TUMOR-NECROSIS-FACTOR RECEPTOR SUPERFAMILY

Tumor necrosis factor (TNF) is a powerful cytokine which is involved i n the immune and pro-inflammatory response. The TNF receptors (TNF-R1 and TNF-R2) are the sole mediators of TNF signaling. The receptors con sist of a disulfide rich domain which recognizes TNF, a transmembrane helix, and a cytoplasmic domain. Signaling occurs when a TNF trimer bi nds two or three receptors in an extracellular complex which permits a ggregation and activation of the cytoplasmic domains. The complex is t hen endocytosed where it dissociates at low pH. We have now determined the structure of the soluble extracellular domain of TNF-R1 in two cr ystal forms at pH 3.7 in addition to our earlier report of one form at pH 7.5. One low pH form diffracts to 1.85 A and the entire polypeptid e sequence has now been traced for this protein. The C-terminal 20 res idues of the protein which were disordered in all previous structures show a different topology and disulfide connectivity to that seen in t he remainder of the structure. In all crystal forms, the uncomplexed s oluble extracellular domain of the type I TNF-R (sTNF-R1) exists as a dimer. At low pH the dimer buries a large amount of solvent accessible surface (2,900 Angstrom(2)) over 800 Angstrom(2) greater than the are a buried fry TNF complexation. This dimer at low pH is different than both dimers observed in our previous pH 7.5 structure of unliganded sT NF-R1. We suggest that the low pH dimer forms during endocytosis and a s the dimer completely buries the TNF interaction surface, the dimer w ould break up the receptor TVF complex. We have identified two distinc t structural modules in sTNF-R1, a type A and a type B module. We sugg est that these modules are the unit of structural conservation rather than the 6 cysteine subdomain. Although the orientation of these modul es with respect to each other is sensitive to crystal packing, complex ation, and pH, the modules themselves are structurally well conserved between and within the known sTNF-R1 structures. This modular approach will allow Ils to build accurate models for all members of the TNF-R superfamily. (C) 1996 Wiley-Liss, Inc.