of current knowledge on basic biochemistry and pharmacological functions of BCRP as well as its relevance to drug resistance and drug disposition.

HMMpTM: improving transmembrane protein topology prediction using phosphorylation and glycosylation site prediction

During the last two decades a large number of computational methods have been developed for predicting transmembrane protein topology. Current predictors rely on topogenic signals in the protein sequence, such as the distribution of positively charged residues in extra-membrane loops and the existence of N-terminal signals. However, phosphorylation and glycosylation are post-translational modifications (PTMs) that occur in a compartment-specific manner and therefore the presence of a phosphorylation or glycosylation site in a transmembrane protein provides topological information. We examine the combination of phosphorylation and glycosylation site prediction with transmembrane protein topology prediction. We report the development of a Hidden Markov Model based method, capable of predicting the topology of transmembrane proteins and the existence of kinase specific phosphorylation and N/O-linked glycosylation sites along the protein sequence. Our method integrates a novel feature in transmembrane protein topology prediction, which results in improved performance for topology prediction and reliable prediction of phosphorylation and glycosylation sites. The method is freely available at http://bioinformatics.biol.uoa.gr/HMMpTM.

Structural model of the anion exchanger 1 (SLC4A1) and identification of transmembrane segments forming the transport site

The anion exchanger 1 (AE1), a member of bicarbonate transporter family SLC4, mediates an electroneutral chloride/bicarbonate exchange in physiological conditions. However, some point mutations in AE1 membrane-spanning domain convert the electroneutral anion exchanger into a Na(+) and K(+) conductance or induce a cation leak in a still functional anion exchanger. The molecular determinants that govern ion movement through this transporter are still unknown. The present study was intended to identify the ion translocation pathway within AE1. In the absence of a resolutive three-dimensional structure of AE1 membrane-spanning domain, in silico modeling combined with site-directed mutagenesis experiments was done. A structural model of AE1 membrane-spanning domain is proposed, and this model is based on the structure of a uracil-proton symporter. This model was used to design cysteine-scanning mutagenesis on transmembrane (TM) segments 3 and 5. By measuring AE1 anion exchange activity or cation leak, it is proposed that there is a unique transport site comprising TM3-5 and TM8 that should function as an anion exchanger and a cation leak.

Mechanisms of CFTR Folding at the Endoplasmic Reticulum

In the past decade much has been learned about how Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) folds and misfolds as the etiologic cause of cystic fibrosis (CF). CFTR folding is complex and hierarchical, takes place in multiple cellular compartments and physical environments, and involves several large networks of folding machineries. Insertion of transmembrane (TM) segments into the endoplasmic reticulum (ER) membrane and tertiary folding of cytosolic domains begin cotranslationally as the nascent polypeptide emerges from the ribosome, whereas posttranslational folding establishes critical domain-domain contacts needed to form a physiologically stable structure. Within the membrane, N- and C-terminal TM helices are sorted into bundles that project from the cytosol to form docking sites for nucleotide binding domains, NBD1 and NBD2, which in turn form a sandwich dimer for ATP binding. While tertiary folding is required for domain assembly, proper domain assembly also reciprocally affects folding of individual domains analogous to a jig-saw puzzle wherein the structure of each interlocking piece influences its neighbors. Superimposed on this process is an elaborate proteostatic network of cellular chaperones and folding machineries that facilitate the timing and coordination of specific folding steps in and across the ER membrane. While the details of this process require further refinement, we finally have a useful framework to understand key folding defect(s) caused by ΔF508 that provides a molecular target(s) for the next generation of CFTR small molecule correctors aimed at the specific defect present in the majority of CF patients.

Aberrant substrate engagement of the ER translocon triggers degradation by the Hrd1 ubiquitin ligase

The Hrd1 ubiquitin ligase plays a role in quality control of two substrates associated with the Sec61 translocon.

Little is known about quality control of proteins that aberrantly or persistently engage the endoplasmic reticulum (ER)-localized translocon en route to membrane localization or the secretory pathway. Hrd1 and Doa10, the primary ubiquitin ligases that function in ER-associated degradation (ERAD) in yeast, target distinct subsets of misfolded or otherwise abnormal proteins based primarily on degradation signal (degron) location. We report the surprising observation that fusing Deg1, a cytoplasmic degron normally recognized by Doa10, to the Sec62 membrane protein rendered the protein a Hrd1 substrate. Hrd1-dependent degradation occurred when Deg1-Sec62 aberrantly engaged the Sec61 translocon channel and underwent topological rearrangement. Mutations that prevent translocon engagement caused a reversion to Doa10-dependent degradation. Similarly, a variant of apolipoprotein B, a protein known to be cotranslocationally targeted for proteasomal degradation, was also a Hrd1 substrate. Hrd1 therefore likely plays a general role in targeting proteins that persistently associate with and potentially obstruct the translocon.

