Transport of the ADP ATP carrier of mitochondria from the TOM complex to the TIM22.54 complex

Members of the mitochondrial carrier family such as the ADP/ATP carrier (AA C) are composed of three structurally related modules. Here we show that ea ch of the modules contains a mitochondrial import signal recognized by Tim1 0 and Tim12 in the intermembrane space. The first and the second module are translocated across the outer membrane independently of the membrane poten tial, Delta psi, but they are not inserted into the inner membrane. The thi rd module interacts tightly with the TOM complex and thereby prevents compl ete translocation of the precursor across the outer membrane. At this stage , binding of a TIM9.10 complex confers a topology to the translocation inte rmediate which reflects the modular structure of the AAC. The precursor is then transferred to the TIR/19.10.12 complex, still interacting with the TO M complex. Release of the precursor from the TOM complex and insertion into the inner membrane by the TIM22.54 complex requires a Delta psi-responsive signal in the third module.