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Analysis of the subunit stoichiometries in viral entry.

Magnus C, Regoes RR - PLoS ONE (2012)

Bottom Line: Our model framework also shows why it is important to subdivide the question of the number of functional subunits within one trimer into the three different subunit stoichiometries.As an example for how our models can be applied, we reanalyze a data set on subunit stoichiometries.Our study is motivated by the mechanism of HIV entry but the experimental technique and the model framework can be extended to other viral systems as well.

View Article: PubMed Central - PubMed

Affiliation: Integrative Biology, The Swiss Federal Institute of Technology, Zurich, Switzerland. carsten.magnus@zoo.ox.ac.uk

ABSTRACT
Virions of the Human Immunodeficiency Virus (HIV) infect cells by first attaching with their surface spikes to the CD4 receptor on target cells. This leads to conformational changes in the viral spikes, enabling the virus to engage a coreceptor, commonly CCR5 or CXCR4, and consecutively to insert the fusion peptide into the cellular membrane. Finally, the viral and the cellular membranes fuse. The HIV spike is a trimer consisting of three identical heterodimers composed of the gp120 and gp41 envelope proteins. Each of the gp120 proteins in the trimer is capable of attaching to the CD4 receptor and the coreceptor, and each of the three gp41 units harbors a fusion domain. It is still under debate how many of the envelope subunits within a given trimer have to bind to the CD4 receptors and to the coreceptors, and how many gp41 protein fusion domains are required for fusion. These numbers are referred to as subunit stoichiometries. We present a mathematical framework for estimating these parameters individually by analyzing infectivity assays with pseudotyped viruses. We find that the number of spikes that are engaged in mediating cell entry and the distribution of the spike number play important roles for the estimation of the subunit stoichiometries. Our model framework also shows why it is important to subdivide the question of the number of functional subunits within one trimer into the three different subunit stoichiometries. In a second step, we extend our models to study whether the subunits within one trimer cooperate during receptor binding and fusion. As an example for how our models can be applied, we reanalyze a data set on subunit stoichiometries. We find that two envelope proteins have to engage with CD4-receptors and coreceptors and that two fusion proteins must be revealed within one trimer for viral entry. Our study is motivated by the mechanism of HIV entry but the experimental technique and the model framework can be extended to other viral systems as well.

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Related in: MedlinePlus

Predictions for the relative infectivity for different subunit stoichiometries in the basic model (equation 4).For plot (A) and (B) we assume that virions have exactly 10 trimers. For plot (A) and (C) we fix the stoichiometry of entry at , according to our estimate in [15]. (A) Influence of the subunit stoichiometry  on the predictions for the relative infectivity. (B) Higher stoichiometries of entry shift the RI curve to the left (solid curves). Together with the effect of the subunit stoichiometry one can find parameter combinations of  and  that lead to similar predictions (blue curves). (C) Dependence of the relative infectivity on the trimer number distribution.
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pone-0033441-g004: Predictions for the relative infectivity for different subunit stoichiometries in the basic model (equation 4).For plot (A) and (B) we assume that virions have exactly 10 trimers. For plot (A) and (C) we fix the stoichiometry of entry at , according to our estimate in [15]. (A) Influence of the subunit stoichiometry on the predictions for the relative infectivity. (B) Higher stoichiometries of entry shift the RI curve to the left (solid curves). Together with the effect of the subunit stoichiometry one can find parameter combinations of and that lead to similar predictions (blue curves). (C) Dependence of the relative infectivity on the trimer number distribution.

Mentions: The relative infectivities for the subunit stoichiometry , as functions of the fraction of envelope proteins with a mutation are predicted to be sigmoid curves. The smaller the subunit stoichiometry is, the higher the fraction of mutated envelope proteins, , must become to decrease the relative infectivity (Figure 4A). In this Figure, the number of trimers on virions is fixed to 10 ( for , else) and the stoichiometry of entry .


