The Small Heat Shock Protein αB-Crystallin Negatively Regulates Cytochrome c- and Caspase-8-dependent Activation of Caspase-3 by Inhibiting Its Autoproteolytic Maturation*

Caspases are universal effectors of apoptosis. The mitochondrial and death receptor pathways activate distinct apical caspases (caspase-9 and -8, respectively) that converge on the proteolytic activation of the downstream executioner caspase-3. Caspase-9 and -8 cleave procaspase-3 to produce a p24 processing intermediate (composed of its prodomain and large subunit), which then undergoes autoproteolytic cleavage to remove the prodomain from the active protease. Recently, several heat shock proteins have been shown to selectively inhibit the mitochondrial apoptotic pathway by disrupting the activation of caspase-9 downstream of cytochrome c release. We report here that the small heat shock protein αB-crystallin inhibits both the mitochondrial and death receptor pathways. In S-100 cytosolic extracts treated with cytochromec/dATP or caspase-8, αB-crystallin inhibits the autoproteolytic maturation of the p24 partially processed caspase-3 intermediate. In contrast, neither the closely related small heat shock protein family member Hsp27 nor Hsp70 inhibited the maturation of the p24 intermediate. We also demonstrate that αB-crystallin co-immunoprecipitates with the p24 partially processed caspase-3in vivo. Taken together, our results demonstrate that αB-crystallin is a novel negative regulator of apoptosis that acts distally in the conserved cell death machinery by inhibiting the autocatalytic maturation of caspase-3.