Redesigning the procaspase-8 dimer interface for improved dimerization.

Caspase-8 is a cysteine directed aspartate-specific protease that is activated at the cytosolic face of the cell membrane upon receptor ligation. A key step in the activation of caspase-8 depends on adaptor-induced dimerization of procaspase-8 monomers. Dimerization is followed by limited autoproteolysis within the intersubunit linker (IL), which separates the large and small subunits of the catalytic domain. Although cleavage of the IL stabilizes the dimer, the uncleaved procaspase-8 dimer is sufficiently active to initiate apoptosis, so dimerization of the zymogen is an important mechanism to control apoptosis. In contrast, the effector caspase-3 is a stable dimer under physiological conditions but exhibits little enzymatic activity. The catalytic domains of caspases are structurally similar, but it is not known why procaspase-8 is a monomer while procaspase-3 is a dimer. To define the role of the dimer interface in assembly and activation of procaspase-8, we generated mutants that mimic the dimer interface of effector caspases. We show that procaspase-8 with a mutated dimer interface more readily forms dimers. Time course studies of refolding also show that the mutations accelerate dimerization. Transfection of HEK293A cells with the procaspase-8 variants, however, did not result in a significant increase in apoptosis, indicating that other factors are required in vivo. Overall, we show that redesigning the interface of procaspase-8 to remove negative design elements results in increased dimerization and activity in vitro, but increased dimerization, by itself, is not sufficient for robust activation of apoptosis.