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A cDNA clone, designated TRAP (TNF-related activation protein) was isolated from a collection of T cell activation genes. The polypeptide encoded by a mRNA of approx. 2.3 kb is 261 amino acids long with a calculated M(r) of 29.3 kDa. The structural features predict a type II transmembrane protein, but are also compatible with a secreted form. TRAP is highly similar to an identified murine CD40 ligand both at the cDNA (82.8% identity) and the protein (77.4% identity) levels, and related to tumor necrosis factor/lymphotoxin. Expressed in a murine myeloma, TRAP was identified as a ligand for CD40 by binding to a soluble CD40 construct. TRAP mRNA is expressed in a T cell-specific fashion with a maximum at 8 h after stimulation. The TRAP gene is located in the q26.3-q27.1 region of the X chromosome.
Graf et al. (1992) studied this question.