Structural and biophysical properties of FopA, a major outer membrane protein of Francisella tularensis.

Francisella tularensis is an extremely infectious pathogen and a category A bioterrorism agent. It causes the highly contagious zoonosis, Tularemia. Currently, FDA approved vaccines against tularemia are unavailable. F. tularensis outer membrane protein A (FopA) is a well-studied virulence determinant and protective antigen against tularemia. It is a major outer membrane protein (Omp) of F. tularensis. However, FopA-based therapeutic intervention is hindered due to lack of complete structural information for membrane localized mature FopA. In our study, we established recombinant expression, monodisperse purification, crystallization and X-ray diffraction (~6.5 Å) of membrane localized mature FopA. Further, we performed bioinformatics and biophysical experiments to unveil its structural organization in the outer membrane. FopA consists of 393 amino acids and has less than 40% sequence identity to known bacterial Omps. Using comprehensive sequence alignments and structure predictions together with existing partial structural information, we propose a two-domain organization for FopA. Circular dichroism spectroscopy and heat modifiability assay confirmed FopA has a ß-barrel domain consistent with alphafold2's prediction of an eight stranded ß-barrel at the N-terminus. Small angle X-ray scattering (SAXS) and native-polyacrylamide gel electrophoresis revealed FopA purified in detergent micelles is predominantly dimeric. Molecular density derived from SAXS at 31 Å shows putative dimeric N-terminal ß-barrels surrounded by detergent corona and connected to C-terminal domains via flexible linker. Disorder analysis predicts N- and C-terminal domains are interspersed by a long intrinsically disordered region and alphafold2 predicts this region to be largely unstructured. Taken together, we propose a dimeric, two-domain organization of FopA in the outer membrane: the N-terminal ß-barrel is membrane embedded, provides dimerization interface and tethers to membrane extrinsic C-terminal domain via long flexible linker. Structure determination of membrane localized mature FopA is essential to understand its role in pathogenesis and develop anti-tularemia therapeutics. Our results pave the way towards it.

Purification, characterization, and preliminary serial crystallography diffraction advances structure determination of full-length human particulate guanylyl cyclase A receptor.

Particulate Guanylyl Cyclase Receptor A (pGC-A) is a natriuretic peptide membrane receptor, playing a vital role in controlling cardiovascular, renal, and endocrine functions. The extracellular domain interacts with natriuretic peptides and triggers the intracellular guanylyl cyclase domain to convert GTP to cGMP. To effectively develop methods to regulate pGC-A, structural information on the full-length form is needed. However, structural data on the transmembrane and intracellular domains are lacking. This work presents expression and optimization using baculovirus, along with the first purification of functional full-length human pGC-A. In vitro assays revealed the pGC-A tetramer was functional in detergent micelle solution. Based on our purification results and previous findings that dimer formation is required for functionality, we propose a tetramer complex model with two functional subunits. Previous research suggested pGC-A signal transduction is an ATP-dependent, two-step mechanism. Our results show the binding ligand also moderately activates pGC-A, and ATP is not crucial for activation of guanylyl cyclase. Furthermore, crystallization of full-length pGC-A was achieved, toward determination of its structure. Needle-shaped crystals with 3 Å diffraction were observed by serial crystallography. This work paves the road for determination of the full-length pGC-A structure and provides new information on the signal transduction mechanism.

Characterization and computational simulation of human Syx, a RhoGEF implicated in glioblastoma.

