Inner mitochondrial membrane protein insertion

The nuclear-encoded proteins are inserted into both inner and outer mitochondrial membranes, the intermembrane space, and the matrix and there are several different mechanisms involved. As mentioned above there is no apparent requirement for a presequence on proteins which insert specifically into the mitochondrial outer membrane. For proteins destined for the inner mitochondrial membrane, a stop-transfer mechanism is proposed. Thus some information in the peptide must stop the complete transfer of the protein into the mitochondrial matrix, enabling the protein to remain in the inner mitochondrial membrane. For some proteins in the intermembrane space (for example the Rieske iron-sulphur protein associated with the outer face of complex III), a particularly complicated import pathway... 

The Rieske protein in mitochondrial bci complexes is assembled when the protein is incorporated into the complex. The Rieske protein is encoded in the nucleus and synthesized in the cytosol with a mitochondrial targeting presequence, which is required to direct the apoprotein to the mitochondrial matrix. The C-terminus is then targeted back to the outside of the inner mitochondrial membrane where the Rieske cluster is assembled. In addition, the presequence is removed and the protein is processed to its mature size after the protein is inserted into the bci complex. In mammals, the presequence is cleaved in a single step by the core proteins 1 and 2, which are related to the general mitochondrial matrix processing protease (MPP) a and (3 subunits the bovine heart presequence is retained as a 8.0 kDa subunit of the complex (42, 107). In Saccharomyces cerevis-iae, processing occurs in two steps Initially, the yeast MPP removes 22 amino acid residues to convert the precursor to the intermediate form, and then the mitochondrial intermediate protease (MIP) removes 8 residues after the intermediate form is in the bci complex (47). Cleavage by MIP is independent of the assembly of the Rieske cluster Conversion of the intermediate to the mature form was observed in a yeast mutant that did not assemble any Rieske cluster (35). However, in most mutants where the assembly of the Rieske cluster is prevented, the amount of Rieske protein is drastically reduced, most likely because of instability (35, 44). 

Murcha MW et al. (2007) Characterization of the preprotein and amino acid transporter gene family in Arabidopsis. Plant Physiol 143 199-212 Murcha MW, Millar AH, Whelan J (2005a) The N-terminal cleavable extension of plant carrier proteins is responsible for efficient insertion into the inner mitochondrial membrane. J Mol Biol 351 16-25... 

The overall assembly of cytochrome c oxidase on the inner mitochondrial membrane is controlled by a large number of nuclear encoded genes (Tzagoloff and Dieckmann, 1990). Four of these genes, scol, sco2, coxl 1, and coxl 7, encode proteins that appear to be involved in copper incorporation into the catalytic core of the enzyme, though precisely which one(s) is (are) responsible for insertion of copper into the complex remains unclear (Horvath et al., 2000). Scol is anchored in the inner mitochondrial membrane and is essential for the accumulation of Coxl and CoxII subunits as well as the proper assembly of the cytochrome c... 

All three respiratory complexes are typical integral membrane proteins that span the inner mitochondrial membrane. Each consists of several different subunits, the exact number of which is still under debate. The genes of some subunits of cytochrome oxidase and the />c, complex are in mitochondrial DNA (mtDNA). These proteins are synthesised inside the mitochondrion. However, most proteins of these complexes, as well as cytochrome c, are synthesised on cytoplasmic ribosomes and coded by the nuclear genome. This raises intriguing questions of how the latter are imported into the mitochondrion and inserted into the mitochondrial membrane, as well as of how mitochondrial and cytoplasmic transcription and translation are synchronised [3-5]. 

FECH (also known as heme synthase) is an iron-sulfur protein located in the inner mitochondrial membrane. This enzyme inserts ferrous iron into protoporphyrin to form heme During this process, two hydrogens are displaced from the ring nitrogens. Other metals in the divalent state will also act as substrate, yielding the corresponding chelate (e.g., incorporation of Zn into protoporphyrin to yield zinc protoporphyrin). In iron-deficient states Zn successfully competes with Fe in developing red cells so that the concentration of zinc protoporphyrin in erythrocytes increases. Furthermore, other dicarboxylic porphyrins will also serve as substrates (e.g., mesoporphyrin). 

The final pathway for insertion in the inner mitochondrial membrane is followed by multipass proteins that con-... 

One of the more challenging locations, therefore, for consideration of the comprehensive hydrophobic effect in the panoply of biological energy conversions is the electron transport chain embedded within the inner mitochondrial membrane. Essential parts of these protein-based machines insert into and function in very hydrophobic lipid bilayers. Here the ingress and egress of protons for develop-... 

