DYNAMIC INTERACTION OF THE PROTEIN TRANSLOCATION SYSTEMS IN THE INNERAND OUTER MEMBRANES OF YEAST MITOCHONDRIA

Mitochondria contain two distinct protein import systems, one in the o uter and the other in the inner membrane. These systems can act indepe ndently of one another in submitochondrial fractions or if a protein i s transported to the outer membrane or to the intermembrane space. It has been proposed that the two systems associate reversibly when a pro tein is transported across both membranes, but this hypothesis has rem ained unproven. In order to address this question, we have checked whe ther antibodies against a subunit of one system can co-immunoprecipita te subunits of the other system. We find that the two systems associat e stably if a matrix-targeted precursor is arrested during import; no association is seen in the absence of a stuck precursor. These experim ents provide direct evidence that protein import into the mitochondria l matrix is mediated by the reversible interaction of the two transloc ation systems.