Declaration of smallpox eradication by the WHO on 1980 led to discontinuation of the world-wide vaccination campaign. The increasing percentage of unvaccinated individuals, the existence of its causative infectious agent variola virus (VARV) and the recent synthetic achievements, increases the threat of intentional or accidental release and reemergence of smallpox. Control of smallpox would require an emergency vaccination campaign as no other protective measure has been approved to achieve eradication and ensure world-wide protection. Experimental data in surrogate animal models support the assumption, based on anecdotal, uncontrolled historical data, that vaccination up to 4 days postexposure, confers effective protection. The long incubation period and the uncertainty of the exposure status in the surrounding population calls for the development and evaluation of safe and effective methods that would allow to extend the therapeutic window and to reduce the disease manifestations and vaccine adverse reactions. To achieve these goals, we need to evaluate the efficacy of novel and already licensed vaccines as a sole treatment or in conjunction with immune modulators and antiviral drugs. In this review, we address the available data, recent achievements and open questions.

The modification of GFP-CaaL sensor proteins by geranylgeranyl groups in plants requires an active plastidial mevalonate (MVA)-independent pathway. As a consequence, fosmidomycin, a specific inhibitor of 1-deoxy-D-xylulose (DX)-5 phosphate reductoisomerase/DXR, the second enzyme in this so-called methylerythritol phosphate (MEP) pathway, impedes protein prenylation, which results in GFP-labeling of nuclei instead of the plasma membrane. In tobacco cells, a visual screen of conditions that allowed membrane localization of GFP-CaaL proteins in the presence of fosmidomycin, pinpointed jasmonic acid methyl esther (Me-JA) as a chemical able to overcome the inhibition in a gradual manner. Using Arabidopsis protein prenyltransferase loss-of-function mutant lines expressing GFP-CaaL proteins, we demonstrated that Me-JA induces a modification in protein substrate recognition by protein farnesyltransferase (PFT). Similar to Me-JA, farnesol and MVA also alter the protein substrate specificity of PFT, whereas DX and geranylgeraniol have limited or no effect. Our data indicate that stimulation of the MVA pathway adjusts the protein substrate specificity of PFT, which is corroborated with metabolic cross-talk of the origin of the prenyl group used to modify the protein. To illustrate how activation of the MVA pathway impacting PFT substrate specificity benefits a plant under stress conditions, a model is proposed.