Reconstitution of a minimal ESX-5 type VII secretion system suggests a role for PPE proteins in the outer membrane transport of proteins

Mycobacteria utilize type VII secretion systems (T7SSs) to secrete proteins across their highly hydrophobic and diderm cell envelope. Pathogenic mycobacteria have up to five different T7SSs, called ESX-1 to ESX-5, which are crucial for growth and virulence. Here, we use a functionally reconstituted ESX-5 system in the avirulent species Mycobacterium smegmatis that lacks ESX-5, to define the role of each esx-5 gene in system functionality. By creating an array of gene deletions and assessing protein levels of components and membrane complex assembly, we observed that only the five components of the inner membrane complex are required for its assembly. However, in addition to these five core components, active secretion also depends on both the Esx and PE/PPE substrates. Tagging the PPE substrates followed by subcellular fractionation, surface labeling and membrane extraction showed that these proteins localize to the mycobacterial outer membrane. This indicates that they could play a role in secretion across this enigmatic outer barrier. These results provide the first full overview of the role of each esx-5 gene in T7SS functionality. IMPORTANCE Pathogenic mycobacteria, such as the notorious Mycobacterium tuberculosis, are highly successful as pathogens, in part due to their specific and diderm cell envelope, with a mycolic acid-containing outer membrane. The architecture of this highly impermeable membrane is little understood and the proteins that populate it even less so. To transport proteins across their cell envelope, mycobacteria employ a specialized transport pathway called type VII secretion. While recent studies have elucidated the type VII secretion membrane channel that mediates transport across the inner membrane, the identity of the outer membrane channel remains a black box. Here, we show evidence that specific substrates of the type VII pathway could form these channels. Elucidating the pathway and mechanism of protein secretion through the mycobacterial outer membrane will allow its exploitation for the development of novel mycobacterial therapeutics.

Expression and folding of human very-low-density lipoprotein receptor fragments: neutralization capacity toward human rhinovirus HRV2

Minor group human rhinoviruses (HRVs) use members of the low-density lipoprotein receptor family for cell entry. To investigate the utility of receptor fragments as viral inhibitors, various polypeptide segments derived from the ligand binding domain of human very-low-density lipoprotein receptor (VLDLR) were expressed in a soluble form in bacteria. Whereas none of the fragments was active in virus binding immediately after recovery from the cell lysates, constructs encompassing complement type repeats 1-3, 1-6, and 1-8 spontaneously acquired virus binding activity by incubation at 4 degrees C in buffer containing Ca(2+) ions and lacking any redox system. When immobilized receptor-associated protein (RAP), a specific chaperone for VLDLR, was present during the incubation, the yield of protein active in ligand binding was substantially increased. A VLDLR fragment with repeats 4-6 failed to bind virus; however, it bound RAP. Bacterial expression of truncated VLDLR 1-3 at high yield, easy purification, and folding together with high inhibitory activity toward HRV2 makes this protein a promising starting point for the development of an oligopeptide-based antiviral agent. Using sucrose density gradient centrifugation, we demonstrate the formation of virus-receptor complexes. The recombinant receptors can thus be used for structure determination by electron cryo-microscopy.

The cellular receptor to human rhinovirus 2 binds around the 5-fold axis and not in the canyon: a structural view

Human rhinovirus serotype 2 (HRV2) belongs to the minor group of HRVs that bind to members of the LDL-receptor family including the very low density lipoprotein (VLDL)-receptor (VLDL-R). We have determined the structures of the complex between HRV2 and soluble fragments of the VLDL-R to 15 A resolution by cryo-electron microscopy. The receptor fragments, which include the first three ligand-binding repeats of the VLDL-R (V1-3), bind to the small star-shaped dome on the icosahedral 5-fold axis. This is in sharp contrast to the major group of HRVs where the receptor site for ICAM-1 is located at the base of a depression around each 5-fold axis. Homology models of the three domains of V1-3 were used to explore the virus-receptor interaction. The footprint of VLDL-R on the viral surface covers the BC- and HI-loops on VP1.

Recombinant soluble low-density lipoprotein receptor fragment inhibits common cold infection

A fragment of the human low-density lipoprotein receptor encompassing the seven ligand-binding repeats fused to a C-terminal oligo-His tag was expressed in Sf9 insect cells. The melittin signal sequence encoded in the baculovirus vector led to secretion of the protein into the cell supernatant in a soluble form. The receptor fragment bound its natural ligand beta-migrating very-low-density lipoprotein and human rhinovirus serotype 2 in non-reducing ligand blots. Infection of all minor group human rhinovirus serotypes investigated was inhibited by the presence of the receptor fragment during viral challenge of HeLa cells. Infection is inhibited by aggregation of the virions.

