THE HIGH-RESOLUTION, 3-DIMENSIONAL SOLUTION STRUCTURE OF HUMAN INTERLEUKIN-4 DETERMINED BY MULTIDIMENSIONAL HETERONUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY

The high-resolution three-dimensional solution structure of recombinan t human interleukin-4 (IL-4), a protein of approximately 15 kDa which plays a key role in the regulation of B and T lymphocytes, has been de termined using three- and four-dimensional heteronuclear NMR spectrosc opy. The structure is based on a total of 2973 experimental NMR restra ints, comprising 2515 approximate interproton distance restraints, 102 distance restraints for 51 backbone hydrogen bonds, and 356 torsion a ngle restraints. A total of 30 structures was calculated by means of h ybrid distance geometry-simulated annealing, and the atomic rms distri bution about the mean coordinate positions for residues 8-129 is 0.44 +/- 0.03 angstrom for the backbone atoms, 0.83 +/- 0.03 angstrom for a ll atoms, and 0.51 +/- 0.04 angstrom for all atoms excluding disordere d side chains. The N- and C-terminal residues (1-7 and 130-133, respec tively) appear to be disordered. The structure of IL-4 is dominated by a left-handed four-helix bundle with an unusual topology comprising t wo overhand connections. The linker elements between the helices are f ormed by either long loops, small helical turns, or short strands. The latter include a mini anti-parallel beta-sheet. A best fit superposit ion of the NMR structure of IL-4 with the 2.25 angstrom resolution cry stal structure [Wlodawer, A., Pavlovsky, A., & Gutschina, A. (1992) FE BS Lett. 309, 59-64] yields a backbone atomic rms difference of 1.37 a ngstrom which can be mainly attributed to tighter packing of the helic es in the crystal structure. This is indicated by an approximately 20% reduction in the axial separation of three pairs of helices (alpha(A) -alpha(C), alpha(A)-alpha(D), and alpha(C)-alpha(D)) in the crystal st ructure relative to the NMR structure and may reflect the greater flex ibility of the molecule in solution which is reduced in the crystal du e to intermolecular contacts. Comparison of the NMR structure of IL-4 with the X-ray structures of two other related proteins, granulocyte-m acrophage colony stimulating factor [Diedrichs, K., Boone, T., & Karpl us, P. A. (1992) Science 254, 1779-1782] and human growth hormone [de Vos, A. M., Ultsch, M., & Kossiakoff, A. A. (1992) Science 255, 306-31 2], that bind to the same hematopoietic superfamily of cell surface re ceptors reveals a remarkably similar topological fold, despite the abs ence of any significant overall sequence identity, and substantial dif ferences in the relative lengths of the helices, the lengths and the n ature of the various connecting elements, and the pattern and number o f disulfide bridges. Indeed, the C(alpha) atom coordinates of 72 and 7 9 residues of IL-4 can be superimposed on the C(alpha) coordinates of granulocyte-macrophage colony stimulating factor and human growth horm one with rms differences of approximately 1.7 and 2.0 angstrom, respec tively.