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Characterization of structural domains of human osteoclastogenesis inhibitory factor
K Yamaguchi1, M Kinosaki, M Goto
1Research Institute of Life Science, Snow Brand Milk Products Co., Ltd., 519 Ishibashi-machi, Shimotsuga-gun, Tochigi 329-0512, Japan. fvbd7042@mb.infoweb.or.jp
Abstract:
Osteoclastogenesis inhibitory factor (OCIF) is a heparin-binding secretory glycoprotein that belongs to the tumor necrosis factor receptor (TNFR) family. OCIF is present both as a approximately 60-kDa monomer and a disulfide-linked homodimer. We attempted to characterize the seven structural domains of OCIF by determining the capabilities of various OCIF mutants to inhibit osteoclastogenesis, to interact with heparin, and to form dimers. We also examined a potential of domains 5 and 6, death domain homologous regions (DDHs), for inducing cell death by expressing OCIF/Fas fusion proteins. Our results show that: (i) the N-terminal portion of OCIF containing domains 1-4, which have structural similarity to the extracellular domains of the TNFR family proteins, is sufficient 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 responsible for dimer formation; and (iv) the C-terminal portion containing domains 5 and 6, DDHs, has a high potential for mediating a cytotoxic 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.
Insights
Osteoclastogenesis inhibitory factor (OCIF) inhibits bone resorption via its N-terminal domains. The C-terminal domains, particularly DDHs, mediate cytotoxic signals, while domain 7 is crucial for heparin binding and dimerization.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Osteoclastogenesis inhibitory factor (OCIF) is a heparin-binding glycoprotein within the tumor necrosis factor receptor (TNFR) superfamily.
- OCIF exists as a monomer and a disulfide-linked homodimer.
Purpose of the Study:
- To characterize the seven structural domains of OCIF.
- To determine the functional roles of OCIF domains in osteoclastogenesis inhibition, heparin interaction, and dimer formation.
- To investigate the cell death-inducing potential of OCIF domains 5 and 6.
Main Methods:
- Construction and expression of various OCIF mutants.
- Analysis of osteoclastogenesis inhibition.
- Heparin-binding assays.
- Dimerization studies.
- Expression of OCIF/Fas fusion proteins to assess cytotoxicity.
Main Results:
- The N-terminal portion (domains 1-4) of OCIF is sufficient for inhibiting osteoclastogenesis.
- Domain 7 contains the heparin-binding site, but heparin affinity does not correlate with inhibitory activity.
- Cys-400 in domain 7 is essential for OCIF dimer formation.
- The C-terminal domains (5 and 6), homologous to death domains (DDHs), mediate cytotoxic signals when fused with Fas.
Conclusions:
- The N-terminal domains of OCIF are responsible for its osteoclastogenesis inhibitory function.
- Domain 7 plays a key role in OCIF dimerization and heparin interaction.
- The C-terminal DDHs possess cytotoxic potential, suggesting roles in cell signaling pathways.