DIRECT LINK BETWEEN CYTOKINE ACTIVITY AND A CATALYTIC SITE FOR MACROPHAGE-MIGRATION INHIBITORY FACTOR

Macrophage migration inhibitory factor (MIF) is a secreted protein tha t activates macrophages, neutrophils and T cells, and is implicated in sepsis, adult respiratory distress syndrome and rheumatoid arthritis. The mechanism of MIF function, however, is unknown. The three-dimensi onal structure of MIF is unlike that of any other cytokine, but bears striking resemblance to three microbial enzymes, two of which possess an N-terminal proline that serves as a catalytic base. Human MIF also possesses an N-terminal proline (Pro-1) that is invariant among all kn own homologues, Multiple sequence alignment of these MIF homologues re veals additional invariant residues that span the entire polypeptide b ut are in close proximity to the N-terminal proline in the folded prot ein. We find that p-hydroxyphenylpyruvate, a catalytic substrate of MI F binds to the N-terminal region and interacts with Pro-1. Mutation of Pro-1 to a glycine substantially reduces the catalytic and cytokine a ctivity of MIF. We suggest that the underlying biological activity of MIF may be based on an enzymatic reaction, The identification of the a ctive site should facilitate the development of structure-based inhibi tors.