Understanding integration of α-helical membrane proteins: the next steps

Integration of a protein into the endoplasmic reticulum (ER) membrane occurs through a series of multistep reactions that include targeting of ribosome-nascent polypeptide complexes to the ER, attachment of the ribosome to the protein translocation channel, lateral partitioning of α-helical transmembrane spans into the lipid bilayer, and folding of the lumenal, cytosolic and membrane-embedded domains of the protein. However, the molecular mechanisms and kinetics of these steps are still not entirely clear. To obtain a better understanding of the mechanism of membrane protein integration, we propose that it will be important to utilize in vivo experiments to examine the kinetics of membrane protein integration and in vitro experiments to characterize interactions between nascent membrane proteins, protein translocation factors and molecular chaperones.

The oligosaccharyltransferase subunits OST48, DAD1 and KCP2 function as ubiquitous and selective modulators of mammalian N-glycosylation

Protein N-glycosylation is an essential modification that occurs in all eukaryotes and is catalysed by the oligosaccharyltransferase (OST) in the endoplasmic reticulum. Comparative studies have clearly shown that eukaryotic STT3 proteins alone can fulfil the enzymatic requirements for N-glycosylation, yet in many cases STT3 homologues form stable complexes with a variety of non-catalytic OST subunits. Whereas some of these additional components might play a structural role, others appear to increase or modulate N-glycosylation efficiency for certain precursors. Here, we have analysed the roles of three non-catalytic mammalian OST components by studying the consequences of subunit-specific knockdowns on the stability and enzymatic activity of the OST complex. Our results demonstrate that OST48 and DAD1 are required for the assembly of both STT3A- and STT3B-containing OST complexes. The structural perturbations of these complexes we observe in OST48- and DAD1-depleted cells underlie their pronounced hypoglycosylation phenotypes. Thus, OST48 and DAD1 are global modulators of OST stability and hence N-glycosylation. We show that KCP2 also influences protein N-glycosylation, yet in this case, the effect of its depletion is substrate specific, and is characterised by the accumulation of a novel STT3A-containing OST subcomplex. Our results suggest that KCP2 acts to selectively enhance the OST-dependent processing of specific protein precursors, most likely co-translational substrates of STT3A-containing complexes, highlighting the potential for increased complexity of OST subunit composition in higher eukaryotes.

Orientational preferences of neighboring helices can drive ER insertion of a marginally hydrophobic transmembrane helix

α-helical integral membrane proteins critically depend on the correct insertion of their transmembrane α helices into the lipid bilayer for proper folding, yet a surprisingly large fraction of the transmembrane α helices in multispanning integral membrane proteins are not sufficiently hydrophobic to insert into the target membrane by themselves. How can such marginally hydrophobic segments nevertheless form transmembrane helices in the folded structure? Here, we show that a transmembrane helix with a strong orientational preference (N(cyt)-C(lum) or N(lum)-C(cyt)) can both increase and decrease the hydrophobicity threshold for membrane insertion of a neighboring, marginally hydrophobic helix. This effect helps explain the "missing hydrophobicity" in polytopic membrane proteins.

Positive charges on the translocating polypeptide chain arrest movement through the translocon

Polypeptide chains synthesized by membrane-bound ribosomes are translocated through, and integrated into, the endoplasmic reticulum (ER) membrane by means of the protein translocation channel, the translocon. Positive charges on the nascent chain determine the orientation of the hydrophobic segment as it is inserted into the translocon and enhance the stop-translocation of translocating hydrophobic segments. Here we show that positive charges temporarily arrested ongoing polypeptide chain movement through the ER translocon by electrostatic interaction, even in the absence of a hydrophobic segment. The C-terminus of the polypeptide chain was elongated during the arrest, and then the full-length polypeptide chain moved through the translocon. The translocation-arrested polypeptide was not anchored to the membrane and the charges were on the cytoplasmic side of the membrane. The arrest effect was prevented by negatively charged residues inserted into the positive-charge cluster, and it was also suppressed by high salt conditions. We propose that positive charges are independent translocation regulators that are more active than previously believed.

