Role of metal-ligand coordination in the folding pathway of zinc finger peptides

In the zinc fingers of TFIIIA family, two cysteines near the N-terminus and two histidines near the C-terminus are conserved for each finger unit. A cooperative binding of these residues to a Zn(II) ion is essential for the formation of the finger structure consisting of an anti-parallel β-sheet and an α-helix. In order to reveal the folding pathway of the zinc linger, we have investigated, by Raman spectroscopy, the relationship between Zn(II)-ligand binding and conformational change of a 27-mer peptide representing the third finger of mouse transcription factor Zif268. In the absence of Zn(II), the peptide assumes a β-sheet-rich structure. Upon addition of Zn(II), cysteines preferentially bind to Zn(II) prior to the metal coordination of histidines. Both the Zn(II)-cysteine and Zn(II)-histidine binding induce a partial secondary structure transition from β-sheet to α-helix. Exchange of the ligand amino acid residues, i.e., cysteines to histidines and vice versa, produces a striking effect on the folding of the peptide. The β-sheet → α-helix transition is induced only by the Zn(II)-cysteine binding and the ligand exchanged peptide is not capable of folding into the finger structure. The present results demonstrate the importance of the ligand arrangement in the folding of zinc finger.