PH-dependent and ligand induced conformational changes of eucaryotic protein synthesis initiation factor eIF-(iso) 4F: a circular dichroism study

Abstract

The structural features of wheat germ protein synthesis initiation factor eIF-(iso)4F, which has a cap binding protein as one of its two subunits, are unknown. In this study, circular dichroism (CD) spectra and secondary structure prediction were obtained for eIF-(iso)4F and its two subunits, p28 and p86. The α-helix content of eIF-(iso)4F changed from 42% at pH 6.3 to 15% at pH 7.6, the optimum pH for cap binding. The β-sheet content increased from 14% (pH 6.3) to 38% at pH 7.6. The CD spectra of the two subunits, p28 and p86 were also measured and analyzed. The separated subunits both had a higher α-helix content at pH 7.6 than the native protein, giving values of 60% and 34% α-helix for p28 and p86, respectively. Binding of the dinucleotide cap analog to p28 reduced the α-helix content to approximately 8% with an increase in the β sheet content from 10% to 37%. The conformational changes in eIF-(iso)4F upon binding with mRNA are dependent on cap or oligonucleotide structure. A conformation consisting of approximately the same α-helix and β-sheet content can be induced by ligands even at non-optimal pH values. This large conformational transition suggests eIF-(iso)4F binds nucleic acids by interaction of a β-sheet motif and that this conformational transition may have a regulatory role.