Trimeric structure of TNF alpha, produced by Mus musculus, based on PDB structure 2TNF (1.4 Å Resolution). Different colors represent different monomers. Baeyens, KJ et al. (1999). Figure rendered using FirstGlance Jmol.
Tumor necrosis factors (or the TNF family) refer to a group of cytokines that can cause cell death (apoptosis). The first two members of the family to be identified were:
Tumor necrosis factor (TNF), formerly known as TNFα or TNF alpha, is the best-known member of this class. TNF is a monocyte-derived cytotoxin that has been implicated in tumor regression, septic shock, and cachexia. The protein is synthesized as a prohormone with an unusually long and atypical signal sequence, which is absent from the mature secreted cytokine. A short hydrophobic stretch of amino acids serves to anchor the prohormone in lipid bilayers. Both the mature protein and a partially processed form of the hormone can be secreted after cleavage of the propeptide.
Nineteen cytokines have been identified as part of the TNF family on the basis of sequence, functional, and structural similarities. They include:
Tumor Necrosis Factor (TNF) (also known as cachectin  or TNF alpha) is a cytokine that has a wide variety of functions. It can cause cytolysis of certain tumor cell lines; it is involved in the induction of cachexia; it is a potent pyrogen, causing fever by direct action or by stimulation of interleukin-1 secretion; it can stimulate cell proliferation and induce cell differentiation under certain conditions.
4-1BBL, an inducible T cell surface molecule that contributes to T-cell stimulation.
OX40L, a cytokine that co-stimulates T cell proliferation and cytokine production.
TNF-related apoptosis inducing ligand (TRAIL), a cytokine that induces apoptosis.
Model of hydrogen bond between Asn34 of subunit A and Arg82 of subunit C, produced by M. musculus, based on PDB structure 2TNF. The residues participating the hydrogen bond are shown in stick. The short bond length, 2.84Å, highly suggests a strong hydrogen bond that supports the tertiary structure. Baeyens, KJ et al. (1999). Generated in Chimera.
All these cytokines seem to form homotrimeric (or heterotrimeric in the case of LT-alpha/beta) complexes that are recognized by their specific receptors. Strong hydrogen bonds between the monomers stabilize the tertiary structure. One such example is the Asn34-Arg82 hydrogen bond in the M. musculus TNF alpha. The PROSITE pattern for this family is located in a beta-strand in the central section of the protein that is conserved across all members.
All members of the TNF family, with the exception of the secreted lymphotoxin and a proliferation-inducing ligand (APRIL), are type II transmembrane proteins that protrude from immune cells. Such membrane-bound TNF ligands frequently signal back to the immune cells when they contact and bind their cognate receptors on other cells.
^ abcBaeyens KJ, De Bondt HL, Raeymaekers A, Fiers W, De Ranter CJ (April 1999). "The structure of mouse tumour-necrosis factor at 1.4 Å resolution: towards modulation of its selectivity and trimerization". Acta Crystallogr. D Biol. Crystallogr.55 (Pt 4): 772–8. PMID10089307.Cite uses deprecated parameters (help)
^ abKriegler M, Perez C, DeFay K, Albert I, Lu SD (April 1988). "A novel form of TNF/cachectin is a cell surface cytotoxic transmembrane protein: ramifications for the complex physiology of TNF". Cell53 (1): 45–53. doi:10.1016/0092-8674(88)90486-2. PMID3349526.Cite uses deprecated parameters (help)
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^Browning JL, Ngam-ek A, Lawton P, DeMarinis J, Tizard R, Chow EP, Hession C, O'Brine-Greco B, Foley SF, Ware CF (March 1993). "Lymphotoxin beta, a novel member of the TNF family that forms a heteromeric complex with lymphotoxin on the cell surface". Cell72 (6): 847–56. doi:10.1016/0092-8674(93)90574-A. PMID7916655.Cite uses deprecated parameters (help)
^Suda T, Takahashi T, Golstein P, Nagata S (December 1993). "Molecular cloning and expression of the Fas ligand, a novel member of the tumor necrosis factor family". Cell75 (6): 1169–78. doi:10.1016/0092-8674(93)90326-L. PMID7505205.Cite uses deprecated parameters (help)
^Wiley SR, Schooley K, Smolak PJ, Din WS, Huang CP, Nicholl JK, Sutherland GR, Smith TD, Rauch C, Smith CA (December 1995). "Identification and characterization of a new member of the TNF family that induces apoptosis". Immunity3 (6): 673–82. doi:10.1016/1074-7613(95)90057-8. PMID8777713.Cite uses deprecated parameters (help)