Dissection of protein translocation across the mitochondrial outer and inner membranes

Import of nuclear-encoded precursor proteins into the mitochondria is a multistep rocess that is facili-tated by the coordinated interaction of two indepen-dent translocation systems in the mitochondrial outer and inner membranes (Gtick et al. 1991; Pfanner et al. 1992; Segui-Real et al. 1993). The import apparatus of the mitochondrial outer membrane (MOM) consists of at least six protein components hat are organized as a complex (for review, see Kiebler et al. 1993; Lill et al. 1995). This so-called MOM complex mediates insertion of preproteins into the outer membrane and import of some preproteins into the intermembrane space (Mayer et al. 1993). In addition, it can initiate the translocation of matrix-targeted preproteins by trans-ferring the amino-terminal mitochondrial targeting se-quence (presequence) onto the intermembrane space side of the outer membrane (Mayer et al. 1995a). For further import of these preproteins into the inner membrane and the matrix space, the MOM machinery cooperates with the import apparatus of the mito-chondrial inner membrane (MIM). Transfer across this membrane requires a membrane potential, AqJ, and matrix ATP as energy sources. In Saccharomyces cerevisiae three essential components of the inner membrane have been identified which are required for protein translocation. MIM17 (SMS1) (Dekker et al