Distribution and apoptotic function of outer membrane proteins depend on mitochondrial fusion

Cells deficient in mitochondrial fusion have been shown to have defects linked to the exchange of inner membrane and matrix components. Because outer-mitochondrial membrane (OMM) constituents insert directly from the cytoplasm, a role for fusion in their intermitochondrial transfer was unanticipated. Here, we show that fibroblasts lacking the GTPases responsible for OMM fusion, mitofusins 1 and 2 (MFN1 and MFN2), display more heterogeneous distribution of OMM proteins. Proteins with different modes of OMM association display varying degrees of heterogeneity in Mfn1/2(-/-) cells and different kinetics of transfer during fusion in fusion-competent cells. Proapoptotic Bak exhibits marked heterogeneity, which is normalized upon expression of MFN2. Bak is critical for Bid-induced OMM permeabilization and cytochrome c release, and Mfn1/2(-/-) cells show dysregulation of Bid-dependent apoptotic signaling. Bid sensitivity of Bak-deficient mitochondria is regained upon fusion with Bak-containing mitochondria. Thus, OMM protein distribution depends on mitochondrial fusion and is a locus of apoptotic dysfunction in conditions of fusion deficiency.