Apoptosis in motion - An apical, P35-insensitive caspase mediates programmed cell death in insect cells

Activation of caspases by proteolytic processing is a critical step during apoptosis in metazoans. Here we use high resolution time lapse microscopy t o show a tight link between caspase activation and the morphological events delineating apoptosis in cultured SF21 cells from the moth Spodoptera frug iperda, a model insect system. The principal effector caspase, Sf-caspase-1 , is proteolytically activated during SF21 apoptosis. To define the potenti al role of initiator caspases in vivo, we tested the effect of cell-permeab le peptide inhibitors on pro-Sf-caspase-1 processing. Anti-caspase peptide analogues prevented apoptosis induced by diverse signals, including W radia tion and baculovirus infection. IETD-fmk potently inhibited the initial pro cessing of pro-Sf-caspase-1 at the junction (TETD-G) of the large and small subunit, a cleavage that is blocked by inhibitor of apoptosis Op-IAP but n ot pancaspase inhibitor P35, Because Sf-caspase-1 was inhibited poorly by I ETD-CHO, our data indicated that the protease responsible for the first ste p in pro-Sf-caspase-1 activation is a distinct apical caspase, Thus, Sf-cas pase-1 activation is mediated by a novel, P35-resistant caspase, These find ings support the hypothesis that apoptosis in insects, like that in mammals , involves a cascade of caspase activations.