Structural discovery of guanine nucleotide exchange factor (GEF) protein complexes is likely to become increasingly relevant with the development of new therapeutics targeting small GTPases and development of new classes of small molecules that inhibit protein-protein interactions. Syx (also known as PLEKHG5 in humans) is a RhoA GEF implicated in the pathology of glioblastoma (GBM). Here we investigated protein expression and purification of ten different human Syx constructs and performed biophysical characterizations and computational studies that provide insights into why expression of this protein was previously intractable. We show that human Syx can be expressed and isolated and Syx is folded as observed by circular dichroism (CD) spectroscopy and actively binds to RhoA as determined by co-elution during size exclusion chromatography (SEC). This characterization may provide critical insights into the expression and purification of other recalcitrant members of the large class of oncogenic-Diffuse B-cell lymphoma (Dbl) homology GEF proteins. In addition, we performed detailed homology modeling and molecular dynamics simulations on the surface of a physiologically realistic membrane. These simulations reveal novel insights into GEF activity and allosteric modulation by the plekstrin homology (PH) domain. These newly revealed interactions between the GEF PH domain and the membrane embedded region of RhoA support previously unexplained experimental findings regarding the allosteric effects of the PH domain from numerous activity studies of Dbl homology GEF proteins. This work establishes new hypotheses for structural interactivity and allosteric signal modulation in Dbl homology RhoGEFs.

Biophysical characterization and a roadmap towards the NMR solution structure of G0S2, a key enzyme in non-alcoholic fatty liver disease.

In the United States non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease, affecting an estimated 80 to 100 million people. It occurs in every age group, but predominantly in people with risk factors such as obesity and type 2 diabetes. NAFLD is marked by fat accumulation in the liver leading to liver inflammation, which may lead to scarring and irreversible damage progressing to cirrhosis and liver failure. In animal models, genetic ablation of the protein G0S2 leads to alleviation of liver damage and insulin resistance in high fat diets. The research presented in this paper aims to aid in rational based drug design for the treatment of NAFLD by providing a pathway for a solution state NMR structure of G0S2. Here we describe the expression of G0S2 in an E. coli system from two different constructs, both of which are confirmed to be functionally active based on the ability to inhibit the activity of Adipose Triglyceride Lipase. In one of the constructs, preliminary NMR spectroscopy measurements show dominant alpha-helical characteristics as well as resonance assignments on the N-terminus of G0S2, allowing for further NMR work with this protein. Additionally, the characterization of G0S2 oligomers are outlined for both constructs, suggesting that G0S2 may defensively exist in a multimeric state to protect and potentially stabilize the small 104 amino acid protein within the cell. This information presented on the structure of G0S2 will further guide future development in the therapy for NAFLD.

Protein expression and purification of G-protein coupled receptor kinase 6 (GRK6), toward structure-based drug design and discovery for multiple myeloma.

Human G-protein coupled receptor kinase 6 (GRK6) belongs to the GRK4 kinase subfamily of the G protein-coupled receptor kinase family which comprises of GRK1, GRK2, and GRK4. These kinases phosphorylate ligand-activated G-protein coupled receptors (GPCRs), driving heterotrimeric G protein coupling, desensitization of GPCR, and ß-arrestin recruitment. This reaction series mediates cellular signal pathways for cell survival, proliferation, migration and chemotaxis. GRK6 is a kinase target in multiple myeloma since it is highly expressed in myeloma cells compared to epithelial cells and has a significant role in mediating the chemotactic responses of T and B-lymphocytes. To support structure-based drug design, we describe three human GRK6 constructs, GRK6, GRK6His/EK, and GRK6His/TEV, designed for protein expression in Spodoptera frugiperda Sf9 insect cells. The first construct did not contain any purification tag whereas the other two constructs contained the His10 affinity tag, which increased purification yields. We report here that all three constructs of GRK6 were overexpressed in Sf9 insect cells and purified to homogeneity at levels that were suitable for co-crystallization of GRK6 with potential inhibitors. The yields of purified GRK6, GRK6His/EK, and GRK6His/TEV were 0.3 mg, 0.8 mg and 0.7 mg per liter of cell culture, respectively. In addition, we have shown that GRK6His/TEV with the His10 tag removed was highly homogeneous and monodisperse as observed by dynamic light scattering measurement and actively folded as exhibited by circular dichroism spectroscopy. The described methods will support the structure-based development of additional therapeutics against multiple myeloma.

