The calpains are a a family of neutral cysteine proteases that come in two
flavors: calpain I (mu calpain) and calpain II (m-calpain). Calpain I
lives in neurons, while calpain II seems to prefer hanging out in glia.
Calpains need calcium to be proteolytically active, and are also
regulated by calpastatin and activator proteins.
Figure. Ribbon diagram of calpain.
The physiologic role of calpain
seems to be in modulating neurite outgrowth and remodeling of synapses.
Synaptic activity, of course, is coupled to calcium flux across the
membrane, and so it's easy to see how a protease linked to calcium flux
could be useful in the controlled deconstruction of synaptic
architecture. It's also easy to see how a huge flux of calcium--as
occurs during brain ischemia--could cause calpain to just lose its
frippin' mind and go over to the Dark Side.