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Spike protein oligomerization control of Semliki Forest virus fusion
M Lobigs1, J M Wahlberg, H Garoff
1Department of Molecular Biology, Karolinska Institute, Huddinge, Sweden.
Abstract:
We have recently shown, using cleavage-deficient mutants of the p62-E1 membrane protein complex of Semliki Forest virus that p62 cleavage to E2 is necessary for the activation of the fusion function of the complex at pH 5.8 (a pH optimal for virus fusion) (M. Lobigs and H. Garoff, J. Virol. 64:1233-1240, 1990). In this study, we show that the mutant precursor complexes can be induced to activate membrane fusion when treated with more acidic buffers (pH 5.0 and 4.5), which also appear to dissociate most of the p62-E1 complexes and change the conformation of the E1 subunit (the supposed fusion protein of Semliki Forest virus into a form which is resistant to trypsin digestion. These data suggest that p62 cleavage is not essential for membrane fusion per se but that the crucial event activating this process seems to be the apparent dissociation of the heterodimer, which in turn is facilitated by the spike precursor cleavage.
Insights
Cleavage of Semliki Forest virus p62-E1 complex is not essential for membrane fusion. Fusion activation occurs at lower pH, inducing p62-E1 complex dissociation and E1 conformational changes.
Area of Science:
- Virology
- Membrane protein complex function
- Viral fusion mechanisms
Background:
- Previous studies indicated p62 cleavage is necessary for Semliki Forest virus fusion complex activation at pH 5.8.
- The p62-E1 complex mediates membrane fusion, with E1 as the putative fusion protein.
Purpose of the Study:
- To investigate the role of p62 cleavage in viral membrane fusion.
- To determine the conditions that activate fusion in cleavage-deficient p62-E1 mutants.
Main Methods:
- Utilized cleavage-deficient mutants of the Semliki Forest virus p62-E1 complex.
- Examined fusion activation under varying acidic buffer conditions (pH 5.0 and 4.5).
- Assessed p62-E1 complex dissociation and E1 subunit conformation changes via trypsin resistance.
Main Results:
- Mutant p62-E1 complexes induced membrane fusion at more acidic pH (5.0 and 4.5).
- Lower pH induced dissociation of p62-E1 complexes and altered E1 conformation.
- E1 subunit adopted a trypsin-resistant form at acidic pH.
Conclusions:
- p62 cleavage is not strictly required for Semliki Forest virus membrane fusion.
- Heterodimer dissociation, facilitated by spike precursor cleavage, is the key event for fusion activation.
- Acidic conditions promote fusion by dissociating the p62-E1 complex and altering E1 structure.