Author Correction: Membrane directed expression in Escherichia coli of BBA57 and other virulence factors from the Lyme disease agent Borrelia burgdorferi.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Membrane directed expression in Escherichia coli of BBA57 and other virulence factors from the Lyme disease agent Borrelia burgdorferi.

Membrane-embedded proteins are critical to the establishment, survival and persistence in the host of the Lyme disease bacterium Borrelia burgdorferi (Bb), but to date, there are no solved structures of transmembrane proteins representing these attractive therapeutic targets. All available structures from the genus Borrelia represent proteins expressed without a membrane-targeting signal peptide, thus avoiding conserved pathways that modify, fold and assemble membrane protein complexes. Towards elucidating structure and function of these critical proteins, we directed translocation of eleven expression-optimized Bb virulence factors, including the signal sequence, to the Escherichia coli membrane, of which five, BBA57, HtrA, BB0238, BB0323, and DipA, were expressed with C-terminal His-tags. P66 was also expressed using the PelB signal sequence fused to maltose binding protein. Membrane-associated BBA57 lipoprotein was solubilized by non-ionic and zwitterionic detergents. We show BBA57 translocation to the outer membrane, purification at a level sufficient for structural studies, and evidence for an α-helical multimer. Previous studies showed multiple critical roles of BBA57 in transmission, joint arthritis, carditis, weakening immune responses, and regulating other Bb outer surface proteins. In describing the first purification of membrane-translocated BBA57, this work will support subsequent studies that reveal the precise mechanisms of this important Lyme disease virulence factor.

Generation of High-Specificity Antibodies against Membrane Proteins Using DNA-Gold Micronanoplexes for Gene Gun Immunization.

Membrane proteins are the molecular interface of the cell and its environs; however, studies of membrane proteins are highly technically challenging, mainly due to instability of the isolated protein. Towards the production of antibodies that recognize properly folded and stabilized forms of membrane protein antigen, we describe a DNA-based immunization method for mice that expresses the antigen in the membranes of dendritic cells, thus allowing direct presentation to the immune system. This genetic immunization approach employs a highly efficient method of biolistic delivery based on DNA-gold micronanoplexes, which are complexes of micron-sized gold particles that allow dermal penetration and nanometer-sized gold particles that provide a higher surface area for DNA binding than micron gold alone. In contrast to antibodies derived from immunizations with detergent-solubilized protein or with protein fragments, antibodies from genetic immunization are expected to have a high capacity for binding conformational epitopes and for modulating membrane protein activity. © 2018 by John Wiley & Sons, Inc.

Polyclonal Antibody Production for Membrane Proteins via Genetic Immunization.

Antibodies are essential for structural determinations and functional studies of membrane proteins, but antibody generation is limited by the availability of properly-folded and purified antigen. We describe the first application of genetic immunization to a structurally diverse set of membrane proteins to show that immunization of mice with DNA alone produced antibodies against 71% (n = 17) of the bacterial and viral targets. Antibody production correlated with prior reports of target immunogenicity in host organisms, underscoring the efficiency of this DNA-gold micronanoplex approach. To generate each antigen for antibody characterization, we also developed a simple in vitro membrane protein expression and capture method. Antibody specificity was demonstrated upon identifying, for the first time, membrane-directed heterologous expression of the native sequences of the FopA and FTT1525 virulence determinants from the select agent Francisella tularensis SCHU S4. These approaches will accelerate future structural and functional investigations of therapeutically-relevant membrane proteins.

Biophysical Characterization of a Vaccine Candidate against HIV-1: The Transmembrane and Membrane Proximal Domains of HIV-1 gp41 as a Maltose Binding Protein Fusion.

