During apoptosis, proapoptotic BAX and BAK trigger mitochondrial outer membrane 
(MOM) permeabilization by a mechanism that is not yet fully understood. BH3-only 
proteins such as tBID, together with lipids of the MOM, are thought to play a 
key role in BAX and BAK activation. In particular, cardiolipin (CL) has been 
shown to stimulate tBID-induced BAX activation in vitro. However, it is still 
unclear whether this process also relies on CL in the cell, or whether it is 
more dependent on MTCH2, a proposed receptor for tBID present in the MOM. To 
address this issue, we deleted both alleles of cardiolipin synthase in human 
HCT116 cells by homologous recombination, which resulted in a complete absence 
of CL. The CL-deficient cells were fully viable in glucose but displayed 
impaired oxidative phosphorylation and an inability to grow in galactose. Using 
these cells, we found that CL was not required for either tBID-induced BAX 
activation, or for apoptosis in response to treatment with TRAIL. Downregulation 
of MTCH2 in HCT116 cells also failed to prevent recruitment of tBID to 
mitochondria in apoptotic conditions. However, when both CL and MTCH2 were 
depleted, a significant reduction in tBID recruitment was observed, suggesting 
that in HCT116 cells, CL and MTCH2 can have redundant functions in this process.