The Mechanism Of Sam50-Mic19-Mic60 Axis Regulating Mitochondrial Inner Membrane Architecture

Mitochondria are bi-layer membrane organelles surrounded by the outer and inner membranes.The mitochondrial ultra-structure is extremely delicate and highly organized and the maintenance and formation of the mitochondrial inner membrane architecture requires membrane contact between the inner and outer membranes.The sorting and assembly machinery(SAM)complex plays essential role in the membrane insertion of ?-barrel proteins on the mitochondrial outer membrane(OMM).The MICOS(mitochondrial contact site and cristae organizing system)complex is a highly conserved complex that is essential for mitochondrial inner membrane structures.However,how these complexes spatially interact with each other and how the interaction regulates the ultra-structure of mitochondria is still obscure and to be solved.Here,we present that mitochondrial intermembrane protein Mic19 interacts with outer membrane protein Sam50 and inner membrane protein Mic60 to form an interaction axis.The Sam50-Mic19-Mic60 axis mediates the formation of MIB complex and bridges mitochondrial outer-and inner-membrane contact.Respectively,The Mic19 N-terminus interacts with the mitochondrial outer membrane protein Sam50,and the C-terminus interacts with the inner membrane protein Mic60 to establish mitochondrial membrane contacts.Also,the integrity of Mic19 is required for both SAM and MICOS complex stabilization.Interestingly,we first found that Mic19 N-terminal,the Sam50-physical-binding site,will be cleaved by mitochondrial protease OMA1 upon Sam50 depletion or Mic19 over-expression.The processed short form Mic19(S-Mic19)is capable of stabilizing the other MICOS complex subunits but not Sam50.Once the formation of SAM-MICOS supercomplex is "blocked",even if Sam50 was exogenously reintroduced,it still disrupts the mitochondrial inner membrane structure.More importantly,we put forward that the SAM complex is actually the anchor site of the mitochondrial junction structure(CJs)on OMM.At the same time,we showed that Mic25 is an important alternative to Mic19,and over-expression of Mic25 can restore the Sam50-‘bridge protein'-Mic60 interaction axis and reconstruct the cristae junctions in the absence of Mic19.In conclusion,our work demonstrated the essential role of Sam50-Mic19-Mic60 axis in determining the mitochondrial inner membrane structure.The mitochondrial outer membrane protein Sam50 not only mediates the transport of mitochondrial proteins,but also regulates the organization of the mitochondrial architecture,which provides a new perspective to explain the formation and maintenance of mitochondrial cristae and CJs structure.Moreover,whether Mic19 or Mic25 acts as a ‘bridge' protein,only connecting and stabilizing SAM-MICOS complexes can link the outer membrane with inner membrane to establish the mitochondrial membrane structure.