Reliability of nine programs of topological predictions and their application to integral membrane channel and carrier proteins

Stabilization and structure determination of integral membrane proteins by termini restraining

Integral membrane proteins isolated from cellular environment often lose activity and native conformation required for functional analyses and structural studies. Even in their native state, they lack sufficient surfaces to form crystal contacts. Furthermore, most of them are too small for cryogenic electron microscopy detection and too big for solution NMR. To overcome these difficulties, we recently developed a strategy to stabilize the folded state of membrane proteins by restraining their two termini with a self-assembling protein coupler. The termini-restrained membrane proteins from distinct functional families retain their activities and show increased stability and yield. This strategy enables their structure determination at near-atomic resolution by facilitating the entire pipeline from crystallization, crystal identification, diffraction enhancement and phase determination, to electron density improvement. Furthermore, stabilization of membrane proteins enables their biochemical and biophysical characterization. Here we present the protocol of membrane protein engineering (2 weeks), quality assessment (1-2 weeks), protein production (1-6 weeks), crystallization (1-2 weeks), diffraction improvement (1-3 months) and crystallographic data analysis (1 week). This protocol is intended not only for structural biologists, but also for biochemists, biophysicists and pharmaceutical scientists whose research focuses on membrane proteins.

Generation of a Polyclonal Antibody against the Mouse Metal Transporter ZIP8

ZIP8 is a newly identified metal transporter. In human patients, mutations in ZIP8 result in severe manganese deficiency, suggesting a critical role for ZIP8 in regulating systemic manganese homeostasis. In mice, the deletion of ZIP8 recapitulates the symptoms of patients with ZIP8 mutations. However, further studies using mouse models to examine ZIP8′s function were hindered by the lack of suitable antibodies to detect endogenous ZIP8 protein. In this study, we report the design, generation, and validation of a polyclonal antibody against mouse ZIP8. We have demonstrated that the newly generated antibody can be reliably used in immunoblotting analysis to detect endogenous ZIP8 protein in mouse tissues. The successful generation and validation of anti-mouse ZIP8 antibody provide opportunities to further examine the function and regulation of this metal transporter. In addition, our study may provide valuable insights into the future development of antibodies targeting polytopic membrane proteins.

The Hepatitis B Virus Envelope Proteins: Molecular Gymnastics Throughout the Viral Life Cycle

New hepatitis B virions released from infected hepatocytes are the result of an intricate maturation process that starts with the formation of the nucleocapsid providing a confined space where the viral DNA genome is synthesized via reverse transcription. Virion assembly is finalized by the enclosure of the icosahedral nucleocapsid within a heterogeneous envelope. The latter contains integral membrane proteins of three sizes, collectively known as hepatitis B surface antigen, and adopts multiple conformations in the course of the viral life cycle. The nucleocapsid conformation depends on the reverse transcription status of the genome, which in turn controls nucleocapsid interaction with the envelope proteins for virus exit. In addition, after secretion the virions undergo a distinct maturation step during which a topological switch of the large envelope protein confers infectivity. Here we review molecular determinants for envelopment and models that postulate molecular signals encoded in the capsid scaffold conducive or adverse to the recruitment of envelope proteins.

Expansion of the Major Facilitator Superfamily (MFS) to include novel transporters as well as transmembrane-acting enzymes

The Major Facilitator Superfamily (MFS) is currently the largest characterized superfamily of transmembrane secondary transport proteins. Its diverse members are found in essentially all organisms in the biosphere and function by uniport, symport, and/or antiport mechanisms. In 1993 we first named and described the MFS which then consisted of 5 previously known families that had not been known to be related, and by 2012 we had identified a total of 74 families, classified phylogenetically within the MFS, all of which included only transport proteins. This superfamily has since expanded to 89 families, all included under TC# 2.A.1, and a few transporter families outside of TC# 2.A.1 were identified as members of the MFS. In this study, we assign nine previously unclassified protein families in the Transporter Classification Database (TCDB; http://www.tcdb.org) to the MFS based on multiple criteria and bioinformatic methodologies. In addition, we find integral membrane domains distantly related to partial or full-length MFS permeases in Lysyl tRNA Synthases (TC# 9.B.111), Lysylphosphatidyl Glycerol Synthases (TC# 4.H.1), and cytochrome b₅₆₁ transmembrane electron carriers (TC# 5.B.2). Sequence alignments, overlap of hydropathy plots, compatibility of repeat units, similarity of complexity profiles of transmembrane segments, shared protein domains and 3D structural similarities between transport proteins were analyzed to assist in inferring homology. The MFS now includes 105 families.

