The intermembrane space of mitochondria contains the specific mitochondrial intermembrane space assembly (MIA) machinery that operates in the biogenesis pathway of precursor proteins destined to this compartment. The Mia40 component of the MIA pathway functions as a receptor and binds incoming precursors, forming an essential early intermediate in the biogenesis of intermembrane space proteins. The elements that are crucial for the association of the intermembrane space precursors with Mia40 have not been determined. In this study, we found that a region within the Tim9 and Tim10 precursors, consisting of only nine amino acid residues, functions as a signal for the engagement of substrate proteins with the Mia40 receptor. Furthermore, the signal contains sufficient information to facilitate the transfer of proteins across the outer membrane to the intermembrane space. Thus, here we have identified the mitochondrial intermembrane space sorting signal required for delivery of proteins to the mitochondrial intermembrane space.
INTRODUCTIONMitochondria pose a great challenge for the proper delivery of proteins because of their complex architecture. Mitochondrial precursors must find their way to one of the four mitochondrial subcompartments: the outer membrane, intermembrane space, inner membrane, or matrix. As a direct consequence of this complexity, several machineries for the translocation and sorting of mitochondrial precursors have evolved. Interplay between these machineries and specific signals present in the precursors drive different protein targeting pathways (Schatz and Dobberstein, 1996;Emanuelsson and von Heijne, 2001;Jensen and Johnson, 2001;Endo et al., 2003;Koehler, 2004;Oka and Mihara, 2005;Dolezal et al., 2006;Neupert and Herrmann, 2007;Bolender et al., 2008). Initially, mitochondrial precursors are recognized in a signal-dependent manner by specific receptors and are transferred across the barrier of outer mitochondrial membrane by using the translocase of the outer membrane (TOM) complex. On the trans-side of the outer mitochondrial membrane, sorting machineries decode specific signals in precursors, and this results in the branching of protein import pathways. The most well characterized is the presequence pathway across the inner membrane driven by a cleavable and positively charged signal sequence, called a presequence, and the translocase of the inner membrane (TIM) 23 complex Endo et al., 2003;Oka and Mihara, 2005;Neupert and Herrmann, 2007;Bolender et al., 2008). The presequence is cleaved off by a specific protease liberating the mature protein. However, other mitochondrial signals are not proteolytically removed and remain as part of the native mitochondrial protein. One example is the recently identified -signal that is recognized by the sorting and assembly machinery (SAM) complex to sort -barrel proteins to the mitochondrial outer membrane . In other mitochondrial membrane proteins, the membrane domains, anchors, and their surrounding regions are used to some extent for selection of ...
