Abstract
The idea that reversible binding of a small inorganic ion can mimic covalent modification by phosphate is, at first, surprising. However, there is a fairly long history of using such compounds (aluminum fluoride and vanadate as well as beryllofluoride) to form complexes with nucleoside diphosphates at the active sites of ATPases and GTPases (1, 2, 6, 13). Indeed, it was in an attempt to form such a complex with beryllofluoride and ADP at the ATP binding site of the σ54 activator NtrC that Dalai Yan et al. found activation of the protein rather than the anticipated inhibition (15). Subsequent detailed investigation showed that addition of BeF3− yielded an activity level as high as that from phosphorylation in a direct assay of transcriptional activation by NtrC. The interaction with BeF3− is very specific to the active site in the response regulator receiver domain, which is normally activated by phosphotransfer from a histidine kinase as part of a two-component signaling system (14). The aspartate that either is phosphorylated or interacts with the beryllofluoride is part of an active site that greatly enhances the rate of transfer of phosphate from activated phosphodonors, including the phosphohistidine of the kinase. It is the interaction with residues that constitute this active site, including a bound magnesium ion, that facilitates phosphotransfer and beryllofluoride binding.