Subcellular localization and CARD-dependent oligomerization of the death adaptor RAIDD

RAIDD, a caspase recruitment domain (CARD) containing molecule, interacts with procaspase-2 in a CARD-dependent manner. This interaction has been suggested to mediate the recruitment of caspase-2 to the tumour necrosis factor receptor 1 (TNFR1). In this paper we have studied the subcellular localization of RAIDD and its interaction with caspase-2. We demonstrate that endogenous RAIDD is mostly localized in the cytoplasm and to some extent in the nucleus. RAIDD localization is not affected by TNF-treatment of HeLa cells, but in cells ectopically expressing caspase-2, a fraction of RAIDD is recruited to the nucleus. In transfected cells, coexpression of RAIDD and caspase-2 leads to CARD-dependent colocalization of the two proteins to discrete subcellular structures. We further show that overexpression of the RAIDD-CARD results in the formation of filamentous structures due to CARD-mediated oligomerization. These structures were similar to death effector filaments (DEFs) formed by FADD and FLICE death effector domains (DEDs), and partially colocalized with DEFs. Our results suggest that similar to the DED, the RAIDD-CARD has the ability to form higher order complexes, believed to be important in apoptotic execution. We also present evidence that RAIDD-CARD oligomerization may be regulated by intramolecular folding of the RAIDD molecule.