DETERMINATION OF THE SECONDARY STRUCTURE AND FOLDING TOPOLOGY OF HUMAN INTERLEUKIN-4 USING 3-DIMENSIONAL HETERONUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY

The secondary structure of human recombinant interleukin-4 (IL-4) has been investigated by three-dimensional (3D) N-15- and C-13-edited nuclear Overhauser (NOE) spectroscopy on the basis of the H-1, N-15, and C-13 assignments presented in the preceding paper [Powers, R., Garrett, D. S., March, C. J., Frieden, E. A., Gronenborn, A. M., & Clore, G. M. (1992) Biochemistry (preceding paper in this issue)]. Based on the NOE data involving the NH, C(alpha)H, and C(beta)H protons, as well as 3J(HN-alpha) coupling constant, amide exchange, and C-13(alpha) and C-13(beta) secondary chemical shift data, it is shown that IL-4 consists of four long helices (residues 9-21, 45-64, 74-96, and 113-129), two small helical turns (residues 27-29 and 67-70), and a mini antiparallel beta-sheet (residues 32-34 and 110-112). In addition, the topological arrangement of the helices and the global fold could be readily deduced from a number of long-range interhelical NOEs identified in the 3D C-13-edited NOE spectrum in combination with the spatial restrictions imposed by three disulfide bridges. These data indicate that the helices of interleukin-4 are arranged in a left-handed four-helix bundle with two overhand connections.