Differences in the conformational state of a zinc‐finger DNA‐binding protein domain occupied by zinc and copper revealed by electrospray ionization mass spectrometry

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Abstract
Transition metal ions are important in biological regulation partly because they can bind to and stabilize protein surface domain structures in specific conformations that are involved in key molecular recognition events. There are twso C2-C2 type zinc-finger sequences within the highly conserved DNA-binding domain of the estrogen receptor protein (ERDBD). Electrospray ionization (ESI) mass spectrometry has been used to demonstrate that the metal-binding sites within the 71-residue ERDBD can bind either Zn (up to 2) or Cu (up to 4). Evidence for the induction and /or stabilization of a different conformational state with bound Cu is revealed by a characteristic shift in the ESI charge envelope. The 10+ charge state is most abundant for the fully reduced ERDBD apopeptide and the ERDBD-Zn holopeptide (bound Zn does not alter the charge envelope. In contrast, the 8+ charge state is typically the optimjum charge state observed for the ERDBD-Cu holopeptide; indeed, the entire charge envelope is frame-shifted to lower charge states with bound Cu. Interpretation of the altered charge states is simplified because (i) a single type of metal-binding ligand (sulfur) is involved in the case of both Zn and Cu binding, and (ii) the two different metal cations are both divaalent. Thus, it is likely that the dissimilar charge envelopes represent different peptide conformers, each of which is stabilized by a different type of bound metal ion. The covalent bridging or adjacent Cys residues within a peptide is similar to the formation of intramolecular disulfide bonds, a process that we also found to decrease both the optimum and mumber of ERDBD charge states observed by ESI. The existence of distinct conformational states for the ERDBD-Zn and ERDBD-Cu is consistent with the differences in apparent coordinate covalent geometry and stoichiometry for the two bound metal ions. Our findings are aslo consistent with prevous investigations of globular proteins that reveal an increase both in the number of charge states and a shift in the optimum charge state when structure is disrupted by reduction of intramolecular disulfide bonds.

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