Alteration of CFTR transmembrane span integration by disease-causing mutations

Many missense mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR) result in its misfolding, endoplasmic reticulum (ER) accumulation, and, thus, cystic fibrosis. A number of these mutations are located in the predicted CFTR transmembrane (TM) spans and have been projected to alter span integration. However, the boundaries of the spans have not been precisely defined experimentally. In this study, the ER luminal integration profiles of TM1 and TM2 were determined using the ER glycosylation machinery, and the effects of the CF-causing mutations G85E and G91R thereon were assessed. The mutations either destabilize the integrated conformation or alter the TM1 ER integration profile. G85E misfolding is based in TM1 destabilization by glutamic acid and loss of glycine and correlates with the temperature-insensitive ER accumulation of immature full-length CFTR harboring the mutation. By contrast, temperature-dependent misfolding owing to the G91R mutation depends on the introduction of the basic side chain rather than the loss of the glycine. This work demonstrates that CF-causing mutations predicted to have similar effects on CFTR structure actually result in disparate molecular perturbations that underlie ER accumulation and the pathology of CF.

Evidence against extracellular exposure of a highly immunogenic region in the C-terminal domain of the simian immunodeficiency virus gp41 transmembrane protein

The generally accepted model for human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein topology includes a single membrane-spanning domain. An alternate model has been proposed which features multiple membrane-spanning domains. Consistent with the alternate model, a high percentage of HIV-1-infected individuals produce unusually robust antibody responses to a region of envelope, the so-called "Kennedy epitope," that in the conventional model should be in the cytoplasm. Here we show analogous, robust antibody responses in simian immunodeficiency virus SIVmac239-infected rhesus macaques to a region of SIVmac239 envelope located in the C-terminal domain, which in the conventional model should be inside the cell. Sera from SIV-infected rhesus macaques consistently reacted with overlapping oligopeptides corresponding to a region located within the cytoplasmic domain of gp41 by the generally accepted model, at intensities comparable to those observed for immunodominant areas of the surface component gp120. Rabbit serum raised against this highly immunogenic region (HIR) reacted with SIV envelope in cell surface-staining experiments, as did monoclonal anti-HIR antibodies isolated from an SIVmac239-infected rhesus macaque. However, control experiments demonstrated that this surface staining could be explained in whole or in part by the release of envelope protein from expressing cells into the supernatant and the subsequent attachment to the surfaces of cells in the culture. Serum and monoclonal antibodies directed against the HIR failed to neutralize even the highly neutralization-sensitive strain SIVmac316. Furthermore, a potential N-linked glycosylation site located close to the HIR and postulated to be outside the cell in the alternate model was not glycosylated. An artificially introduced glycosylation site within the HIR was also not utilized for glycosylation. Together, these data support the conventional model of SIV envelope as a type Ia transmembrane protein with a single membrane-spanning domain and without any extracellular loops.

Topology models of anion exchanger 1 that incorporate the anti-parallel V-shaped motifs found in the EM structureThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process

We recently published the three-dimensional structure of the membrane domain of human erythrocyte anion exchanger 1 (AE1) at 7.5 Å resolution, solved by electron crystallography. The structure exhibited distinctive anti-parallel V-shaped motifs, which protrude from the membrane bilayer on both sides. Similar motifs exist in the previously reported structure of a bacterial chloride channel (ClC)-type protein. Here, we propose two topology models of AE1 that reflect the anti-parallel V-shaped structural motifs. One is assumed to have structural similarity with the ClC protein and the other is only assumed to have internal repeats, as is often the case with transporters. Both models are consistent with most topological results reported thus far for AE1, each having advantages and disadvantages.

A sugar chain at a specific position in the nascent polypeptide chain induces forward movement during translocation through the translocon

Nascent polypeptide chains synthesized by membrane bound ribosomes are cotranslationally translocated through and integrated into the endoplasmic reticulum translocon. Hydrophobic segments and positive charges on the chain are critical to halt the ongoing translocation. A marginally hydrophobic segment, which cannot be inserted into the membrane by itself, can be a transmembrane segment depending on its downstream positive charges. In certain conditions, positive charges even 60 residues downstream cause the marginally hydrophobic segment to span the membrane by inducing the segment to slide back from the lumen. Here we systematically examined the effect of a core sugar chain on the fate of a marginally hydrophobic segment using a cell-free translation and translocation system. A sugar chain added within 12 residues upstream of the marginally hydrophobic segment prevents the sliding back and promotes forward movement of the polypeptide chain. The sugar chain apparently functions as a ratchet to keep the polypeptide chain in the lumen. We propose that the sugar chain is a third topology determinant of membrane proteins, in addition to a hydrophobic segment and positive charges of the nascent chain.

