SOLUTION DYNAMICS AND SECONDARY STRUCTURE OF MURINE LEUKEMIA INHIBITORY FACTOR - A 4-HELIX CYTOKINE WITH A RIGID CD-LOOP

Leukemia inhibitory factor (LIF) is a hematopoietic cytokine which eli cits its effects on diverse cell types via both gp130 and a more speci fic LIF receptor. Recombinant, murine LIF was studied by multidimensio nal homonuclear and H-1-N-15 heteronuclear NMR and 95% of backbone ami de resonances assigned. Definition of the secondary structure by chemi cal shift data and NOE connectivities shows a four-or-helix bundle fol d (helices A-D) in solution, with an additional flexible turn of helix in the AB loop. Subtle differences an seen in the conformations of he lices A and D from those defined in the crystal structure [Robinson, R , C., Grey, L. M., Staunton, D., Vankelcom, H., Vernallis, A. B., More au, J.-F., Stuart, D. I., Heath, J. K,, et Jones, E. Y. (1994) Cell 77 , 1101-1116]. The dynamics of the polypeptide backbone of LIF were ass essed from N-15 T-1 and T-2 relaxation times and N-15-H-1 heteronuclea r NOEs of the amide groups. Using model-free formalism, the overall ro tational correlation time of LIF in solution is calculated to be 9.7 p s, The four alpha-helices are relatively rigid, and high mobility is o bserved for N-terminal residues (Ser I-Asn 21) and the AB loop. In con trast to several closely related cytokines, the long CD loop is relati vely rigid. This may have implications for interactions with the speci fic LIF receptor, which binds in this region.