Identification of functional regions defining different activity in caspase-3 and caspase-7 within cells

Caspases are central to apoptosis, and the principal executioner caspases, caspase-3 and -7, were reported to be similar in activity, primary structure, and three-dimensional structure. Here, we identified different activity in caspase-3 and -7 within cells and examined the relationship between their structure and function using human cells expressing almost equal amounts of exogenous caspase-3, caspase-7, and/or chimeric constructs after down-regulation of endogenous caspase-3 and -7 expression. Caspase-3 (produced in human cells) showed much stronger cleaving activity than caspase-7 against a low molecular weight substrate in vitro dependent on four specific amino acid regions. Within cells, however, an additional three regions were required for caspase-3 to exert much stronger protease activity than caspase-7 against cellular substrates. Three of the former four regions and the latter three regions were shown to form two different three-dimensional structures that were located at the interface of the homodimer of procaspase-7 on opposite sides. In addition, procaspase-3 and -7 revealed specific homodimer-forming activity within cells dependent on five amino acid regions, which were included in the regions critical to the cleaving activity within cells. Thus, human caspase-3 and -7 exhibit differences in protease activity, specific homodimer-forming activity, and three-dimensional structural features, all of which are closely interrelated.