XB-ART-55054PLoS One January 1, 2018; 13 (6): e0199227.
Influenza D virus M2 protein exhibits ion channel activity in Xenopus laevis oocytes.
BACKGROUND: A new type of influenza virus, known as type D, has recently been identified in cattle and pigs. Influenza D virus infection in cattle is typically asymptomatic; however, its infection in swine can result in clinical disease. Swine can also be infected with all other types of influenza viruses, namely A, B, and C. Consequently, swine can serve as a "mixing vessel" for highly pathogenic influenza viruses, including those with zoonotic potential. Currently, the only antiviral drug available targets influenza M2 protein ion channel is not completely effective. Thus, it is necessary to develop an M2 ion channel blocker capable of suppressing the induction of resistance to the genetic shift. To provide a basis for developing novel ion channel-blocking compounds, we investigated the properties of influenza D virus M2 protein (DM2) as a drug target. RESULTS: To test the ion channel activity of DM2, the DNA corresponding to DM2 with cMyc-tag conjugated to its carboxyl end was cloned into the shuttle vector pNCB1. The mRNA of the DM2-cMyc gene was synthesized and injected into Xenopus oocytes. The translation products of DM2-cMyc mRNA were confirmed by immunofluorescence and mass spectrometry analyses. The DM2-cMyc mRNA-injected oocytes were subjected to the two-electrode voltage-clamp (TEVC) method, and the induced inward current was observed. The midpoint (Vmid) values in Boltzmann modeling for oocytes injected with DM2-cMyc RNA or a buffer were -152 and -200 mV, respectively. Assuming the same expression level in the Xenopus oocytes, DM2 without tag and influenza C virus M2 protein (CM2) were subjected to the TEVC method. DM2 exhibited ion channel activity under the condition that CM2 ion channel activity was reproduced. The gating voltages represented by Vmid for CM2 and DM2 were -141 and -146 mV, respectively. The reversal potentials observed in ND96 for CM2 and DM2 were -21 and -22 mV, respectively. Compared with intact DM2, DM2 variants with mutation in the YxxxK motif, namely Y72A and K76A DM2, showed lower Vmid values while showing no change in reversal potential. CONCLUSION: The M2 protein from newly isolated influenza D virus showed ion channel activity similar to that of CM2. The gating voltage was shown to be affected by the YxxxK motif and by the hydrophobicity and bulkiness of the carboxyl end of the molecule.
PubMed ID: 29927982
PMC ID: PMC6013169
Article link: PLoS One
Species referenced: Xenopus
Genes referenced: cnbp myc
Article Images: [+] show captions
|Fig 1. Amino acid sequences.A. Primary structures of influenza C virus M2 protein (CM2; Sequence ID, YP_089658.1 (C/Ann Arbor/1/1950)) and influenza D virus M2 protein (DM2; AFJ19025 (D/Swine/Oklahoma/1334/2011)). CM2 contains a 24-amino acid signal sequence (−24 to −1). Mature CM2 starts with Cys1 and contains extracellular, transmembrane (underlined), and internal domains in order from the amino to carboxyl terminals. Amino acid substitutions between Yamagata [BAA03793.1 (C/Yamagata/1/1988)] and Ann Arbor isolates are indicated in brackets. CM2 has disulfide-linked oligomerization sites (“*”; Cys1, Cys6, Cys20), a glycosylation site (Asn11, “g”), and a palmitoylation site (Cys65, “p”). A phosphorylation site involved in efficient virus replication is also indicated (Ser68, “k”) . In DM2, the predicted transmembrane domain is indicated by an underline. The YxxxK motif, Tyr72, and Lys76 in D is indicated by bold face. DM2 has a potential glycosylation site at Asn39 indicated by “g”. A C-terminal variant in D cMyc has a spacer GAG and a cMyc-tag EQKLISEEDL. A readthrough C-terminal variant of DM2, DM2 RT, has an extra peptide from the vector pNCB1. B. Structures of M2 proteins. AM2. The NMR structure (residues 22–62; PDB ID, 2L0J) and crystallographic structure (21–46; 4QKL) of influenza A virus M2 protein [AAA43303 [A/Udorn/1972(H3N2)]]. BM2. The NMR structure (1–33; 2KIX) of influenza B virus M2 protein [ACF54325.1 (B/Taiwan/70061/2006)]. CM2. The structure of influenza C virus M2 protein (27–46) was obtained by site-specific infrared dichroism . DM2. Influenza D virus M2 protein is focused on in this study. Solved or predicted structures are underlined. Amino acid residues providing experimental structure information are indicated by underlines. Primary structures of mutated DM2 are added with the isoelectric point (pI) and the hydrophobicity value (GRAVY) for the shaded helix domain. In GRAVY, greater positive values indicate higher hydrophobicity. C. Predicted secondary structure of carboxyl ends in DM2 constructs. Predicted secondary structures E and H correspond to β-strand and α-helix, respectively.|
|Fig 2. 3D structure of M2 protein.A. The structure of AM2 was solved experimentally by nuclear magnetic resonance spectroscopy (NMR; structures were deposited in the Protein Data Bank with ID code 2L0J; model 1 is shown while the NMR structure contains 8 isomers). AM2 adopts a homo-tetramer configuration, and the middle of the assembly contains a pore. The valve residues His37 and Trp41 face inward within the pore on each monomer. B. The monomer structure of AM2 with valve residues, which control ion flow by pseudo-cation—pi interaction. C. A theoretical model of DM2 in monomer format.|
|Fig 3. Confocal microscopy of oocytes.A. An oocyte injected with phosphate-buffered saline. B. An oocyte injected with RNA encoding c-Myc-tagged DM2. Oocytes were treated with anti-cMyc antibodies (mouse 9E10) and then donkey anti-mouse Alexa Fluor 488 antibodies.|
|Fig 4. Mass spectrometry and peptide identification.Presented the MS/MS fragmentation spectrum of the 36 amino acids peptide identified, Arg123 –Lys158. The oocyte injected with RNA encoding DM2–cMyc was subjected to tryptic digestion followed by mass spectrometry using a nano-LC-MS/MS set-up. At 1255.91, a peptide of 36-amino acid long which includes the C-terminal region, the linker, and a portion of the cMyc sequence (Fig 1) was confidently identified with m/z value. The 20 and 17 successive peptide N-terminus retaining Y (blue) and peptide N-terminus retaining B fragment ions (red) matched the peptide sequence with a parent ion mass error of 1.5 ppm. The identified amino acid sequence for Y peptide shown is backward (blue). The S1 Table shows the fragmentation table for the corresponding peptide Arg123 –Lys158 representing both b- and y-ions.|
|Fig 5. Western blot analysis.One each of the uninjected and the DM2 cMyc RNA injected oocyte was subjected to the Western Blot. The cMyc tag specific mouse monoclonal antibody 9E 10 was used to detect the expressed protein. The protein size is given in kDa.|
|Fig 6. Induced current recorded by two-electrode voltage-clamp (TEVC) method at pH 8.5.A. Induced current and given membrane potential. An oocyte injected with either RNA or phosphate-buffered saline was subjected to a TEVC recording, and the current was measured for 2 s while the potential was kept constant at each of several membrane potentials with 10 mV intervals. A relaxation time of 5 s was set before the next measurement while the membrane was held at Vm = 0 mV. Short bars indicate values at -110 mV and the corresponding current for eye references. B-E. Overlapping traces for activating and tail current vs. holding potential. An average of 5 measurements were plotted with the SD. Solid bars indicate p<0.05 in the t-test. Cartoons represent the molecular organizations of M2 proteins. The green circle and D represents DIDS.|
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