Antoine P. Maillard

Rabies virus glycoprotein can fold in two alternative, antigenically distinct conformations depending on membrane-anchor type

Rabies virus glycoprotein (G) is a trimeric type I transmembrane glycoprotein that mediates both receptor recognition and low pH-induced membrane fusion. We have previously demonstrated that a soluble form of the ectodomain of G (G(1-439)), although secreted, is folded in an alternative conformation, which is monomeric and antigenically distinct from the native state of the complete, membrane-anchored glycoprotein. This has raised questions concerning the role of the transmembrane domain (TMD) in the correct native folding of the ectodomain. Here, we show that an ectodomain anchored in the membrane by a glycophosphatidylinositol is also folded in an alternative conformation, whereas replacement of the TMD of G by other peptide TMDs results in correct antigenicity of G. However, mutants with an insertion of a hydrophilic linker between the ectodomain and the TMD also fold in an alternative conformation. The influence of the membrane-anchor type on G ectodomain trimerization and folding is discussed.

Structural Basis for Metal Sensing by Cnrx.

CnrX is the metal sensor and signal modulator of the three-protein transmembrane signal transduction complex CnrYXH of Cupriavidus metallidurans CH34 that is involved in the setup of cobalt and nickel resistance. We have determined the atomic structure of the soluble domain of CnrX in its Ni-bound, Co-bound, or Zn-bound form. Ni and Co ions elicit a biological response, while the Zn-bound form is inactive. The structures presented here reveal the topology of intraprotomer and interprotomer interactions and the ability of metal-binding sites to fine-tune the packing of CnrX dimer as a function of the bound metal. These data suggest an allosteric mechanism to explain how the complex is switched on and how the signal is modulated by Ni or Co binding. These results provide clues to propose a model for signal propagation through the membrane in the complex.

Differential stability and fusion activity of Lyssavirus glycoprotein trimers

The oligomeric structure and the fusion activity of lyssavirus glycoprotein (G) was studied by comparing G from Mokola virus (GMok) and rabies virus (PV strain) (GPV), which are highly divergent lyssaviruses. G expressed at the surface of BSR cells upon either plasmid transfection or virus infection are shown to be mainly trimeric after cross-linking experiments. However, solubilization by a detergent (CHAPS) and analysis in sucrose sedimentation gradient evidenced that GMok trimer is less stable than GPV trimer. A chimeric glycoprotein (G Mok-PV) associating the N-terminal half of GMok to the C-terminal half part of GPV formed trimers with an intermediate stability, indicating that the G C-terminal domain is essential in trimer stability. A cell to cell fusion assay revealed that GMok (and not G Mok-PV) was able to induce fusion at a higher pH (0.5 pH unit) than GPV. Such differences in the oligomeric structure stability and in the fusion activity of lyssavirus glycoproteins may partly account for the previously reported differences of their immunogenic and pathogenic properties.

X-ray structure of the metal-sensor CnrX in both the apo- and copper-bound forms.

Both the X‐ray structures of the apo‐ and the copper‐bound forms of the metal‐sensor domain (residues 31–148) of CnrX from Cupriavidus metallidurans CH34 were obtained at 1.74 Å resolution from a selenomethionine derivative. This four‐helix hooked‐hairpin is the first structure of a metal‐sensor in an ECF‐type signaling pathway. The copper ion is bound in a type 2‐like center with a 3N1O coordination in the equatorial plane and shows an unprecedented remote fifth axial ligand with Met93 contributing a weak S–Cu bond. The signal onset cannot be explained by conformational changes associated with CnrX metallation.

Spectroscopic characterization of two peptides derived from the stem of rabies virus glycoprotein.

Rabies virus glycoprotein (G) is a trimeric type I transmembrane glycoprotein that mediates both receptor recognition and low pH-induced membrane fusion. Electron microscopy has indicated that the ectodomain of protein G is made of a globular head and a stem. In order to characterize the putative stem region at the molecular level, we designed two peptides, P(S) and P(L), which were produced as GST fusion proteins in bacteria. Peptide P(S) extends from amino acid (aa) 374 to aa 428 whereas peptide P(L) extends from aa 368 down to the end of the ectodomain of G (aa 439). Their secondary and quaternary structures have been studied with spectroscopic and biophysical methods. We show that these isolated peptides are monomeric and poorly structured in aqueous solution. However, circular dichroism (CD) in presence of 2,2,2-trifluoroethanol and NMR data indicate that this region may adopt a alpha-helical conformation in the complete glycoprotein.

Spectroscopic characterization of the metal-binding sites in the periplasmic metal-sensor domain of CnrX from Cupriavidus metallidurans CH34.

CnrX, the dimeric metal sensor of the three-protein transmembrane signal transduction complex CnrYXH of Cupriavidus metallidurans CH34, contains one metal-binding site per monomer. Both Ni and Co elicit a biological response and bind the protein in a 3N2O1S coordination sphere with a nearly identical octahedral geometry as shown by the X-ray structure of CnrXs, the soluble domain of CnrX. However, in solution CnrXs is titrated by 4 Co-equiv and exhibits an unexpected intense band at 384 nm that was detected neither by single-crystal spectroscopy nor under anaerobiosis. The data from a combination of spectroscopic techniques (spectrophotometry, electron paramagnetic resonance, X-ray absorption spectroscopy) showed that two sites correspond to those identified by crystallography. The two extra binding sites accommodate Co(II) in an octahedral geometry in the absence of oxygen and are occupied in air by a mixture of low-spin Co(II) as well as EPR-silent Co(III). These extra sites, located at the N-terminus of the protein, are believed to participate to the formation of peroxo-bridged dimers. Accordingly, we hypothesize that the intense band at 384 nm relies on the formation of a binuclear μ-peroxo Co(III) complex. These metal binding sites are not physiologically relevant since they are not detected in full-length NccX, the closest homologue of CnrX. X-ray absorption spectroscopy demonstrates that NccX stabilizes Co(II) in two-binding sites similar to those characterized by crystallography in its soluble counterpart. Nevertheless, the original spectroscopic properties of the extra Co-binding sites are of interest because they are susceptible to be detected in other Co-bound proteins.