Import of carrier proteins into the mitochondrial inner membrane mediated by Tim22

TRANSLOCATION of mitochondrial preproteins across the inner membrane is facilitated by the TIM machinery^1-8. Tim23 binds to matrix targeting signals and initiates membrane potential-dependent import⁸. Tim23 and Tim17 are constituents of a translocation channel across the inner membrane⁶. Tim44 is associated with this channel at the matrix side⁶, and Tim44 recruits mitochondrial Hsp70 and its co-chaperone Mge1, which drive protein translocation into the matrix using ATP as an energy source^9-14. Tim22 is a new component of the import machinery of mitochondria, which shares sequence similarity with both Tim23 and Tim17. Here we report that Tim22 is required for the import of proteins of the mitochondrial ADP/ ATP carrier (AAC) family into the inner membrane. Members of the yeast AAC family are synthesized without matrix targeting signals^15,16. Tim22 is in an assembly of high relative molecular mass that is distinct from the Tim23-Timl7 complex⁶. Import of proteins of the AAC family is independent of Tim23, and import of matrix targeting signals containing preproteins is independent ofTim22.