Human caspase-7 activity and regulation by its N-terminal peptide

Central to the execution phase of apoptosis are the two closely related caspase-3 and -7. They share common substrate specificity and structure, but differ completely in the sequence of their respective N-terminal regions including their N-peptides, a 23-28 residue segment that are removed during zymogen activation. We show that the N-peptide of caspase-7 plays no role in the fundamental activation or properties of the active protease in vitro. However, the N-peptide modifies the properties of caspase-7 in vivo. In ectopic expression experiments, caspase-7 constructs with no N-peptide are far more lethal than constructs that have an uncleavable peptide. Moreover, the N-peptide of caspase-7 must be removed before efficient activation of the zymogen can occur in vivo. These disparate requirements for the N-peptide argue that it serves to physically sequester the caspase-7 zymogen in a cytosolic location that prevents access by upstream activators (caspase-8, -9, and -10). The N-peptide must first be removed, probably by caspase-3, before efficient conversion and activation of the zymogen can occur in vivo.