Phosphorylation of I kappa B-alpha inhibits its cleavage by caspase CPP32 in vitro

I kappa B proteins function as direct regulators of Rel/NF-kappa B transcription complexes. We show that the cell-death protease CPP32 (caspase-3) in vitro specifically cleaved chicken and human I kappa B-alpha at a conserved Asp-Ser sequence. This cleavage site appears to be identical to the site at which chicken I kappa B-alpha is cleaved in vivo in temperature-sensitive v-Rel-transfarmed chicken spleen cells undergoing apoptosis. Other caspases, namely interleukin-1 beta-converting enzyme (caspase-1) and Ich-1 (caspase-2), did not cleave I kappa B-alpha. CPP32 also cleaved mammalian I kappa B-beta in vitro at the analogous Asp-Ser sequence. Cleavage of I kappa B-alpha by CPP32 was blocked by serine phosphorylation of I kappa B-alpha. Cleavage of I kappa B-alpha by a CPP32-like protease could generate a constitutive inhibitor of Rel transcription complexes. This report provides evidence for a direct biochemical interaction between the NF-kappa B signaling pathway and a cell-death protease signaling pathway.