PH-DEPENDENT AND LIGAND-INDUCED CONFORMATIONAL-CHANGES OF EUKARYOTIC PROTEIN-SYNTHESIS INITIATION-FACTOR EIF-(ISO) 4F - A CIRCULAR-DICHROISM STUDY

The structural features of wheat germ protein synthesis initiation fac tor eIF-(iso)4F, which has a cap binding protein as one of its two sub units, 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 alpha-helix content of eIF-(iso)4F chan ged from 42% at pH 6.3 to 15% at pH 7.6, the optimum pH for cap bindin g. The beta-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 a nd analyzed. The separated subunits both had a higher alpha-helix cont ent at pH 7.6 than the native protein, giving values of 60% and 34% al pha-helix for p28 and p86. respectively. Binding of the dinucleotide c ap analog to p28 reduced the alpha-helix content to approximately 8% w ith an increase in the beta sheet content from 10% to 37%. The conform ational changes in eIF-(iso)4F upon binding with mRNA are dependent on cap or oligonucleotide structure. A conformation consisting of approx imately the same alpha-helix and beta-sheet content can be induced by ligands even at non-optimal pH values. This large conformational trans ition suggests eIF-(iso)4F binds nucleic acids by interaction of a bet a-sheet motif and that this conformational transition may have a regul atory role.