Death domain mutagenesis of KILLER/DR5 reveals residues critical for apoptotic signaling

The Fas/tumor necrosis factor (TNF)/TRAIL receptors signal death through a cytoplasmic death domain (DD) containing six alpha-helices with positively charged helix 2 interacting with negatively charged helix 3 of another DD. DD mutation occurs in head/neck and lung cancer (TRAIL receptor KILLER/DR5) and in lpr mice (Fas). We examined the apoptotic potential of known KILLER/DR5 lung tumor-derived mutants (n = 6) and DD mutants (n = 18) generated based on conservation with DR4, Fas, Fas-associated death domain (FADD), and tumor necrosis factor receptor 1 (TNFR1). With the exception of Arg-330 required in Fas or FADD for aggregation or for TNFR1 cytotoxicity, surprisingly major loss-of-function KILLER/DR5 alleles (W325A, L334A (lpr-like), I339A, and W360A) contained hydrophobic residues. Loss-of-function of I339A (highly conserved) has not been reported in DDs. Charged residue mutagenesis revealed the following points. 1) E326A, conserved in DR4, is dispensable for death; the homologous residue is positively charged in Fas, TNFR1, and FADD and is critical for DD interactions. 2) K331A, D336A, E338A, K340A, K343A, and D351A have partial loss-of-function suggesting multiple charges stabilize receptor-adapter interactions. Analysis of the tumor-derived KILLER/DR5 mutants revealed the following. 1) L334F has partial loss-of-function versus L334A, whereas E338K has major loss-of-function versus E338A, examples where alanine and tumor-specific substitutions have divergent phenotypes. 2) Unexpectedly, S324F, E326K, K386N, and D407Y have no loss-of-function with tumor-specific or alanine substitutions. Loss-of-function KILLER/DR5 mutants were deficient in recruitment of FADD and caspase 8 to TRAIL death-inducing signaling complexes. The results reveal determinants within KILLER/DR5 for death signaling and drug design.