The above describes the major pathway of proteins destined for the mitochondrial matrix. However, certain proteins insert into the outer mitochoiidrial membrane facilitated by the TOM complex. Others stop in the intermembrane space, and some insert into the inner membrane. Yet others proceed into the matrix and then return to the inner membrane or intermembrane space. A number of proteins contain two signaling sequences—one to enter the mitochondrial matrix and the other to mediate subsequent relocation (eg, into the inner membrane). Certain mitochondrial proteins do not contain presequences (eg, cytochrome Cy which locates in the inter membrane space), and others contain internal presequences. Overall, proteins employ a variety of mechanisms and routes to attain their final destinations in mitochondria. 

Rehling P, Model K, Brandner K, Kovermann P, Sickmann A, Meyer HE, Kuhlbrandt W, Wagner R, Truscott KN, Pfanner N (2003) Protein insertion into the mitochondrial inner membrane by a twin-pore translocase. Science 299 1747-1751 Richards TA, Cavalier-Smith T (2005) Myosin domain evolution and the primary divergence of eukaryotes. Nature 436 1113-1118... 

The space between the inner and outer mitochondrial membrane can be reached by proteins that have two signal peptides. The first inserts the protein into the membrane and is cleaved in the matrix the second remains and directs the protein to the intermembrane space. 

Inner and outer mitochondrial membranes are specialized modifications of a large family of membranes that includes the nuclear and the plasma membranes and the membranes of the endoplasmic reticulum. Mitochondrial membranes function as selective barriers that are also capable of active transport and provide a framework on which catalytic proteins (e.g., those of the respiratory chain) are tightly inserted in a pattern compatible with maximum efficiency. Like other membranes, mitochondrial membranes are composed of lipids and protein molecules—the fundamental building blocks. However, some features are unique to the mitochondria. For example, the mitochondrial membranes contain a lipid cardiolipin not found in the plasma membrane. 

An electrophoresis gel of the bovine heart complex is shown in Figure 21.14. The total mass of the protein in the complex, composed of 13 subunits, is 204 kD. Subunits I through III, the largest ones, are encoded by mitochondrial DNA, synthesized in the mitochondrion, and inserted into the inner membrane from the matrix side. The smaller subunits are coded by nuclear DNA and synthesized in the cytosol. 

The first condition is met by having a series of four protein complexes inserted into the mitochondrial inner membrane, each made up of a number of electron (and sometimes proton) acceptors of increasing redox potential. Three of them (Complexes I, III and IV) are presented in cartoon form in Figure 5.17. Complex I, referred to more prosaically as... 

Kerscher O, Holder J, Srinivasan M, Leung RS, Jensen RE (1997) The Tim54p-Tim22p complex mediates insertion of proteins into the mitochondrial inner membrane. J Cell Biol 139 1663-1675... 

Koehler CM et al. (2000) Timl8p, a new subunit of the TIM22 complex that mediates insertion of imported proteins into the yeast mitochondrial inner membrane. Mol Cell Biol 20 1187-1193... 

Stuart R (2002) Insertion of proteins into the inner membrane of mitochondria the role of the Oxal complex. Biochim Biophys Acta 1592 79-87 Stuart RA, Cyr DM, Craig EA, Neupert W (1994) Mitochondrial molecular chaperones their role in protein translocation. Trends Biochem Sci 19 87-92 Sutak R et al. (2004) Mitochondrial-type assembly of FeS centers in the hydrogenosomes of the amitochondriate eukaryote Trichomonas vaginalis. Proc Natl Acad Sci USA 101 10368-10373... 

Metalloprotein protein that binds a specific metal ion and requires that metal ion for proper function Metal transporter transmembrane protein responsible for the translocation of metal ions across a lipid bilayer MTMl mitochondrial inner membrane transporter needed for activating SOD2 with manganese SCO Copper carrying molecule, possibly the copper chaperone or copper insertion factor for cytochrome oxidase SMF2 intracellular metal transporter essential for manganese trafficking... 

The first condition is met by having a series of four protein complexes, inserted into the mitochondrial inner membrane, each made up of a number of electron (and sometimes proton) acceptors of increasing redox potential. Three of them (Complexes I, III, and IV) are presented in cartoon form in Figure 5.18. Complex I, referred to more prosaically as NADH-Coenzyme Q oxidoreductase, transfers electrons stepwise from NADH, through a flavo-protein (containing FMN as cofactor) to a series of iron—sulfur clusters (of which more in Chapter 13) and ultimately to coenzyme Q, a lipid-soluble quinone, which transfers its electrons to Complex III. The AE o for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol, and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

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