CHARACTERIZATION OF STRUCTURAL DOMAINS OF HUMAN OSTEOCLASTOGENESIS INHIBITORY FACTOR

Osteoclastogenesis inhibitory factor (OCIF) is a heparin-binding secre tary glycoprotein that belongs to the tumor necrosis factor receptor ( TNFR) family. OCIF is present both as a similar to 60-kDa monomer and a disulfide-linked homodimer, We attempted to characterize the seven s tructural domains of OCIF by determining the capabilities of various O CIF mutants to inhibit osteoclastogenesis, to interact with heparin, a nd to form dimers. We also examined a potential of domains 5 and 6, de ath domain homologous regions (DDHs), for inducing cell death by expre ssing OCIF/Fas fusion proteins, Our results show that: (i) the N-termi nal portion of OCIF containing domains 1-4, which have structural simi larity to the extracellular domains of the TNFR family proteins, is su fficient to inhibit osteoclastogenesis; (ii) a heparin-binding site is located in domain 7, and affinity for heparin does not correlate with the inhibitory activity; (iii) Cys-400 in domain 7 is the residue res ponsible for dimer formation; and (iv) the C-terminal portion containi ng domains 5 and 6, DDHs, has a high potential for mediating a cytotox ic signal when it is expressed in cells as an OCIF/Fas fusion protein in which the transmembrane region of Fas is inserted in front of DDHs.