Determination of the Secondary Structure and Folding Topology of Human Interleukin-4 Using Three-Dimensional Heteronuclear Magnetic Resonance Spectroscopy

The secondary structure of human recombinant interleukin-4 (IL-4) has been investigated by three-dimensional (3D) ¹⁵N- and ¹³C-edited nuclear Overhauser (NOE) spectroscopy on the basis of the ¹H, ¹⁵N, and ¹³C 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^αH, and C^βH protons, as well as ³J_Hnα coupling constant, amide exchange, and ¹³C^α and ¹³C^β secondary chemical shift data, it is shown that IL-4 consists offour 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 β-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 oflong-range interhelical NOEs identified in the 3D ¹³C-edited NOE spectrum in combination with the spatial restrictions imposed by three disulfide bridges. These data indicate that the helices ofinterleukin-4 are arranged in a left-handed four-helix bundle with two overhand connections.