The 4E binding proteins (4E-BP1 and 4E-BP2) inhibit translation by binding to the limiting, proto-oncogenic initiation factor eIF4E. 4E-BPs produced in Escherichia coli had little or no folded structure, measured by NMR and CD. However, these proteins inhibited translation in reticulocyte lysate. Furthermore, they bound to isolated mouse eIF4E, showing a few broader, dispersed new NMR signals but no general increase in chemical shift dispersion. A peptide with the sequence of 4E-BP1 residues 49-68 was sufficient to bind eIF4E and to inhibit translation in reticulocyte lysate. These results suggest that a short central region of the 4E-BPs is responsible for eIF4E binding and translation inhibition while the remainder is unfolded and flexible.