Effect of M1 protein and low pH on nuclear transport of influenza virus ribonucleoproteins
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M Bui1, G Whittaker, A Helenius
1Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Abstract:
Influenza virus enters its host cell by receptor-mediated endocytosis followed by acid-activated membrane fusion in endosomes. The viral ribonucleoprotein particles (vRNPs) delivered into the cytosol then dissociate from the matrix protein, M1, and from each other, after which they are individually imported into the nucleus via the nuclear pores. For some time, it has been believed that the low pH in endosomes may, in some way, trigger the capsid disassembly events necessary for nuclear transport. This report provides direct evidence that the association of M1 with vRNPs is sensitive to mildly acidic pH within the infected cell. Recombinant M1, expressed in cultured cells, was found to associate with vRNPs and inhibit their nuclear import. Brief acidification of the cytosolic compartment eliminated the interfering activity and allowed the incoming vRNPs to enter the nucleus. Newly assembled progeny M1-vRNP complexes in the cytosol of infected cells were also dissociated by brief acidification. Acidic pH was thus found to serve as a switch that allowed M1 to carry out its multiple functions in the uncoating, nuclear transport, and assembly of vRNPs.
Insights
Acidic pH triggers influenza virus uncoating and nuclear transport. This study shows that low pH in endosomes and the cytosol dissociates the matrix protein M1 from viral ribonucleoprotein particles (vRNPs), enabling nuclear import and viral assembly.
Area of Science:
- Virology
- Cell Biology
- Molecular Biology
Background:
- Influenza virus infection involves endocytosis and acid-activated fusion within endosomes.
- Viral ribonucleoprotein particles (vRNPs) are released into the cytosol and require nuclear import.
- The role of low pH in triggering vRNP uncoating and nuclear transport has been hypothesized.
Purpose of the Study:
- To provide direct evidence for the pH-sensitivity of matrix protein M1 association with vRNPs.
- To investigate the effect of acidic pH on vRNP nuclear import and M1 dissociation.
- To elucidate the function of acidic pH as a trigger for influenza virus uncoating and assembly.
Main Methods:
- Expression of recombinant M1 protein in cultured cells.
- Analysis of M1-vRNP complex formation and dissociation under varying pH conditions.
- Monitoring of vRNP nuclear import in response to cytosolic acidification.
Main Results:
- Recombinant M1 associates with vRNPs and inhibits their nuclear import.
- Mildly acidic pH disrupts the M1-vRNP association, facilitating nuclear import.
- Acidification also dissociates newly assembled M1-vRNP complexes in infected cells.
Conclusions:
- The association of influenza virus matrix protein M1 with vRNPs is sensitive to mildly acidic pH.
- Acidic pH acts as a critical switch, regulating M1's function in vRNP uncoating, nuclear transport, and viral assembly.
- This pH-dependent mechanism is essential for the influenza virus life cycle.