STRUCTURES OF THE EXTRACELLULAR DOMAIN OF THE TYPE-I TUMOR-NECROSIS-FACTOR RECEPTOR

Background: Tumor necrosis factor (TNF) is a powerful cytokine that is involved in immune and pro-inflammatory responses, Two TNF receptors that belong to the cysteine-rich low affinity nerve growth factor rece ptor family (TNF-R1 and TNF-R2) are the sole mediators of TNF signalli ng. Signalling is thought to occur when a trimer of TNF binds to the e xtracellular domains of two or three receptor molecules, which permits aggregation and activation of the cytoplasmic domains. The complex is then internalized within an endocytic vesicle, whereupon it dissociat es at low pH. Structure of the soluble extracellular domain of the rec eptor (sTNF-R1) both in the unliganded and TNF-bound state have previo usly been determined. In both instances, the fourth subdomain of the r eceptor was found to be partly disordered. In the unliganded state at pH 7.5, the extracellular domain forms two distinct types of dimer, pa rallel and antiparallel; the antiparallel dimer occludes the TNF-bindi ng. Results: We have determined the structure of sTNF-R1 in two crysta l forms in high salt at pH 3.7. The orthorhombic crystals diffract to 1.85 Angstrom and the entire polypeptide is well ordered. In contrast, the C-terminal 32 residues are disordered in the hexagonal crystals. In the orthorhombic form, these residues exhibit a topology and disulp hide connectivity that differs from the other three cysteine-rich doma ins in the molecule. In both forms, the interlace is considerably more extensive than that used in complex formation with LT alpha. This 'lo w pH' dimer is different from both of the dimers observed in crystals grown at pH 7.5. Conclusions: The occurrence of the antiparallel dimer s in both low pH crystal forms suggest that they are not an artefact o f crystal packing. Such dimers may form in the low pH environment of t he endosome, Because the dimer contact surface occludes the TNF-bindin g site, formation of this dimer would dissociate the TNF-receptor comp lex within the endosome. Three of the four cysteine-rich domains of TN F-R1 are constructed from two distinct structural modules, termed A1 a nd B2. The fourth subdomain comprises an A1 module followed by an unus ual C2 module. Although the orientation of these modules with respect to each other is sensitive to crystal packing, ligand binding, pH and ionic strength, the modules are structurally well conserved between an d within the known sTNF-R1 structures. (C) Current Biology Ltd