Identification of the cytolinker plectin as a major early in vivo substrate for caspase 8 during CD95- and tumor necrosis factor receptor-mediated apoptosis

Caspase 8 plays an essential role in the execution of death receptor-mediat ed apoptosis. To determine the localization of endogenous caspase 8, me use d a panel of subunit-specific anti-caspase 8 monoclonal antibodies in confo cal immunofluorescence microscopy. In the human breast carcinoma cell line MCF7, caspase 8 predominantly colocalized with and bound to mitochondria. A fter induction of apoptosis through CD95 or tumor necrosis factor receptor I, active caspase 8 translocated to plectin, a major cross-linking protein of the three main cytoplasmic filament systems, whereas the caspase 8 prodo main remained bound to mitochondria. Plectin was quantitatively cleaved by caspase 8 at Asp 2395 in the center of the molecule in all cells tested. Cl eavage of plectin clearly preceded that of other caspase substrates such as poly(ADP-ribose) polymerase, gelsolin, cytokeratins, or lamin B. In primar y fibroblasts from plectin-deficient mice, apoptosis-induced reorganization of the actin cytoskeleton, as seen in wild-type cells, was severely impair ed, suggesting that during apoptosis, plectin is required for the reorganiz ation of the microfilament system.