Multifaceted roles of porin in mitochondrial protein and lipid transport

Mitochondria are essential eukaryotic organelles responsible for primary cellular energy production. Biogenesis, maintenance, and functions of mitochondria require correct assembly of resident proteins and lipids, which require their transport into and within mitochondria. Mitochondrial normal functions also require an exchange of small metabolites between the cytosol and mitochondria, which is primarily mediated by a metabolite channel of the outer membrane (OM) called porin or voltage-dependent anion channel. Here, we describe recently revealed novel roles of porin in the mitochondrial protein and lipid transport. First, porin regulates the formation of the mitochondrial protein import gate in the OM, the translocase of the outer membrane (TOM) complex, and its dynamic exchange between the major form of a trimer and the minor form of a dimer. The TOM complex dimer lacks a core subunit Tom22 and mediates the import of a subset of mitochondrial proteins while the TOM complex trimer facilitates the import of most other mitochondrial proteins. Second, porin interacts with both a translocating inner membrane (IM) protein like a carrier protein accumulated at the small TIM chaperones in the intermembrane space and the TIM22 complex, a downstream translocator in the IM for the carrier protein import. Porin thereby facilitates the efficient transfer of carrier proteins to the IM during their import. Third, porin facilitates the transfer of lipids between the OM and IM and promotes a back-up pathway for the cardiolipin synthesis in mitochondria. Thus, porin has roles more than the metabolite transport in the protein and lipid transport into and within mitochondria, which is likely conserved from yeast to human.