Antifolding activity of hsp60 couples protein import into the mitochondrial matrix with export to the intermembrane space

Import of cytochrome b2 to the mitochondrial intermembrane space: The tightly folded heme‐binding domain makes import dependent upon matrix ATP

“…We reexamined these findings in light of the possible influence of the heme-binding domain. pb2(l-167)-DHFR is targeted to the intermembrane space and processed in two steps like authentic cytochrome b2 (Rassow et al, 1989;Click et al, 1992b;Koll et al, 1992). However, pb2( 1-167)-DHFR has an incomplete hemebinding domain ( Fig.…”

“…long presequences of 80 and 61 residues, respectively (Sadler et al, 1984;Guiard, 1985). In both cases the N-terminal portion of the presequence resembles a mitochondrial matrix-targeting signal; the C-terminal portion, which contains a stretch of uncharged amino acids, is responsible for targeting to the intermembrane space (van Loon et al, 1986Jensen et al, 1992;Koll et al, 1992;Beasley et al, 1993;Schwarz et al, 1993). Cleavage of the presequence occurs in two steps, the first being catalyzed by the general matrix-processing peptidase, the second by a different protease at the outer face of the inner membrane (Gasser et al, 1982;Ohashi et al, 1982;Reid et al, 1982;Teintze et al, 1982;Behrens et al, 1991;Schneider et al, 1991).…”

“…Although it is generally agreed that these bipartite presequences contain information for intramitochondrial sorting, there is less agreement about how mitochondria decode this information (reviewed in Glick et al [1992a]). The "conservative sorting" model states that cytochromes b2 and c, are first imported across the inner membrane by the usual matrix-targeting pathway and then are reexported to the intermembrane space by a second, bacterialtype translocation system (Hartl et al, 1987;Koll et al, 1992). Our results favor the alternative "stop-transfer" model, in which the C-terminal portion of the presequence arrests translocation through the inner membrane, so that the mature part of the protein crosses only the outer membrane to reach the intermembrane space (van Loon & Schatz, 1987;Glick et al, 1992b;Wachter et al, 1992).…”

“…Depletion of matrix ATP allows precursor proteins to cross the outer membrane but prevents complete translocation of precursors across the inner membrane (Hwang & Schatz, 1989;Hwang et al, 1991;Rassow & Pfanner, 1991). Thus, a stop-transfer mechanism might operate independently of matrix ATP, whereas a conservative sorting pathway should require matrix ATP for the initial import across the inner membrane (Glick et al, 1992b;Koll et al, 1992). We showed previously that cytochrome c1 is imported and processed normally in ATP-depleted mitochondria, indicating that this protein never passes through the matrix .…”

Mitochondrial protein import: Specific recognition and membrane translocation of preproteins

“…The sorting of these precursor proteins destined for the intermembrane space can be explained in different ways. According to the "conservative sorting" model (Hartl et al, 1987;Koll et al, 1992;Schwarz et al, 1993) the preproteins are imported across both membranes into the matrix and are simultaneously or subsequently exported across the inner membrane directed by the hydrophobic sorting signal acting as an export signal. In the alternative "stop-transfer" model (van Loon & Schatz, 1987;Glick et al, 1992) the hydrophobic part of the bipartite presequence acts as an anchor which arrests preproteins upon import through the inner membrane, subsequent sorting occurs by a selective translocation across the outer membrane into the intermembrane space.…”