Metamorphic Protein IscU Changes Conformation by <i>cis</i>–<i>trans</i> Isomerizations of Two Peptidyl–Prolyl Peptide Bonds

Abstract

IscU from <i>Escherichia coli</i>, the scaffold protein for iron–sulfur cluster biosynthesis and transfer, populates two conformational states with similar free energies and with lifetimes on the order of 1 s that interconvert in an apparent two-state reaction. One state (S) is structured, and the other (D) is largely disordered; however, both play essential functional roles. We report here nuclear magnetic resonance studies demonstrating that all four prolyl residues of apo-IscU (P14, P35, P100, and P101) are <i>trans</i> in the S state but that two absolutely conserved residues (P14 and P101) become <i>cis</i> in the D state. The peptidyl–prolyl peptide bond configurations were determined by analyzing assigned chemical shifts and were confirmed by measurements of nuclear Overhauser effects. We conclude that the S ⇄ D interconversion involves concerted <i>trans</i>–<i>cis</i> isomerization of the N13–P14 and P100–P101 peptide bonds. Although the D state is largely disordered, we show that it contains an ordered domain that accounts for the stabilization of two high-energy <i>cis</i> peptide bonds. Thus, IscU may be classified as a metamorphic protein