Peptides Derived of Kunitz-Type Serine Protease Inhibitor as Potential Vaccine Against Experimental Schistosomiasis

Schistosomiasis is a significant public health problem in sub-Saharan Africa, China, Southeast Asia, and regions of South and Central America affecting about 189 million people. Kunitz-type serine protease inhibitors have been identified as important players in the interaction of other flatworm parasites with their mammalian hosts. They are involved in host blood coagulation, fibrinolysis, inflammation, and ion channel blocking, all of them critical biological processes, which make them interesting targets to develop a vaccine. Here, we evaluate the protective efficacy of chemically synthesized T- and B-cell peptide epitopes derived from a kunitz protein from Schistosoma mansoni. Putative kunitz-type protease inhibitor proteins were identified in the S. mansoni genome, and their expression was analyzed by RNA-seq. Gene expression analyses showed that the kunitz protein Smp_147730 (Syn. Smp_311670) was dramatically and significantly up-regulated in schistosomula and adult worms when compared to the invading cercariae. T- and B-cell epitopes were predicted using bioinformatics tools, chemically synthesized, and formulated in the Adjuvant Adaptation (ADAD) vaccination system. BALB/c mice were vaccinated and challenged with S. mansoni cercariae. Kunitz peptides were highly protective in vaccinated BALB/c mice showing significant reductions in recovery of adult females (89-91%) and in the numbers of eggs trapped in the livers (77-81%) and guts (57-77%) of mice. Moreover, liver lesions were significantly reduced in vaccinated mice (64-65%) compared to infected control mice. The vaccination regime was well-tolerated with both peptides. We propose the use of these peptides, alone or in combination, as reliable candidates for vaccination against schistosomiasis.

TcNST2encodes a Golgi-localized UDP-galactose transporter inTrypanosoma cruzi

Survival and infectivity of trypanosomatids rely on cell-surface and secreted glycoconjugates, many of which contain a variable number of galactose residues. Incorporation of galactose to proteins and lipids occurs along the secretory pathway from UDP-galactose (UDP-Gal). Before being used in glycosylation reactions, however, this activated sugar donor must first be transported across the endoplasmic reticulum and Golgi membranes by a specific nucleotide sugar transporter (NST). In this study, we identified an UDP-Gal transporter (named TcNST2 and encoded by the TcCLB.504085.60 gene) fromTrypanosoma cruzi, the etiological agent of Chagas disease. TcNST2 was identified by heterologous expression of selected putative nucleotide sugar transporters in a mutant Chinese Hamster Ovary cell line.TcNST2mRNA levels were detected in allT. cruzilife-cycle forms, with an increase in expression in axenic amastigotes. Confocal microscope analysis indicated that the transporter is specifically localized to the Golgi apparatus. A three-dimensional model of TcNST2 suggested an overall structural conservation as compared with members of the metabolite transporter superfamily and also suggested specific features that could be related to its activity. The identification of this transporter is an important step toward a better understanding of glycoconjugate biosynthesis and the role NSTs play in this process in trypanosomatids.

Brettanomyces bruxellensis SSU1 Haplotypes Confer Different Levels of Sulfite Tolerance When Expressed in a Saccharomyces cerevisiae SSU1 Null Mutant

