THE MITOCHONDRIAL RECEPTOR COMPLEX - MOM22 IS ESSENTIAL FOR CELL VIABILITY AND DIRECTLY INTERACTS WITH PREPROTEINS

A multisubunit complex in the mitochondrial outer membrane is responsi ble for targeting and membrane translocation of nuclear-encoded prepro teins. This receptor complex contains two import receptors, a general insertion pore and the protein Mom22. It was unknown if Mom22 directly interacts with preproteins, and two views existed about the possible functions of Mom22: a central role in transfer of preproteins from bot h receptors to the general insertion pore or a more limited function d ependent on the presence of the receptor Mom19. For this report, we id entified and cloned Saccharomyces cerevisiae MOM22 and investigated wh ether it plays a direct role in targeting of preproteins. A preprotein accumulated at the mitochondrial outer membrane was cross-linked to M om22. The cross-linking depended on the import stage of the preprotein , Overexpression of Mom22 suppressed the respiratory defect of yeast c ells lacking Mom19 and increased preprotein import into mom19 Delta mi tochondria, demonstrating that Mom22 can function independently of Mom 19. Overexpression of Mom22 even suppressed the lethal phenotype of a double deletion of the two import receptors known so far (mom19 Delta mom72 Delta). Deletion of the MOM22 gene was lethal for yeast cells, i dentifying Mom22 as one of the few mitochondrial membrane proteins ess ential for fermentative growth. These results suggest that Mom22 plays an essential role in the mitochondrial receptor complex. It directly interacts with preproteins in transit and can perform receptor-like ac tivities.