THE SUBUNIT STRUCTURE OF HUMAN MACROPHAGE-MIGRATION INHIBITORY FACTOR- EVIDENCE FOR A TRIMER

The subunit structure of human macrophage migration inhibitory factor (MIF) has been studied by preliminary X-ray analysis of wild-type and selenomethionine-MIF and dynamic light scattering, Crystal form I of M IF belongs to space group P2(1)2(1)2(1) and is grown from 2 M ammonium sulfate at pH 8.5. Angstrom native data set has been collected to 2.4 Angstrom resolution, Self-rotation studies and V-m values indicate th at three molecules per asymmetric unit are present, A data set to 2.8 Angstrom resolution has been collected for crystal form II, which belo ngs to space group P3(1)21 or P3(2)21 and grows from 2 M ammonium sulf ate, 2% polyethylene glycol (average molecular mass 400), 0.1 M HEPES, pH 7.5, Three, four, five or six monomers in the asymmetric unit are consistent with V-m values for this crystal form, Analysis of crystal form II containing selenomethionine-MIF indicates nine selenium sites are present per asymmetric unit, Dynamic light scattering of MIF sugge sts that the major form of the protein in solution is a trimer, The re sults of these studies are in contrast to previous reports indicating that MIF is a monomer or dimer, The subunit arrangement of MIF is simi lar to that of tumor necrosis factor and suggests that signal transduc tion might require trimerization of receptor subunits.