TRAF1 is cleaved by caspases

During TNF-alpha or Fas ligand-induced apoptosis TRAF1 can be processed into two fragments (Imler M et al. 2000; Leo E et al. 2001). Caspases-3, -6 and most efficiently caspase-8 cleave TRAF1 in vitro (Imler M et al. 2000; Leo E et al. 2001). Cleavage of TRAF1 occurs at the Asp-163 residue. A mutant TRAF1 (Asp163Ala) was not processed by either caspase (Imler M et al. 2000). The C-terminal cleavage product of TRAF1 was found to inhibit the induction of NFkB when co-expressed with NFkB inducers (such as TNFR1, DR3, TNFR2, Fas etc.) in human embryonic kidney 293 (HEK293) cells or upon treatment with TNF (Imler M et al. 2000; Henkler F et al. 2003). Furthermore, co-transfection of VSV-tagged IKKbeta and TRAF1(truncated or wild-type) into HEK293 or Jurkat T-cells followed by anti-VSV immunoprecipitation coupled with GST-IkB alpha immunocomplex kinase assays for IKK activity revealed that the caspase-generated TRAF1-fragment, but not TRAF1 itself inhibited IKK activation (Henkler F et al. 2003).

在TNF-α或Fas配体诱导的细胞凋亡过程中，TRAF1可被加工成两个片段（Imler M et al. 2000；Leo E et al. 2001）。体外实验表明，Caspase-3、-6以及最为有效的Caspase-8可以切割TRAF1（Imler M et al. 2000；Leo E et al. 2001）。TRAF1的切割发生在Asp-163残基处。突变型TRAF1（Asp163Ala）无法被任何一种Caspase加工（Imler M et al. 2000）。研究发现，TRAF1的C端切割产物在与NFkB诱导剂（如TNFR1、DR3、TNFR2、Fas等）共同表达于人胚胎肾293（HEK293）细胞中，或经TNF处理后，可以抑制NFkB的诱导（Imler M et al. 2000；Henkler F et al. 2003）。此外，将标记有VSV标签的IKKbeta与截短型或野生型TRAF1共同转染至HEK293或Jurkat T细胞中，随后通过抗VSV免疫沉淀与GST-IkB alpha免疫复合物激酶活性检测，发现由Caspase产生的TRAF1片段而非TRAF1本身可以抑制IKK的激活（Henkler F et al. 2003）。