Solution structure of human type-alpha transforming growth factor determined by heteronuclear NMR spectroscopy and...

Human type-alpha transforming growth factor (hTGF alpha) is a small mitogenic protein containing 50 amino acids and 3 disulfide bonds. Homo- and heteronuclear NMR spectra were used to determine nearly complete sequence-specific 1H and 15N resonance assignments for hTGF alpha under three conditions: pH 6.5 and a temperature of 10 degrees C, pH 6.5 and a temperature of 30 degrees C, and pH 3.5 and a temperature of 30 degrees C. The 15N-enriched samples of hTGF alpha allowed determination of many 3J(HN-H alpha) vicinal coupling constants. Solution structures of human type-alpha transforming growth factor (hTGF alpha) at pH 6.5 and a temperature of 10 degrees C were determined from NMR data using molecular structure generation calculations and restrained energy minimization. These structures are based on 425 conformational constraints, including 357 NOE-derived upper-bound distance constraints, constraints on the ranges of 26 dihedral angles based on measurements of vicinal coupling constants, 42 upper- and lower-bound constraints associated with 6 hydrogen bonds and 3 disulfide bonds, and several stereospecific 1H resonance assignments. The overall structure is similar to that described recently for hTGF alpha by other groups [Kline et al. (1990) Biochemistry 29, 7805-7813; Harvey et al. (1991). Eur. J. Biochem. 198, 555-562], but there are differences in some structural details. The resonance frequencies, vicinal coupling constants, and NOEs form the basis for comparisons of the solution structure of hTGF alpha at neutral and acidic pH. At pH 3.5 the protein structure is partially disordered, with most of the hydrogen-bonded backbone structure still intact. The hTGF alpha structure is also compared with that of murine epidermal growth factor. Coordinates for the set of hTGF alpha structures described in this paper have been deposited in the Protein Data Bank.