The three-dimensional structure of TNF has been determined at 0.29 nm using the technique of X-ray crystallography. Published data on site-directed mutagenesis and antibody binding may now be assessed in the light of the structure, thus the links between structure and function for TNF may be addressed.
TNF is a compact trimer composed of three identical subunits of 157 amino acids. The mainchain topology for a single subunit is essentially a β-sandwich structure formed by two anti-parallel β-pleated sheets. This mainchain fold corresponds to the ‘jelly roll’ motif observed in viral coat proteins such as VP1, VP2 and VP3 of rhinovirus, or the hemagglutinin molecule of influenza. TNF is the first non-viral protein to contain this motif. The subunits associate tightly about a threefold axis interacting through a simple edge-to-face packing of the β-sandwich to form the solid, conical shaped trimer. A large number of the residues conserved between the amino acid sequences of TNF and lymphotoxin lie within the β-sandwich or at the threefold axis of the trimer. This implies the presence of the same β-sandwich motif in the lymphotoxin monomer and preservation of the edge-to-face mode of trimeric association.
The detailed three dimensional structure for TNF explains a wide range of observations, including data on antibody binding and site directed mutagenesis. The currently available evidence points to a region of biological importance situated at the interface between two subunits on the lower half of the trimer.