Analysis of the subunit stoichiometries in viral entry.

Magnus C, Regoes RR - PLoS ONE (2012)

Predictions for the relative infectivity for different subunit stoichiometries in the basic model (equation 4).For plot (A) and (B) we assume that virions have exactly 10 trimers. For plot (A) and (C) we fix the stoichiometry of entry at , according to our estimate in [15]. (A) Influence of the subunit stoichiometry  on the predictions for the relative infectivity. (B) Higher stoichiometries of entry shift the RI curve to the left (solid curves). Together with the effect of the subunit stoichiometry one can find parameter combinations of  and  that lead to similar predictions (blue curves). (C) Dependence of the relative infectivity on the trimer number distribution.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3316572&req=5

pone-0033441-g004: Predictions for the relative infectivity for different subunit stoichiometries in the basic model (equation 4).For plot (A) and (B) we assume that virions have exactly 10 trimers. For plot (A) and (C) we fix the stoichiometry of entry at , according to our estimate in [15]. (A) Influence of the subunit stoichiometry on the predictions for the relative infectivity. (B) Higher stoichiometries of entry shift the RI curve to the left (solid curves). Together with the effect of the subunit stoichiometry one can find parameter combinations of and that lead to similar predictions (blue curves). (C) Dependence of the relative infectivity on the trimer number distribution.
Mentions: The relative infectivities for the subunit stoichiometry , as functions of the fraction of envelope proteins with a mutation are predicted to be sigmoid curves. The smaller the subunit stoichiometry is, the higher the fraction of mutated envelope proteins, , must become to decrease the relative infectivity (Figure 4A). In this Figure, the number of trimers on virions is fixed to 10 ( for , else) and the stoichiometry of entry .

Bottom Line: Our model framework also shows why it is important to subdivide the question of the number of functional subunits within one trimer into the three different subunit stoichiometries.As an example for how our models can be applied, we reanalyze a data set on subunit stoichiometries.Our study is motivated by the mechanism of HIV entry but the experimental technique and the model framework can be extended to other viral systems as well.

View Article: PubMed Central - PubMed

Affiliation: Integrative Biology, The Swiss Federal Institute of Technology, Zurich, Switzerland. carsten.magnus@zoo.ox.ac.uk

ABSTRACT
Virions of the Human Immunodeficiency Virus (HIV) infect cells by first attaching with their surface spikes to the CD4 receptor on target cells. This leads to conformational changes in the viral spikes, enabling the virus to engage a coreceptor, commonly CCR5 or CXCR4, and consecutively to insert the fusion peptide into the cellular membrane. Finally, the viral and the cellular membranes fuse. The HIV spike is a trimer consisting of three identical heterodimers composed of the gp120 and gp41 envelope proteins. Each of the gp120 proteins in the trimer is capable of attaching to the CD4 receptor and the coreceptor, and each of the three gp41 units harbors a fusion domain. It is still under debate how many of the envelope subunits within a given trimer have to bind to the CD4 receptors and to the coreceptors, and how many gp41 protein fusion domains are required for fusion. These numbers are referred to as subunit stoichiometries. We present a mathematical framework for estimating these parameters individually by analyzing infectivity assays with pseudotyped viruses. We find that the number of spikes that are engaged in mediating cell entry and the distribution of the spike number play important roles for the estimation of the subunit stoichiometries. Our model framework also shows why it is important to subdivide the question of the number of functional subunits within one trimer into the three different subunit stoichiometries. In a second step, we extend our models to study whether the subunits within one trimer cooperate during receptor binding and fusion. As an example for how our models can be applied, we reanalyze a data set on subunit stoichiometries. We find that two envelope proteins have to engage with CD4-receptors and coreceptors and that two fusion proteins must be revealed within one trimer for viral entry. Our study is motivated by the mechanism of HIV entry but the experimental technique and the model framework can be extended to other viral systems as well.

Show MeSH
Related in: MedlinePlus