E Cleavage of the BAK α1-α2 loop potentiates MOMP. Membrane fractions from Bax-/-Bak-/- MEFs expressing BAK or BAKthrombin were incubated or not with thrombin in the presence or absence of cBID (10 nM) for the indicated times. Membrane (P) and supernatant (S) fractions were separated and immunoblotted for cytochrome c or BAK 4B5. 