Crystallization and preliminary X-ray analysis of human rhinovirus serotype 2 (HRV2)

Human rhinoviruses, the major cause of mild recurrent infections of the upper respiratory tract, are small icosahedral particles. Over 100 different serotypes have been identified. The majority (91 serotypes) use intercellular adhesion molecule 1 as the cell-attachment site; ten serotypes (the minor group) bind to members of the low-density lipoprotein receptor. Three different crystal forms of the minor-group human rhinovirus serotype 2 (HRV2) were obtained by the hanging-drop vapour-diffusion technique using ammonium sulfate and sodium/potassium phosphate as precipitants. Monoclinic crystals, space group P2(1), diffracted at least to 2.8 A resolution, and two complete virus particles were located in the crystal asymmetric unit. A second type of crystals had a compact cubic like morphology and diffracted beyond 2.5 A resolution. These crystals belong to a primitive orthorhombic space group, with unit-cell parameters a = 309.3, b = 353.5, c = 759.6 A, and contain one virus particle in the asymmetric unit. A third type of crystals, with a prismatic shape and belonging to space group I222, was also obtained under similar crystallization conditions. These latter crystals, with unit-cell parameters a = 308.7, b = 352.2, c = 380.5 A, diffracted to high resolution (beyond 1.8 A) and contained 15 protomers per asymmetric unit; this requires that three perpendicular crystal twofold axes coincide with three of the viral particle's dyad axes.

Soluble LDL minireceptors. Minimal structure requirements for recognition of minor group human rhinovirus

Soluble human low density lipoprotein minireceptors with less than seven ligand binding repeats (as are present in the native membrane receptor) were expressed in Sf9 insect cells with a hexa-His tag fused to the C terminus. The recombinant truncated proteins were affinity purified from the tissue culture supernatants by Ni-NTA column chromatography. Minireceptors with more than two repeats bound to rabbit beta very low density lipoprotein and could thus be further purified by affinity chromatography. Binding and cell protection assays indicated that two ligand binding repeats are sufficient for attachment of minor group human rhinoviruses to immobilized receptors, whereas at least three ligand binding repeats are required to protect HeLa cells against viral infection.

Very-low-density lipoprotein receptor fragment shed from HeLa cells inhibits human rhinovirus infection

The large family of human rhinoviruses, the main causative agents of the common cold, is divided into the major and the minor group based on receptor specificity. Major group viruses attach to intercellular adhesion molecule 1 (ICAM-1), a member of the immunoglobulin superfamily, whereas minor group viruses use low-density lipoprotein receptors (LDLR) for cell entry. During early attempts aimed at isolating the minor group receptor, we discovered that a protein with virus binding activity was released from HeLa cells upon incubation with buffer at 37 degreesC (F. Hofer, B. Berger, M. Gruenberger, H. Machat, R. Dernick, U. Tessmer, E. Kuechler, and D. Blaas, J. Gen. Virol. 73:627-632, 1992). In light of the recent discovery of several new members of the LDLR family, we reinvestigated the nature of this protein and present evidence for its being derived from the human very-low density lipoprotein receptor (VLDLR). A soluble VLDLR fragment encompassing the eight complement type repeats and representing the N-terminal part of the receptor was then expressed in the baculovirus system; both the shed protein and the recombinant soluble VLDLR bind minor group viruses and inhibit viral infection of HeLa cells in a concentration-dependent manner.

Recombinant soluble low density lipoprotein receptor fragment inhibits minor group rhinovirus infection in vitro

A fragment of the low density lipoprotein receptor encompassing the seven ligand binding repeats was expressed in Sf9 insect cells as a fusion protein with a carboxyl-terminally linked hexa-his tag by using a baculovirus vector. Up to 10 mg/l of the fusion protein was secreted into the medium. The material was soluble in the absence of detergent and active in binding beta very low density lipoprotein and a member of the minor group of human rhinoviruses (HRV2) in ligand blots from sodium dodecyl sulfate-polyacrylamide gels run under nonreducing conditions. The receptor fragment specifically inhibits viral infection of HeLa cells by minor group HRVs in a concentration-dependent manner. Viral infectivity is neutralized by aggregation.

Structure of a neutralizing antibody bound monovalently to human rhinovirus 2

The structure of a complex between human rhinovirus 2 (HRV2) and the Fab fragment of neutralizing monoclonal antibody (MAb) 3B10 has been determined to 25-A resolution by cryoelectron microscopy and three-dimensional reconstruction techniques. The footprint of 3B10 on HRV2 is very similar to that of neutralizing MAb 8F5, which binds bivalently across the icosahedral twofold axis. However, the 3B10 Fab fragment (Fab-3B10) is bound in an orientation, inclined at approximately 45 degrees to the surface of the virus capsid, which is compatible only with monovalent binding of the antibody. The canyon around the fivefold axis is not directly obstructed by the bound Fab. The X-ray structures of a closely related HRV (HRV1A) and a Fab fragment were fitted to the density maps of the HRV2-Fab-3B10 complex obtained by cryoelectron microscope techniques. The footprint of 3B10 on the viral surface is largely on VP2 but also covers the VP3 loop centered on residue 3064 and the VP1 loop centered on residue 1267. MAb 3B10 can interact directly with VP2 residue 2164, the site of an escape mutation on VP2, and with VP1 residues 1264 to 1267, the site of a deletion escape mutation. Deletion of these residues shortens the VP1 loop, moving it away from the MAb binding site. All structural and biochemical evidence indicates that MAb 3B10 binds to a conformation epitope on HRV2.