Molecular characterisation of ‘transmembrane protein 192’ (TMEM192), a novel protein of the lysosomal membrane

Transmembrane protein 192 (TMEM192) has been previously identified in proteomic analyses of lysosomal membranes. TMEM192 does not exhibit any significant homology to known protein families and possesses four potential transmembrane segments. To approach the molecular role of TMEM192, a detailed biochemical characterisation of this protein was performed. Expression constructs of fusion proteins containing TMEM192 and appended epitope tags were constructed. In HeLa cells these proteins were detected in membranes of lysosomes/late endosomes. To examine endogenous TMEM192, a TMEM192-specific antibody was generated and validated. With this antibody colocalisation of endogenous TMEM192 with lysosomal and late endosomal markers was demonstrated. Using Percoll density gradient centrifugation and immunoblotting, co-sedimentation of major portions of both TMEM192 and the lysosomal proteins LAMP-2 and cathepsin D into high-density fractions was observed. Interestingly, in contrast to many other lysosomal proteins no N-glycosylation of TMEM192 could be detected. Western blotting of reduced and non-reduced samples and co-immunoprecipitation experiments indicated TMEM192 to be a homodimer with one or more interchain disulphide bridges. TMEM192 was found to be strongly expressed in human kidney, liver, lung and pancreas tissue. The widespread tissue distribution could suggest an important role of TMEM192 for lysosomal function.

Positive charges of translocating polypeptide chain retrieve an upstream marginal hydrophobic segment from the endoplasmic reticulum lumen to the translocon

Positive charges of nascent chain facilitate membrane spanning of a marginally hydrophobic segment, even when separated by 70 residues from the segment. The segment is exposed to the lumen and then slides back into the membrane. They not only fix the hydrophobic segment in the membrane, but exert a much more dynamic action than previously realized.

Positively charged amino acid residues are well recognized topology determinants of membrane proteins. They contribute to the stop-translocation of a polypeptide translocating through the translocon and to determine the orientation of signal sequences penetrating the membrane. Here we analyzed the function of these positively charged residues during stop-translocation in vitro. Surprisingly, the positive charges facilitated membrane spanning of a marginally hydrophobic segment, even when separated from the hydrophobic segment by 70 residues. In this case, the hydrophobic segment was exposed to the lumen, and then the downstream positive charges triggered the segment to slide back into the membrane. The marginally hydrophobic segment spanned the membrane, but maintained access to the water environment. The positive charges not only fix the hydrophobic segment in the membrane at its flanking position, but also have a much more dynamic action than previously realized.

Structure of the membrane domain of human erythrocyte anion exchanger 1 revealed by electron crystallography

The membrane domain of human erythrocyte anion exchanger 1 (AE1) works as a Cl(-)/HCO(3)(-) antiporter. This exchange is a key step for CO(2)/O(2) circulation in the blood. In spite of their importance, structural information about AE1 and the AE (anion exchanger) family are still very limited. We used electron microscopy to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking, at 7.5-A resolution. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of the ClC chloride channel, a Cl(-)/H(+) antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into the membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of the ClC transporter (helices B+C and J+K), are well fitted to those AE1 densities after simple domain movement.

Helical image reconstruction of the outward-open human erythrocyte band 3 membrane domain in tubular crystals

The C-terminal membrane domain of erythrocyte band 3 functions as an anion exchanger. Here, we report the three-dimensional (3D) structure of the membrane domain in an inhibitor-stabilized, outward-open conformation at 18A resolution. Unstained, frozen-hydrated tubular crystals containing the membrane domain of band 3 purified from human red blood cells (hB3MD) were examined using cryo-electron microscopy and iterative helical real-space reconstruction (IHRSR). The 3D image reconstruction of the tubular crystals showed the molecular packing of hB3MD dimers with dimensions of 60 x 110 A in the membrane plane and a thickness of 70A across the membrane. Immunoelectron microscopy and carboxyl-terminal digestion demonstrated that the intracellular surface of hB3MD was exposed on the outer surface of the tubular crystal. A 3D density map revealed that hB3MD consists of at least two subdomains and that the outward-open form is characterized by a large hollow area on the extracellular surface and continuous density on the intracellular surface.

