Mitochondria import nuclear‐encoded precursor proteins to four different subcompartments. Specific import machineries have been identified that direct the precursor proteins to the mitochondrial outer membrane, inner membrane or matrix, respectively. However, a machinery dedicated to the import of mitochondrial intermembrane space (IMS) proteins has not been found so far. We have identified the essential IMS protein Mia40 (encoded by the Saccharomyces cerevisiae open reading frame YKL195w). Mitochondria with a mutant form of Mia40 are selectively inhibited in the import of several small IMS proteins, including the essential proteins Tim9 and Tim10. The import of proteins to the other mitochondrial subcompartments does not depend on functional Mia40. The binding of small Tim proteins to Mia40 is crucial for their transport across the outer membrane and represents an initial step in their assembly into IMS complexes. We conclude that Mia40 is a central component of the protein import and assembly machinery of the mitochondrial IMS.
The beta-barrel proteins of mitochondria are synthesized on cytosolic ribosomes. The proteins are imported by the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been assumed that the SAM(core) complex with the subunits Sam35, Sam37 and Sam50 represents the last import stage common to all beta-barrel proteins, followed by splitting in a Tom40-specific route and a route for other beta-barrel proteins. We have identified new components of the beta-barrel assembly machinery and show that the major beta-barrel pathway extends beyond SAM(core). Mdm12/Mmm1 function after SAM(core) yet before splitting of the major pathway. Mdm12/Mmm1 have been known for their role in maintenance of mitochondrial morphology but we reveal assembly of beta-barrel proteins as their primary function. Moreover, Mdm10, which functions in the Tom40-specific route, can associate with SAM(core) as well as Mdm12/Mmm1 to form distinct assembly complexes, indicating a dynamic exchange between the machineries governing mitochondrial beta-barrel assembly. We conclude that assembly of mitochondrial beta-barrel proteins represents a major function of the morphology proteins Mdm12/Mmm1.
Tom40 forms the central channel of the preprotein translocase of the mitochondrial outer membrane (TOM complex). The precursor of Tom40 is encoded in the nucleus, synthesized in the cytosol, and imported into mitochondria via a multi-step assembly pathway that involves the mature TOM complex and the sorting and assembly machinery of the outer membrane (SAM complex). We report that opening of the mitochondrial intermembrane space by swelling blocks the assembly pathway of the -barrel protein Tom40. Mitochondria with defects in small Tim proteins of the intermembrane space are impaired in the Tom40 assembly pathway. Swelling as well as defects in the small Tim proteins inhibit an early stage of the Tom40 import pathway that is needed for formation of a Tom40-SAM intermediate. We propose that the biogenesis pathway of -barrel proteins of the outer mitochondrial membrane not only requires TOM and SAM components, but also involves components of the intermembrane space.Mitochondria contain ϳ800 -1500 different proteins (1-4). 99% of these proteins are encoded in the nucleus and synthesized as precursors on cytosolic ribosomes (5-9). The precursor proteins are recognized by receptors of the mitochondrial outer membrane and are translocated by the general import pore (GIP) 1 across the membrane. The receptors and the GIP assemble to form a high molecular weight complex, termed the translocase of the outer mitochondrial membrane (TOM complex) (10 -14). The three receptors, Tom20, Tom22, and Tom70, show a distinct yet overlapping specificity for different classes of mitochondrial precursor proteins. With the help of the small Tom protein, Tom5, the precursors are transferred to the core of the TOM complex, the GIP formed by the essential protein Tom40 (15, 16). Two additional small Tom proteins, Tom6 and Tom7, are involved in the stability and assembly of the TOM complex. Tom40, the three small Tom proteins, and the receptors Tom22 and Tom20 are associated in a complex of about 450 kDa, whereas the receptor Tom70 is only loosely associated with the other Tom proteins (17)(18)(19)(20)(21)(22).After passing the TOM complex, the precursor proteins are transferred to the other mitochondrial compartments, the intermembrane space, inner membrane, and matrix. Two main classes of precursor proteins can be distinguished (5-8, 13). Preproteins with amino-terminal cleavable targeting sequences (presequences) are transferred from the TOM complex to the presequence translocase (TIM23 complex) of the inner membrane and its associated protein import motor, PAM. Noncleavable precursor proteins with multiple internal targeting signals, like the hydrophobic metabolite carriers of the inner membrane, are transferred from the TOM complex to small Tim proteins of the intermembrane space that guide them through this aqueous compartment. The hydrophobic proteins are then delivered to the protein insertion machinery (carrier translocase, TIM22 complex) of the inner membrane (5,6,23,24). Two soluble complexes of small Tim proteins are known in ...
The translocase of the outer membrane (TOM complex) is the central entry gate for nuclear-encoded mitochondrial precursor proteins. All Tom proteins are also encoded by nuclear genes and synthesized as precursors in the cytosol. The channel-forming -barrel protein Tom40 is targeted to mitochondria via Tom receptors and inserted into the outer membrane by the sorting and assembly machinery (SAM complex). A further outer membrane protein, Mim1, plays a less defined role in assembly of Tom40 into the TOM complex. The three receptors Tom20, Tom22, and Tom70 are anchored in the outer membrane by a single transmembrane ␣-helix, located at the N terminus in the case of Tom20 and Tom70 (signal-anchored) or in the C-terminal portion in the case of Tom22 (tail-anchored). Insertion of the precursor of Tom22 into the outer membrane requires pre-existing Tom receptors while the import pathway of the precursors of Tom20 and Tom70 is only poorly understood. We report that Mim1 is required for efficient membrane insertion and assembly of Tom20 and Tom70, but not Tom22. We show that Mim1 associates with SAM core components to a large SAM complex, explaining its role in late steps of the assembly pathway of Tom40. We conclude that Mim1 is not only required for biogenesis of the -barrel protein Tom40 but also for membrane insertion and assembly of signal-anchored Tom receptors. Thus, Mim1 plays an important role in the efficient assembly of the mitochondrial TOM complex.The essential biochemical function of mitochondria depends on the uptake of cytosolic-synthesized precursor proteins. The vast majority of precursor proteins are imported by the preprotein translocase of the outer mitochondrial membrane (TOM 4 complex). Subsequently the precursor proteins are sorted to the different mitochondrial subcompartments, the outer and inner membranes, the intermembrane space and the matrix (1-7). The TOM complex is composed of seven different subunits. Tom40 forms the protein-conducting channel across the outer membrane (8 -10). The three receptors Tom20, Tom22, and Tom70 expose domains on the cytosolic side of the outer membrane, recognize the precursor proteins and direct them to the Tom40 channel (11). In addition, three small Tom proteins, Tom5, Tom6, and Tom7, are associated with the Tom40 core of the complex (12-14). Tom40 forms a transmembrane -barrel, while all other Tom components are embedded in the membrane via a single transmembrane ␣-helix (9 -10, 15). The ␣-helical membrane anchor is localized in the C-terminal portion of Tom22 and the small Tom proteins and thus those proteins belong to the tail-anchored proteins (15-19). Tom20 and Tom70 are integrated into the outer membrane by an N-terminal ␣-helix, which together with flanking regions is important for both intracellular targeting and membrane anchoring, and thus these proteins are called signal-anchored proteins (20 -23).All Tom proteins are synthesized as precursor proteins on cytosolic ribosomes and imported into mitochondria. The import pathway of the precursor of t...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.