Intermembrane space proteins

Prediction of mitochondrial targeting signals is not an easy task. The proposed amphiphilic nature is not clear enough. Nakai and Kanehisa (1992) developed a simple method based on the amino acid composition of the segment of most amino-terminal 20 residues. In addition, a simple rule to discriminate the bipartite signal of intermembrane-space proteins was also included in PSORT. 

Gabriel et al. (2007) Novel mitochondrial intermembrane space proteins as substrates of the MIA import pathway. J Mol Biol 365 612-620 Gakh O, Cavadini P, Isaya G (2002) Mitochondrial processing peptidases. Biochim Biophys Acta 1592 63-77... 

Mitochondrial pathway of caspase activation has been demonstrated in various experimental models and humans to contribute significantly to car-diomyocyte apoptosis in the heart [41-44], Studies suggest that mitochondrial-mediated apoptosis contributes to cardiomyocyte loss through intermembrane space proteins release, Bcl-2 protein involvement, and procaspase activation [41, 45, 46], Due to the energy required by heart muscle, mitochondria are particularly abundant in cardiomyocytes. Under physiological conditions, prosurvival mechanisms exist to protect the myocardium from inappropriately triggered apoptosis [3],... 

Figure 17.7. Two types of mitochondrial membrane permeabilization. Upper Bax/Bak pore leads to release of intermembrane space proteins, but the inner membrane is intact. Lower PTP (permeability transition pore) opening destroys the impermeability of the inner mitochondrial membrane (IMM). The pore opening causes influx of solutes and water into the matrix resulting in swelling. The mitochondrial swelling ruptures outer mitochondrial membrane (OMM). CypD, cyclophilin D.

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

Intermembrane-Space Proteins Two pathways deliver cy tosollc proteins to the space between the inner and outer mitochondrial membranes. The major pathway is followed by... 

To determine whether the bulk of Opal is localized to the mitochondrial matrix or to the intermembrane space, we used a protease protection assay in combination with the digitonin extraction procedure described above. Intermembrane space proteins are cleaved by exogenously added proteases when mitochondria are treated with a relatively low concentration of digitonin, while proteolysis of matrix proteins requires a higher concentration of digitonin. This method is adapted from Click et al. (1992). 

Griparic, L., van der Wei, N. N., Orozco, I. J., Peters, P. J., and van der Bliek, A. M. (2004). Loss of the intermembrane space protein Mgml/OPAl induces swelling and localized constrictions along the lengths of mitochondria. J. Biol. Chem. 279, 18792-18798. 

Wong, E. D., Wagner, J. A., Gorsich, S. W., McCaffery, J. M., Shaw, J. M., and Nunnari, J. (2000). The dynamin-related GTPase, Mgmlp, is an intermembrane space protein required for maintenance of fusion competent mitochondria. J. Cell Biol. 151, 341-352. 

Mitochondria are surrounded by a simple outer membrane and a more complex inner membrane (Figure 21.1). The space between the inner and outer membranes is referred to as the intermembrane space. Several enzymes that utilize ATP (such as creatine kinase and adenylate kinase) are found in the intermembrane space. The smooth outer membrane is about 30 to 40% lipid and 60 to 70% protein, and has a relatively high concentration of phos-phatidylinositol. The outer membrane contains significant amounts of porin —a transmembrane protein, rich in /3-sheets, that forms large channels across the membrane, permitting free diffusion of molecules with molecular weights of about 10,000 or less. Apparently, the outer membrane functions mainly to... 

FIGURE 21.11 The structure of UQ-cyt c reductase, also known as the cytochrome hci complex. The alpha helices of cytochrome b (pale green) define the transmembrane domain of the protein. The bottom of the structure as shown extends approximately 75 A into the mitochondrial matrix, and die top of the structure as shown extends about 38 A into the intermembrane space. (Photograph kindly provided by Di Xia and Johann Deismhofer [From Xia, D., Yn, C.-A., Kim, H., Xia,J-Z., Kachnrin, A. M., Zhang, L., Yn,... 