Photocleavage-based affinity purification and printing of cell-free expressed proteins: application to proteome microarrays

Proteome microarrays hold great promise for various biotechnological and biomedical applications, including mapping protein-protein interactions, drug discovery, and biomarker discovery. However, the need to express, purify, and print thousands of functional proteins at high density on a microarray substrate presents challenges that limit their widespread availability and use. We report the development of new methods, based on photocleavage, for the purification and printing of nascent proteins. Photocleavable biotin (PC-biotin) is incorporated into nascent proteins by misaminoacylated transfer RNAs (tRNAs) used in a coupled transcription/translation rabbit reticulocyte cell-free expression system. Proteins were affinity isolated onto (strept)avidin-coated beads and then photoreleased (PC-SNAG). Compared with polyhistidine tag-based affinity purification, PC-SNAG provided a higher purity yet a comparable yield using a glutathione-S-transferase (GST) test protein. Antibody-mediated PC-SNAG is also demonstrated. PC-SNAG proteins were found to exhibit native enzymatic activity and were suitable for the printing of ordered protein microarrays used in protein-protein interaction assays. Alternatively, when beads carrying photocleavably tethered proteins were placed in close proximity to an activated planar surface and then illuminated, proteins were transferred directly to the surface (PC-PRINT) to form discrete spots whose dimensions match those of the beads. PC-PRINT can provide an inexpensive method to fabricate very large-scale, high-density proteome microarrays. Moreover, transferring the proteins off the beads significantly reduces background autofluorescence observed with common bead types. To decode nascent proteins that are deposited by PC-PRINT from individual beads, the feasibility of using photocleavable quantum dot codes is demonstrated.

Molecular code for protein insertion in the endoplasmic reticulum membrane is similar for N(in)-C(out) and N(out)-C(in) transmembrane helices

Transmembrane alpha-helices in integral membrane proteins can have two orientations in the membrane: N(in)-C(out) or N(out)-C(in). Previous studies of model N(out)-C(in) transmembrane segment have led to a detailed, quantitative picture of the "molecular code" that relates amino acid sequence to membrane insertion efficiency in vivo [Hessa T, et al. (2007) Molecular code for transmembrane helix recognition by the Sec61 translocon. Nature 450:1026-1030], but whether the same code applies also to N(in)-C(out) transmembrane helices is unknown. Here, we show that the contributions of individual amino acids to the overall efficiency of membrane insertion are similar for the two kinds of helices and that the threshold hydrophobicity for membrane insertion can be up to approximately 1 kcal/mol lower for N(in)-C(out) compared with N(out)-C(in) transmembrane helices, depending on the neighboring helices.

Membrane topology and post-translational modification of the Saccharomyces cerevisiae essential protein Rot1

ROT1 is an essential gene that has been related to cell wall biosynthesis, the actin cytoskeleton and protein folding. In order to help to understand its molecular function, we carried out a characterization of the Rot1 protein. It is primarily located at the endoplasmic reticulum-nuclear membrane facing the lumen. Rot1 migrates more slowly than expected, which might suggest post-translational modification. Our results indicate that Rot1 is a protein that is neither GPI-anchored nor O-glycosylated. In contrast, it is N-glycosylated. By a directed mutagenesis of several Asn residues, we identified that the protein is simultaneously glycosylated at N103, N107 and N139. Although the mutation of these three N sites is not lethal, cellular growth is impaired. Sequence analysis predicts a transmembrane domain at the C-terminus. This fragment affects neither the targeting of the Rot1 protein to the ER nor its N-glycosylation, although it is important for the anchoring of the protein to the membrane and for its functionality. The existence of a signal sequence at the N-terminus has been suggested. However, deletion of this fragment impedes neither translocation to the ER nor N-glycosylation, but it is required for cell viability. Finally, we found that Rot1 is translocated to the ER by an SRP-independent post-translational mechanism which depends on Sec62.

Membrane topology of the Drosophila OR83b odorant receptor

By analogy to mammals, odorant receptors (ORs) in insects, such as Drosophila melanogaster, have long been thought to belong to the G-protein coupled receptor (GPCR) superfamily. However, recent work has cast doubt on this assumption and has tentatively suggested an inverted topology compared to the canonical N(out) - C(in) 7 transmembrane (TM) GPCR topology, at least for some Drosophila ORs. Here, we report a detailed topology mapping of the Drosophila OR83b receptor using engineered glycosylation sites as topology markers. Our results are inconsistent with a classical GPCR topology and show that OR83b has an intracellular N-terminus, an extracellular C-terminus, and 7TM helices.

