Polypeptide Chain Fold of Human Transforming Growth Factor α Analogous to Those of Mouse and Human Epidermal Growth Factors As Studied by Two-Dimensional ¹H NMR

The ¹H NMR spectrum of human transforming growth factor α (TGF-α) was analyzed almost completely by the sequential assignment method using two-dimensional NMR techniques. On the basis of the nearly complete sequence-specific resonance assignment, secondary and tertiary structures of human TGF-α in solution (pH 4.9, 28 °C) were determined to satisfy the upper limits of proton-proton distances derived from nuclear Overhauser effect experiments. Although human TGF-α and mouse epidermal growth factor (EGF) share 27% homology in amino acid sequence, the backbone chain folds in the two growth factors are quite similar. The structure and function of TGF-α is well characterized by the “mitten model” previously proposed for mouse EGF. The gross shape of the TGF-α molecule resembles a mitten. TGF-α interacts with the receptor as a mitten would grasp an object. However, there is an appreciable structural difference between the two growth factors in the back of the mitten that is formed by the N-terminal polypeptide segment. This is consistent with the evidence that the backs of these molecules are not involved in the receptor binding.