D BAK wild-type, or BAK with an engineered α1-α2 loop thrombin cleavage site were stably expressed in Bax-/-Bak-/- MEFs. Membrane fractions were incubated with thrombin prior to incubation with copper phenanthroline (CuPhe) to induce disulphide-linkage. Samples were run on SDS-PAGE under non-reducing or reducing conditions and immunoblotted for BAK with an antibody recognising the BAK N-terminus upstream of the thrombin cleavage site (aa23-38) or an antibody recognising the BH3 domain (4B5) downstream of the cleavage site. Schematic indicates the nature of the BAK protein detected by each antibody. Disulphide-linkage indicated by a hashed line. Note that the N-terminal portion of thrombin cleaved BAK could not be detected with the N-terminal antibody unless it was intramolecularly disulphide-linked. *cBID (100 nM) was added to activate BAK during thrombin cleavage. Data representative of two independent experiments. 