BH3 domain) of the BH3-only proteins binds to other Bcl-2 family members thereby influencing their conformation. This interaction facilitates the release of cytochrome C and other mitochondrial proteins from the intermembrane space of mitochondria. Despite much effort the exact biochemical mechanism which governs this release is not yet fully understood. The release of cytochrome C facilitates the formation of the apoptosome, the second platform for apoptosis initiation besides the DISC. At the apoptosome which is also a multi-protein complex the initiator caspase-9 is activated. At this point the two pathways converge. 

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

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 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. 

Figure 12.2 Copper chaperone function, (a) Copper homeostasis in Enterococcus hirae is affected by the proteins encoded by the cop operon. CopA, Cu1+-import ATPase CopB, Cu1+-export ATPase CopY, Cu1+-responsive repressor copZ, chaperone for Cu1+ delivery to CopY. (b) The CTR family of proteins transports copper into yeast cells. Atxlp delivers copper to the CPx-type ATPases located in the post Golgi apparatus for the maturation of Fet3p. (c) Coxl7p delivers copper to the mitochondrial intermembrane space for incorporation into cytochrome c oxidase (CCO). (d) hCTR, a human homologue of CTR, mediates copper-ion uptake into human cells. CCS delivers copper to cytoplasmic Cu/Zn superoxide dismutase (SOD1). Abbreviations IMM, inner mitochondrial membrane OMM, outer mitochondrial membrane PM, plasma membrane PGV, post Golgi vessel. Reprinted from Harrison et al., 2000. Copyright (2000), with permission from Elsevier Science.

Mitochondria (45-56) are organelles possessing a double membrane, the inner of which is invaginated as cristae. An intermembrane space exists between the inner and outer membranes. The inner membrane consists of an unusually high amount of protein and possesses spherically shaped particles approx 9 nm in diameter. These particles appear to be equivalent to F0, Fb and adenosine triphosphatase. In contrast to the inner membrane, the outer membrane is smooth and appears to be connected to the smooth er. This membrane is permeable to all molecules of 10,000 Dalton or less. A mitochondrial matrix is enclosed by the inner membrane and consists of a ground substance of particles, nucleoids, ribosomes, and electron-transparent regions containing DNA. 

The disposition of the different metal centres of bovine heart CcOx is represented in Figure 14.9. The dimetallic CuA site receives electrons directly from cytochrome c, and is located in a globular domain of subunit II, which protrudes into the intermembrane space (the periplasmic space in bacteria). This centre, which was widely believed to be mononuclear is a dicopper site (Figure 14.10) in which the coppers are bridged by two cysteine sul-furs each copper in addition has two other protein ligands. In the one electron-reduced form,... 

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

Most of the mitochondrial proteins are nuclear encoded and thus must be targeted into mitochondria and sorted into some of their components after their synthesis at the cytosol. Because mitochondria have two membranes, there are four localization sites the matrix, the inner membrane, the intermembrane space, and the outer membrane (Fig. 6). Although there has been considerable progress in our understanding of these processes, some questions still remain. Moreover, the total picture is rather complicated and contains many exceptions. A simplified view is presented here based mainly on the view of Pfanner and Mihara (Mihara and Omura, 1996 Pfanner et al., 1997 Pfanner, 1998). There are also a number of other excellent reviews on this subject (Schatz, 1996 Stuart and Neupert, 1996 Neupert, 1997 Roise, 1997). 

Figure 3. Possible mechanisms of actions of Bcl-2 members. Two prevailing models through which Bcl-2 membas trigger cytochrome c release have been suggested. In both models phospholipids in the bilayer stnicture either individually and/or collectively induce a conformational change in Bcl-2 members, allowing them to insert into the outer mitochondrial membrane. In model 1 proapoptotic proteins destabilize the outer mitochondrial membrane, oligomerize and form channels through which cytochrome c and other proteins of the intermembrane space can escape.BcI-2 proteins such as Bax or tBid act in concert with other proteins of the BcI-2 family to form channels. In model 2 Bcl-2 members such as Bax interact with residoit proteins in the outer membrane (OM) such as the voltage-dependent anion...

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