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BID, a proapoptotic BCL-2 family member, plays an essential role in the tumor necrosis factor alpha (TNF-␣)/Fas death receptor pathway in vivo. Activation of the TNF-R1 receptor results in the cleavage of BID into truncated BID (tBID), which translocates to the mitochondria and induces the activation of BAX or BAK. In TNF-␣-activated FL5.12 cells, tBID becomes part of a 45-kDa cross-linkable mitochondrial complex. Here we describe the biochemical purification of this complex and the identification of mitochondrial carrier homolog 2 (Mtch2) as part of this complex. Mtch2 is a conserved protein that is similar to members of the mitochondrial carrier protein family. Our studies with mouse liver mitochondria indicate that Mtch2 is an integral membrane protein exposed on the surface of mitochondria. Using blue-native gel electrophoresis we revealed that in viable FL5.12 cells Mtch2 resides in a protein complex of ca. 185 kDa and that the addition of TNF-␣ to these cells leads to the recruitment of tBID and BAX to this complex. Importantly, this recruitment was partially inhibited in FL5.12 cells stably expressing BCL-X L . These results implicate Mtch2 as a mitochondrial target of tBID and raise the possibility that the Mtch2-resident complex participates in the mitochondrial apoptotic program.

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