Okay, it's back to the metaphor! Whether you're Dr. Hall trying to nuke the Wildfire laboratory to prevent the Andromeda Strain from eating the world, or Ripley trying to blow up yet another space ship to stop the Aliens, or Captain Kirk sacrificing the Enterprise to waste a few Klingons….you're going to need a bomb. That is, you need an executioner. If you're a worm, that means you need a ced-3 gene product. If you're Ripley or Captain Kirk, you need the mammalian equivalent, which is a caspase.

Caspases are a family of proteases, and they come in a lot of different flavors. But they all have several things in common:

1. They are synthesized in their inactive form   as monomers with prodomains of varying length;
2. They require cleavage to be activated, and
3. When cleaved they form active heterotetramers. In the animation below, notice how cleavage of the prodomain of the caspase brings about exposure and completion of the active site, and how homodimerization creates a monster.

Animation. How cleavage activates caspases to screw up your day.