The polypeptide chain of eukaryotic initiation factor 5A occurs in two distinct conformations in the absence of the...

Eukaryotic initiation factor 5A (eIF-5A) requires posttranslational modification of lysine at position 50 to hypusine for its biological activity. We have expressed an unmodified variant of eIF-5A in Escherichia coli and show that it has structural properties different from those of the native protein in terms of its near- and far-UV circular dichroism spectra and its equilibrium unfolding transition with guanidinium chloride. In contrast to the hypusine-modified protein, which unfolds in a two-state process, the complex unfolding transition of unmodified eIF-5A suggests that this variant occurs in two differently folded conformations, F1 and F2. Both conformations are populated under near-physiological conditions at a ration of 60 to 40, respectively. Equilibrium unfolding consists of parallel events: unfolding of F1 to one or several intermediate states (I), and unfolding of F2 to the unfolded state (U). Although the establishment of each of these individual equilibria is fast, the interconversion is slow at guanidinium chloride concentrations between 0 M and 3 M. Kinetic analysis reveals activation energies of 24.3 kcal mol-1 for the reaction of F1 and F2 and 24.1 kcal mol-1 for the reaction of F2 to F1. Both F1 and F2 possess well-defined secondary and tertiary structure. However, the tertiary structures of the two conformations differ as indicated by their distinct near-UV circular dichroism spectra. These differences may be restricted to the C-terminal part of the protein as 2-dimensional 1H-NMR spectra of unmodified eIF-5A reveal no doubled set of proton resonances for aromatic amino acid and histidine residues, of which almost all are located in the N-terminal region.(ABSTRACT TRUNCATED AT 250 WORDS)