The membrane proximal region (MPR, residues 649-683) and transmembrane domain (TMD, residues 684-705) of the gp41 subunit of HIV-1's envelope protein are highly conserved and are important in viral mucosal transmission, virus attachment and membrane fusion with target cells. Several structures of the trimeric membrane proximal external region (residues 662-683) of MPR have been reported at the atomic level; however, the atomic structure of the TMD still remains unknown. To elucidate the structure of both MPR and TMD, we expressed the region spanning both domains, MPR-TM (residues 649-705), in Escherichia coli as a fusion protein with maltose binding protein (MBP). MPR-TM was initially fused to the C-terminus of MBP via a 42 aa-long linker containing a TEV protease recognition site (MBP-linker-MPR-TM). Biophysical characterization indicated that the purified MBP-linker-MPR-TM protein was a monodisperse and stable candidate for crystallization. However, crystals of the MBP-linker-MPR-TM protein could not be obtained in extensive crystallization screens. It is possible that the 42 residue-long linker between MBP and MPR-TM was interfering with crystal formation. To test this hypothesis, the 42 residue-long linker was replaced with three alanine residues. The fusion protein, MBP-AAA-MPR-TM, was similarly purified and characterized. Significantly, both the MBP-linker-MPR-TM and MBP-AAA-MPR-TM proteins strongly interacted with broadly neutralizing monoclonal antibodies 2F5 and 4E10. With epitopes accessible to the broadly neutralizing antibodies, these MBP/MPR-TM recombinant proteins may be in immunologically relevant conformations that mimic a pre-hairpin intermediate of gp41.

NMR Structure of Francisella tularensis Virulence Determinant Reveals Structural Homology to Bet v1 Allergen Proteins.

Tularemia is a potentially fatal bacterial infection caused by Francisella tularensis, and is endemic to North America and many parts of northern Europe and Asia. The outer membrane lipoprotein, Flpp3, has been identified as a virulence determinant as well as a potential subunit template for vaccine development. Here we present the first structure for the soluble domain of Flpp3 from the highly infectious Type A SCHU S4 strain, derived through high-resolution solution nuclear magnetic resonance (NMR) spectroscopy; the first structure of a lipoprotein from the genus Francisella. The Flpp3 structure demonstrates a globular protein with an electrostatically polarized surface containing an internal cavity-a putative binding site based on the structurally homologous Bet v1 protein family of allergens. NMR-based relaxation studies suggest loop regions that potentially modulate access to the internal cavity. The Flpp3 structure may add to the understanding of F. tularensis virulence and contribute to the development of effective vaccines.

Purification and biophysical characterization of the CapA membrane protein FTT0807 from Francisella tularensis.

The capA gene (FTT0807) from Francisella tularensis subsp. tularensis SCHU S4 encodes a 44.4 kDa integral membrane protein composed of 403 amino acid residues that is part of an apparent operon that encodes at least two other membrane proteins, CapB, and CapC, which together play a critical role in the virulence and pathogenesis of this bacterium. The capA gene was overexpressed in Escherichia coli as a C-terminal His6-tagged fusion with a folding reporter green fluorescent protein (frGFP). Purification procedures using several detergents were developed for the fluorescing and membrane-bound product, yielding approximately 30 mg of pure protein per liter of bacterial culture. Dynamic light scattering indicated that CapA-frGFP was highly monodisperse, with a size that was dependent upon both the concentration and choice of detergent. Circular dichroism showed that CapA-frGFP was stable over the range of 3-9 for the pH, with approximately half of the protein having well-defined α-helical and ß-sheet secondary structure. The addition of either sodium chloride or calcium chloride at concentrations producing ionic strengths above 0.1 M resulted in a small increase of the α-helical content and a corresponding decrease in the random-coil content. Secondary-structure predictions on the basis of the analysis of the sequence indicate that the CapA membrane protein has two transmembrane helices with a substantial hydrophilic domain. The hydrophilic domain is predicted to contain a long disordered region of 50-60 residues, suggesting that the increase of α-helical content at high ionic strength could arise because of electrostatic interactions involving the disordered region. CapA is shown to be an inner-membrane protein and is predicted to play a key cellular role in the assembly of polysaccharides.