Inefficient glycosylation leads to high steady-state levels of actively degrading cardiac triadin-1

In junctional sarcoplasmic reticulum, binding to cardiac triadin-1 provides a mechanism by which the Ca(2+)-release channel/ryanodine receptor may link with calsequestrin to regulate Ca(2+) release. Calsequestrin and triadin-1 both contain N-linked glycans, but about half of triadin-1 in the heart remains unglycosylated. To investigate mechanisms for this incomplete glycosylation, we overexpressed triadin-1 as a series of glycoform variants in non-muscle cell lines and neonatal heart cells using plasmid and adenoviral vectors. We showed that the characteristic incomplete glycosylation stemmed from properties of the glycosylation sequence that are conserved among triadin splice variants, including the close proximity of Asn(75) to the sarcoplasmic reticulum inner membrane. Although triadin-1 appeared by SDS-PAGE analysis as a 35/40-kDa doublet in all cells, variations occurred in the relative levels of the two glycoforms depending on the cell type and whether overexpression involved a plasmid or adenoviral vector. Treatment of triadin-1 with the proteasome inhibitor MG-132 led to striking changes in the relative levels of triadin-1 that indicated active breakdown of unglycosylated, but not glycosylated, triadin-1. Besides substantial increases in the relative levels of unglycosylated triadin-1, proteasome inhibition led to an accumulation of two new modified forms of triadin-1 that were seen with triadin-1 only when it is not glycosylated on Asn(75). Effects of tunicamycin and endoglycosidase H confirmed that these novel isoforms represent two alternative N-linked glycosylation sites, indicating that an alternative topology occurs infrequently leading to yet other glycoforms with short half-lives.

Structure-function relationships of the NCKX2 Na+/Ca2+-K+ exchanger

K+-dependent Na+/Ca2+ exchangers (NCKX) have been shown to play important roles in physiological processes as diverse as phototransduction in rod photoreceptors, motor learning and memory in mice, and skin pigmentation in humans. Most structure-function studies on NCKX proteins have been carried out on the NCKX2 isoform, but sequence similarity suggests that the results obtained with the NCKX2 isoform are likely to apply to all NCKX1-5 members of the human SLC24 gene family. Here we review our recent work on the NCKX2 protein concerning the topological arrangement of transmembrane segments carrying out cation transport, and concerning residues important for transport function and cation binding.

Dual topology of the processed hepatitis C virus protein NS4B is influenced by the NS5A protein

Among the least-known hepatitis C virus proteins is the non-structural protein 4B (NS4B). It localizes to the endoplasmic reticulum (ER) membrane and induces membrane changes, resulting in a membranous web that is reported to be the locale for virus replication. A model was presented previously for the topology of recombinant HCV NS4B of the 1a genotype based on in vitro data. In this model, the N-terminal tail of a considerable fraction of the NS4B molecules was translocated into the ER lumen via a post-translational process, giving the protein a dual transmembrane topology. It is now reported that translocation of the N terminus also occurs for processed NS4B expressed in cells in the context of the polyprotein. In the presence of NS5A, however, a lower degree of translocation was observed, which may indicate that NS5A influences the topology of NS4B. In vitro expression studies of NS4B from all major genotypes demonstrated that translocation of the N terminus to the ER lumen is conserved across genotypes. This clearly suggests an important function for this feature. Furthermore, when disrupting a previously reported amphipathic helix (AH) in the N terminus of NS4B, translocation was inhibited. As a disrupted AH also abolished the ability of NS4B to rearrange membranes, these data indicate for the first time an association between translocation of the N terminus and membrane rearrangement. Finally, the present experiments also confirm the predicted location of the first luminal loop to be around aa 112.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

Mutations in band 3 and cation leaky red cells

We have recently shown that amino acid substitutions in the membrane domain of band 3 (anion exchanger 1, SLC4A1) are associated with hereditary stomatocytosis (HSt), a red cell condition in which the cells leak sodium and potassium ions. These substitutions appear to convert band 3 from an anion exchanger into a cation channel. In this review, I will first give some background on the structure and function of normal band 3 and describe our findings in red cells from HSt patients. Then I will compare the properties of the HSt band 3 to those of Southeast Asian Ovalocytosis (SAO) band 3 and discuss the implications for the structure of band 3, the quality control of protein expression in red cells and the cation permeability of normal human red cells.