Functional independence of the protein translocation machineries in mitochondrial outer and inner membranes: passage of preproteins through the intermembrane space.

Abstract

The protein translocation machineries of the outer and inner mitochondrial membranes usually act in concert during translocation of matrix and inner membrane proteins. We considered whether the two machineries can function independently of each other in a sequential reaction. Fusion proteins (pF-CCHL) were constructed which contained dual targeting information, one for the intermembrane space present in cytochrome c heme lyase (CCHL) and the other for the matrix space contained in the signal sequence of the precursor of F 1 -ATPase β-subunit (pF 1 β). In the absence of a membrane potential, Δψ, the fusion proteins moved into the intermembrane space using the CCHL pathway. In contrast, in the presence of Δψ they followed the pF 1 β pathway and eventually were translocated into the matrix. The fusion protein pF51-CCHL containing 51 amino acids of pF 1 β, once transported into the intermembrane space in the absence of a membrane potential, could be further chased into the matrix upon re-establishing Δψ. The sequential and independent movement of the fusion protein across the two membranes demonstrates that the translocation machineries act as distinct entities. Our results support a model in which the two translocation machineries can function independently of each other, but generally interact in a dynamic fashion to achieve simultaneous translocation across both membranes. In addition, the results provide information about the targeting sequences within CCHL. The protein does not contain a signal for retention in the intermembrane space; rather, it lacks matrix targeting information, and therefore is unable to undergo Δψ-dependent interaction with the protein translocation apparatus in the inner membrane.