Functional Analysis of Trail Protein Crystallographic Data: Selectivity and Apoptosis of Tumor Cells

There are several tumor necrosis factor proteins (TNF) responsible for signaling cell death (TNF-R1, DR3, DR4, DR5, and DR6). The binding of TNF related apoptosis-inducing ligand (TRAIL) to some of these receptors initiates the cell death pathway and causes apoptosis. TRAIL is a homotrimer which binds three like death receptors (Figure 1). This leads to trimerization of the death receptors and signaling for cell death. DcR2 do not signal for cell apoptosis (or any other cell death). These receptors can compete for TRAIL with DR4 and DR5 and potentially act as a protection mechanism for target cells. TRAIL is widely expressed in the body, as are DR4 and DR5 receptors. Interestingly, only receptors of tumor cells are sensitive to TRAIL binding. There are several residues which seem to have important roles in the function of TRAIL-induced apoptosis. Glu 147, Glu 151, Arg 154, and Asp 175 are all conserved in the structure of active DR5 receptors. In particular, Glu 147 appears to make a salt bridge with Arg 149 on TRAIL. When compared to other TNF family proteins, the loop containing Arg 149 on TRAIL is considerably longer (Figure 2). Mutagenesis of a 19 amino acid sequence of TRAIL, including that of Arg 149, to a shorter SSL sequence mimicking other TNF proteins, showed 95% reduction in binding with DR5. This shows the importance of the longer loop on TRAIL to facilitate this interaction. Glu 236, Tyr 237) also shows a reduction larger than 5 fold in apoptotic activity.