Oxyanion binding alters conformation and quaternary structure of the C-terminal domain of the transcriptional regulator ModE - Implications for molybdate-dependent regulation, signaling, storage, and transport

The molybdate-dependent transcriptional regulator ModE of Escherichia coli functions as a sensor of intracellular molybdate concentration and a regula tor for the transcription of several operons that control the uptake and ut ilization of molybdenum. We present two high-resolution crystal structures of the C-terminal oxyanion-binding domain in complex with molybdate and tun gstate. The ligands bind between subunits at the dimerization interface, an d analysis reveals that oxyanion selectivity is determined primarily by siz e. The relevance of the structures is indicated by fluorescence measurement s, which show that the oxyanion binding properties of the C-terminal domain of ModE are similar to those of the full-length protein. Comparisons with the apoprotein structure have identified structural rearrangements that occ ur on binding oxyanion. This molybdate-dependent conformational switch prom otes a change in shape and alterations to the surface of the protein and ma y provide the signal for recruitment of other proteins to construct the mac hinery for transcription. Sequence and structure-based comparisons lead to a classification of molybdate-binding proteins.