The addition of SO₂ is practiced in the wine industry to mitigate the risk of microbial spoilage and to extend wine shelf-life. Generally, this strategy does not interfere with primary alcoholic fermentation, as wine strains of Saccharomyces cerevisiae exhibit significant SO₂ tolerance, largely driven by the efflux pump Ssu1p. One of the key yeast species responsible for wine spoilage is Brettanomyces bruxellensis, which also exhibits strain-dependent SO₂ tolerance, although this occurs via unknown mechanisms. To evaluate the factors responsible for the differential sulfite tolerance observed in B. bruxellensis strains, we employed a multifaceted approach to examine both expression and allelic differences in the BbSSU1 gene. Transcriptomic analysis following exposure to SO₂ highlighted different inducible responses in two B. bruxellensis strains. It also revealed disproportionate transcription of one putative BbSSU1 haplotype in both genetic backgrounds. Here, we confirm the functionality of BbSSU1 by complementation of a null mutant in a S. cerevisiae wine strain. The expression of four distinct BbSSU1 haplotypes in the S. cerevisiae ΔSSU1 mutant revealed up to a 3-fold difference in conferred SO₂ tolerance. Substitution of key amino acids distinguishing the encoded proteins was performed to evaluate their relative contribution to SO₂ tolerance. Protein modeling of two haplotypes which differed in two amino acid residues suggested that these substitutions affect the binding of Ssu1p ligands near the channel opening. Taken together, preferential transcription of a BbSSU1 allele that encodes a more efficient Ssu1p transporter may represent one mechanism that contributes to differences in sulfite tolerances between B. bruxellensis strains.IMPORTANCE Brettanomyces bruxellensis is one of the most important wine spoilage microorganisms, with the use of sulfite being the major method to control spoilage. However, this species displays a wide intraspecies distribution in sulfite tolerance, with some strains capable of tolerating high concentrations of SO₂, with relatively high concentrations of this antimicrobial needed for their control. Although SO₂ tolerance has been studied in several organisms and particularly in S. cerevisiae, little is known about the mechanisms that confer SO₂ tolerance in B. bruxellensis Here, we confirmed the functionality of the sulfite efflux pump encoded by BbSSU1 and determined the efficiencies of four different BbSSU1 haplotypes. Gene expression analysis showed greater expression of the haplotype conferring greater SO₂ tolerance. Our results suggest that a combination of BbSSU1 haplotype efficiency, copy number, and haplotype expression levels likely contributes to the diverse SO₂ tolerances observed for different B. bruxellensis strains.

Comparative genomic analyses of transport proteins encoded within the red algae Chondrus crispus, Galdieria sulphuraria, and Cyanidioschyzon merolae11

Galdieria sulphuraria and Cyanidioschyzon merolae are thermo-acidophilic unicellular red algal cousins capable of living in volcanic environments, although the former can additionally thrive in the presence of toxic heavy metals. Bioinformatic analyses of transport systems were carried out on their genomes, as well as that of the mesophilic multicellular red alga Chondrus crispus (Irish moss). We identified transport proteins related to the metabolic capabilities, physiological properties, and environmental adaptations of these organisms. Of note is the vast array of transporters encoded in G. sulphuraria capable of importing a variety of carbon sources, particularly sugars and amino acids, while C. merolae and C. crispus have relatively few such proteins. Chondrus crispus may prefer short chain acids to sugars and amino acids. In addition, the number of encoded proteins pertaining to heavy metal ion transport is highest in G. sulphuraria and lowest in C. crispus. All three organisms preferentially utilize secondary carriers over primary active transporters, suggesting that their primary source of energy derives from electron flow rather than substrate-level phosphorylation. Surprisingly, the percentage of inorganic ion transporters encoded in C. merolae more closely resembles that of C. crispus than G. sulphuraria, but only C. crispus appears to signal via voltage-gated cation channels and possess a Na⁺ /K⁺ -ATPase and a Na⁺ exporting pyrophosphatase. The results presented in this report further our understanding of the metabolic potential and toxic compound resistances of these three organisms.

Genome-Wide Semi-Automated Annotation of Transporter Systems

Usually, transport reactions are added to genome-scale metabolic models (GSMMs) based on experimental data and literature. This approach does not allow associating specific genes with transport reactions, which impairs the ability of the model to predict effects of gene deletions. Novel methods for systematic genome-wide transporter functional annotation and their integration into GSMMs are therefore necessary. In this work, an automatic system to detect and classify all potential membrane transport proteins for a given genome and integrate the related reactions into GSMMs is proposed, based on the identification and classification of genes that encode transmembrane proteins. The Transport Reactions Annotation and Generation (TRIAGE) tool identifies the metabolites transported by each transmembrane protein and its transporter family. The localization of the carriers is also predicted and, consequently, their action is confined to a given membrane. The integration of the data provided by TRIAGE with highly curated models allowed the identification of new transport reactions. TRIAGE is included in the new release of merlin, a software tool previously developed by the authors, which expedites the GSMM reconstruction processes.

