The structurally unique cytokine, macrophage migration inhibitory factor (MIF), exerts glucocorticoid inhibitory properties, alongside cell cycle and immune stimulative activities. Novel MIF inhibitors, such as the naturally occurring phenethyl isothiocyanate (PEITC), have been seen to alleviate disease-associated phenotypes, including those of rheumatoid arthritis, cancer and ulcerative colitis. We have previously published the X-ray crystal structure of covalently bound MIF: PEITC (4F2K)¹; here we present an improved structure based on a higher resolution data set.

Crystals grown in identical conditions to those of the original structure, were collected on beamline MX1 of the Australian Synchrotron. Higher resolution (1.48 Å v 1.53 Å) with greater completeness (94.1% v 81.4%) and a superior R_merge (0.047 v 0.057) resulted.  

Employing anisotropic b-factor refinement performed using CCP4, final structure R_cryst of 0.14 and R_free of 0.16 were determined. Higher R values were seen for structure 4F2K, collected on the Rigaku 007HF rotating anode with an R-Axis IV++ image plate detector (Department of Biochemistry) with R_cryst of 0.19 (0.50 in the highest resolution shell); R_free of 0.23 (0.59). As expected, 4F2K is similar to the new structure with an RMSD of 0.140. The results presented here confirm the previously deduced MIF: PEITC complex.

Modelled transmembrane CD74 (MIF receptor) is predicted to interact in the catalytic pocket of MIF, the same site as that of the irreversible PEITC. The immediate area is affected by formation of PEITC, with the first turn of the first helix and the preceding loop (residues 31-34) adjacent to the binding pocket displaced 1.6 Å from the ß-sheet core. Tyr37 sits directly in front of this binding pocket, with vast deviation of it's position seen in MIF: PEITC. These changes may account for the inhibition of MIF: CD74 interaction by PEITC.

This work was supported by a University of Otago Research Grant.