Mitochondria-derived vesicles (MDVs) have been shown to transport cargo from the mitochondria to the peroxisomes. Mitochondria and peroxisomes share common functions in the oxidation of fatty acids and the reduction of damaging peroxides. Their biogenesis is also linked through both the activation of master transcription factors such as PGC-1alpha and the common use of fission machinery, including DRP1, Mff, and hFis1. We have previously shown that MDVs are formed independently of the known mitochondrial fission GTPase Drp1 and are enriched for a mitochondrial small ubiquitin-like modifier (SUMO) E3 ligase called MAPL (mitochondrial-anchored protein ligase). Here, we demonstrate that the retromer complex, a known component of vesicle transport from the endosome to the Golgi apparatus, regulates the transport of MAPL from mitochondria to peroxisomes. An unbiased screen shows that Vps35 and Vps26 are found in complex with MAPL, and confocal imaging reveals Vps35 recruitment to mitochondrial vesicles. Silencing of Vps35 or Vps26A leads to a significant reduction in the delivery of MAPL to peroxisomes, placing the retromer within a novel intracellular trafficking route and providing insight into the formation of MAPL-positive MDVs.

The mechanisms that link environmental and intracellular stimuli to mitochondrial functions, including fission/fusion, ATP production, metabolite biogenesis, and apoptosis, are not well understood. Here, we demonstrate that the nutrient-sensing mechanistic/mammalian target of rapamycin complex 1 (mTORC1) stimulates translation of mitochondrial fission process 1 (MTFP1) to control mitochondrial fission and apoptosis. Expression of MTFP1 is coupled to pro-fission phosphorylation and mitochondrial recruitment of the fission GTPase dynamin-related protein 1 (DRP1). Potent active-site mTOR inhibitors engender mitochondrial hyperfusion due to the diminished translation of MTFP1, which is mediated by translation initiation factor 4E (eIF4E)-binding proteins (4E-BPs). Uncoupling MTFP1 levels from the mTORC1/4E-BP pathway upon mTOR inhibition blocks the hyperfusion response and leads to apoptosis by converting mTOR inhibitor action from cytostatic to cytotoxic. These data provide direct evidence for cell survival upon mTOR inhibition through mitochondrial hyperfusion employing MTFP1 as a critical effector of mTORC1 to govern cell fate decisions.

Activation of the innate immune response triggered by dsRNA viruses occurs through the assembly of the Mitochondrial Anti-Viral Signaling (MAVS) complex. Upon recognition of viral dsRNA, the cytosolic receptor RIG-I is activated and recruited to MAVS to activate the immune signaling response. We here demonstrate a strict requirement for a mitochondrial anchored protein ligase, MAPL (also called MUL1) in the signaling events that drive the transcriptional activation of antiviral genes downstream of Sendai virus infection, both in vivo and in vitro. A biotin environment scan of MAPL interacting polypeptides identified a series of proteins specific to Sendai virus infection; including RIG-I, IFIT1, IFIT2, HERC5 and others. Upon infection, RIG-I is SUMOylated in a MAPL-dependent manner, a conjugation step that is required for its activation. Consistent with this, MAPL was not required for signaling downstream of a constitutively activated form of RIG-I. These data highlight a critical role for MAPL and mitochondrial SUMOylation in the early steps of antiviral signaling.