Sab‐mediated signaling regulates mitochondrial fission

Mitochondria are dynamic, highly integrated organelles that exist in a constant state of fusion and fission. While some components of these processes are known, the mechanism regulating mitochondrial dynamics have yet to be resolved. Our long‐term goal is to understand how signal transduction on the mitochondrial outer membrane (MOM) influences mitochondrial physiology. Intimate crosstalk exists between mitochondria and other subcellular compartments, in particular the nucleus. Since the MOM is the interface of mitochondria and the cell, signaling complexes at the MOM are appropriately positioned to convey messages to and from mitochondria. Moreover, MOM signaling complexes are in close proximity to proteins involved in crucial aspects of mitochondrial physiology, such as dynamics, energetics, and protein import; likewise, MOM signaling complexes have access to cytosolic signaling proteins. Alterations in these signaling cascades may profoundly impact mitochondria and the cell. We have found that c‐Jun N‐terminal kinase (JNK) signaling on the MOM is mediated by the scaffold protein Sab. Selective inhibition of the JNK‐Sab interaction reduced mitochondrial dysfunction, and increased the length of mitochondria within mammalian cells. Over‐expression of Sab resulted in fragmentation of the mitochondrial network. Based on these observations, we hypothesized that mitochondrial JNK (mito‐JNK) signaling may promote mitochondrial fission. To examine the contribution of mito‐JNK signaling to fission, we ectopically expressed Sab in HeLa cells and murine embryonic fibroblasts (MEFs) to enrich mito‐JNK signaling. Increased Sab expression was sufficient to fragment mitochondria in both cell types. This effect was enhanced by the introduction of chemicals known to promote mitochondrial fission. Affinity purification of epitope‐tagged Sab expressed in MEFs revealed Sab was part of a protein complex with fission proteins Mff, Fis1, MiD49/51, and Drp1. Sab also interacted with mitofusin 2 (Mfn2). Inhibition of mito‐JNK signaling prevented the interaction between Sab and the fission proteins; moreover, silencing Sab expression reduced interactions among the fission components themselves. Therefore, we propose that Sab may serve as a nucleation site for the fission machinery on the MOM. Inhibition of the JNK‐Sab interaction also prevented the phosphorylation and degradation of Mfn2 and reduced the amount of mitochondrial fission in vitro . However, expression of active JNK was not sufficient to induce mitochondrial fragmentation alone suggesting another protein may contribute to Sab‐mediated fission. Extracellular regulated kinases (ERK1/2) have been demonstrated to affect mitochondria dynamics. Biochemical analyses reveal that ERK1/2 can bind to a kinase binding motif (KIM2) on Sab. Over‐expression of Sab reveals a decrease in Mitofusin 1 (Mfn1) levels. ERK signaling has been shown to promote fission by inducing the degradation of Mfn1. Taken together, our results suggest that two prominent protein kinases JNK and ERK can regulate mitochondrial fission by modifying fusion and fission proteins in a Sab‐dependent manner. Our studies illustrate the connectivity of cellular stress response pathways and mitochondrial dynamics.