As observed in our initial virtual screening, the molecular docking analyses suggest that MPA only binds to the closed form of the enzyme. No significant binding of MPA was detected when the molecular docking experiments were performed on the open forms of the C. albicans, S. cerevisiae and Chlorella virus RNA guanylyltransferases bound with GMP. It is tempting to speculate that the opening of the active site of the enzyme that is normally observed following the hydrolysis of GTP to produce the enzyme-GMP covalent intermediate [23] is inhibited by MPA. Following the formation of the GMP adduct, the enzyme must open up to provide access for the incoming mRNA substrate, since this site is blocked off in the closed form of the enzyme [23] . The presence of multiple interactions between MPA and residues of both the Nterminal and C-terminal domains of the enzyme likely inhibits this critical conformational change that is required for the binding of RNA. In order to validate this hypothesis, cross-linking assays were used to monitor the binding of radiolabelled RNA to the enzyme-GMP complex with or without MPA. An RNA harboring a 59triphosphate end was used as a ligand instead of the classical RNA with a 59-diphosphate since the latter would lead to the transfer of the GMP moiety onto the RNA. Using this approach, an apparent K d of 30 mM could be estimated for the binding of the RNA to the enzyme-GMP complex (data not shown). However, the binding of RNA to the enzyme-GMP/MPA complex was severely decreased in the presence of MPA (Fig. 5A ). An IC 50 value of 410 mM could be estimated from the inhibition assays (Fig. 5B) . These results demonstrate that the presence of MPA inhibits the binding of the enzyme-GMP complex to RNA.