RESONANCE ASSIGNMENTS, SECONDARY STRUCTURE AND TOPOLOGY OF LEUKEMIA INHIBITORY FACTOR IN SOLUTION

The chemical shift assignments and secondary structure of a murine-hum an chimera, MH35, of leukaemia inhibitory factor (LIF), a 180-residue protein of molecular mass 20 kDa, have been determined from multidimen sional heteronuclear NMR spectra acquired on a uniformly C-13, N-15-la belled sample. Secondary structure elements were defined on the basis of chemical shifts, NH-(CH)-H-alpha coupling constants, medium-range N OEs and the location of slowly exchanging amide protons. The protein c ontains four alpha-helices, the relative orientations of which were de termined on the basis of long-range, interhelical NOEs. The four helic es are arranged in an up-up-down-down orientation, as found in other f our-helical bundle cytokines. The overall topology of MH35-LIF is simi lar to thar of the X-ray crystallographic structure for murine LIF [Ro binson et al. (1994) Cell, 77, 1101-1116]. Differences between the X-r ay structure and the solution structure are evident in the N-terminal tail, where the solution structure has a trans-Pro(17) compared with t he cis-Pro(17) found in the crystal structure and the small antiparall el beta-sheet encompassing residues in the N-terminus and CD loop in t he crystal structure is less stable.