Molecular and structural characteristics of multidrug resistance-associated protein 7 in Chinese liver fluke Clonorchis sinensis

Multidrug resistance-associated protein 7 (MRP7, ABCC10) is a C subfamily member of the ATP-binding cassette (ABC) superfamily. MRP7 is a lipophilic anion transporter that pumps endogenous and xenobiotic substrates from the cytoplasm to the extracellular milieu. Here, we cloned and characterized CsMRP7 as a novel ABC transporter from the Chinese liver fluke, Clonorchis sinensis. Full-length cDNA of CsMRP7 was 5174 nt, encoded 1636 amino acids (aa), and harbored a 147-bp 5'-untranslated region (5'-UTR) and 116-bp 3'-UTR. Phylogenetic analysis confirmed that CsMRP7 was closer to the ABCC subfamily than the ABCB subfamily. Tertiary structures of the N-terminal region (1-322 aa) and core region (323-1621 aa) of CsMRP7 were generated by homology modeling using glucagon receptor (PDB ID: 5ee7_A) and P-glycoprotein (PDB ID: 4f4c_A) as templates, respectively. CsMRP7 nucleotide-binding domain 2 (NBD2) was conserved more than NBD1, which was the sites of ATP binding and hydrolysis. Like typical long MRPs, CsMRP7 has an additional membrane-spanning domain 0 (MSD0) and cytoplasmic loop, along with a common structural fold consisting of MSD1-NBD1-MSD2-NBD2 as a single polypeptide assembly. MSD0, MSD1, and MSD2 consisted of TM1-7, TM8-13, and TM14-19, respectively. The CsMRP7 transcript was more abundant in the metacercariae than in the adult worms. Truncated NBD1 (39 kDa) and NBD2 (44 kDa) were produced in bacteria and mouse immune sera were raised. CsMRP7 was localized in the apical side of the intestinal epithelium, sperm in the testes and seminal receptacle, receptacle membrane, and mesenchymal tissue around intestine in the adult worm. These results provide molecular information and insights into structural and functional characteristics of CsMRP7 and homologs of flukes.

Characterization of the Tetraspan Junctional Complex (4JC) superfamily

Connexins or innexins form gap junctions, while claudins and occludins form tight junctions. In this study, statistical data, derived using novel software, indicate that these four junctional protein families and eleven other families of channel and channel auxiliary proteins are related by common descent and comprise the Tetraspan (4 TMS) Junctional Complex (4JC) Superfamily. These proteins all share similar 4 transmembrane α-helical (TMS) topologies. Evidence is presented that they arose via an intragenic duplication event, whereby a 2 TMS-encoding genetic element duplicated tandemly to give 4 TMS proteins. In cases where high resolution structural data were available, the conclusion of homology was supported by conducting structural comparisons. Phylogenetic trees reveal the probable relationships of these 15 families to each other. Long homologues containing fusions to other recognizable domains as well as internally duplicated or fused domains are reported. Large "fusion" proteins containing 4JC domains proved to fall predominantly into family-specific patterns as follows: (1) the 4JC domain was N-terminal; (2) the 4JC domain was C-terminal; (3) the 4JC domain was duplicated or occasionally triplicated and (4) mixed fusion types were present. Our observations provide insight into the evolutionary origins and subfunctions of these proteins as well as guides concerning their structural and functional relationships.

Properties and Phylogeny of 76 Families of Bacterial and Eukaryotic Organellar Outer Membrane Pore-Forming Proteins

We here report statistical analyses of 76 families of integral outer membrane pore-forming proteins (OMPPs) found in bacteria and eukaryotic organelles. 47 of these families fall into one superfamily (SFI) which segregate into fifteen phylogenetic clusters. Families with members of the same protein size, topology and substrate specificities often cluster together. Virtually all OMPP families include only proteins that form transmembrane pores. Nine such families, all of which cluster together in the SFI phylogenetic tree, contain both α- and β-structures, are multi domain, multi subunit systems, and transport macromolecules. Most other SFI OMPPs transport small molecules. SFII and SFV homologues derive from Actinobacteria while SFIII and SFIV proteins derive from chloroplasts. Three families of actinobacterial OMPPs and two families of eukaryotic OMPPs apparently consist primarily of α-helices (α-TMSs). Of the 71 families of (putative) β-barrel OMPPs, only twenty could not be assigned to a superfamily, and these derived primarily from Actinobacteria (1), chloroplasts (1), spirochaetes (8), and proteobacteria (10). Proteins were identified in which two or three full length OMPPs are fused together. Family characteristic are described and evidence agrees with a previous proposal suggesting that many arose by adjacent β-hairpin structural unit duplications.

The human transmembrane proteome

Background

Transmembrane proteins have important roles in cells, as they are involved in energy production, signal transduction, cell-cell interaction, cell-cell communication and more. In human cells, they are frequently targets for pharmaceuticals; therefore, knowledge about their properties and structure is crucial. Topology of transmembrane proteins provide a low resolution structural information, which can be a starting point for either laboratory experiments or modelling their 3D structures.

Results

Here, we present a database of the human α-helical transmembrane proteome, including the predicted and/or experimentally established topology of each transmembrane protein, together with the reliability of the prediction. In order to distinguish transmembrane proteins in the proteome as well as for topology prediction, we used a newly developed consensus method (CCTOP) that incorporates recent state of the art methods, with tested accuracies on a novel human benchmark protein set. CCTOP utilizes all available structure and topology data as well as bioinformatical evidences for topology prediction in a probabilistic framework provided by the hidden Markov model. This method shows the highest accuracy (98.5 % for discrinimating between transmembrane and non-transmembrane proteins and 84 % for per protein topology prediction) among the dozen tested topology prediction methods. Analysis of the human proteome with the CCTOP indicates that it contains 4998 (26 %) transmembrane proteins. Besides predicting topology, reliability of the predictions is estimated as well, and it is demonstrated that the per protein prediction accuracies of more than 60 % of the predictions are over 98 % on the benchmark sets and most probably on the predicted human transmembrane proteome too.

Conclusions

Here, we present the most accurate prediction of the human transmembrane proteome together with the experimental topology data. These data, as well as various statistics about the human transmembrane proteins and their topologies can be downloaded from and can be visualized at the website of the human transmembrane proteome (http://htp.enzim.hu).

Reviewers

This article was reviewed by Dr. Sandor Pongor, Dr. Michael Galperin and Dr. Pascale Gaudet (nominated by Dr Michael Galperin).

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-015-0061-x) contains supplementary material, which is available to authorized users.

Mutational analysis of the Shigella flexneri O-antigen polymerase Wzy: identification of Wzz-dependent Wzy mutants

The O-antigen (Oag) component of lipopolysaccharide (LPS) is a major virulence determinant of Shigella flexneri and is synthesized by the O-antigen polymerase, WzySf. Oag chain length is regulated by chromosomally encoded WzzSf and pHS-2 plasmid-encoded WzzpHS2. To identify functionally important amino acid residues in WzySf, random mutagenesis was performed on the wzySf gene in a pWaldo-TEV-GFP plasmid, followed by screening with colicin E2. Analysis of the LPS conferred by mutated WzySf proteins in the wzySf-deficient (Δwzy) strain identified 4 different mutant classes, with mutations found in periplasmic loop 1 (PL1), PL2, PL3, and PL6, transmembrane region 2 (TM2), TM4, TM5, TM7, TM8, and TM9, and cytoplasmic loop 1 (CL1) and CL5. The association of WzySf and WzzSf was investigated by transforming these mutated wzySf plasmids into a wzySf- and wzzSf-deficient (Δwzy Δwzz) strain. Comparison of the LPS profiles in the Δwzy and Δwzy Δwzz backgrounds identified WzySf mutants whose polymerization activities were WzzSf dependent. Colicin E2 and bacteriophage Sf6c sensitivities were consistent with the LPS profiles. Analysis of the expression levels of the WzySf-GFP mutants in the Δwzy and Δwzy Δwzz backgrounds identified a role for WzzSf in WzySf stability. Hence, in addition to its role in regulating Oag modal chain length, WzzSf also affects WzySf activity and stability.