Receptor sites involved in posttranslational transport of apocytochrome c into mitochondria: specificity, affinity, and number of sites

Aim32 is a dual-localized 2Fe-2S mitochondrial protein that functions in redox quality control

Yeast is a facultative anaerobe and uses diverse electron acceptors to maintain redox-regulated import of cysteine-rich precursors via the mitochondrial intermembrane space assembly (MIA) pathway. With the growing diversity of substrates utilizing the MIA pathway, understanding the capacity of the intermembrane space (IMS) to handle different types of stress is crucial. We used MS to identify additional proteins that interacted with the sulfhydryl oxidase Erv1 of the MIA pathway. Altered inheritance of mitochondria 32 (Aim32), a thioredoxin-like [2Fe-2S] ferredoxin protein, was identified as an Erv1-binding protein. Detailed localization studies showed that Aim32 resided in both the mitochondrial matrix and IMS. Aim32 interacted with additional proteins including redox protein Osm1 and protein import components Tim17, Tim23, and Tim22. Deletion of Aim32 or mutation of conserved cysteine residues that coordinate the Fe-S center in Aim32 resulted in an increased accumulation of proteins with aberrant disulfide linkages. In addition, the steady-state level of assembled TIM22, TIM23, and Oxa1 protein import complexes was decreased. Aim32 also bound to several mitochondrial proteins under nonreducing conditions, suggesting a function in maintaining the redox status of proteins by potentially targeting cysteine residues that may be sensitive to oxidation. Finally, Aim32 was essential for growth in conditions of stress such as elevated temperature and hydroxyurea, and under anaerobic conditions. These studies suggest that the Fe-S protein Aim32 has a potential role in general redox homeostasis in the matrix and IMS. Thus, Aim32 may be poised as a sensor or regulator in quality control for a broad range of mitochondrial proteins.

Mitochondrial cytochrome c biogenesis: no longer an enigma

Cytochromes c (cyt c) and c1 are heme proteins that are essential for aerobic respiration. Release of cyt c from mitochondria is an important signal in apoptosis initiation. Biogenesis of c-type cytochromes involves covalent attachment of heme to two cysteines (at a conserved CXXCH sequence) in the apocytochrome. Heme attachment is catalyzed in most mitochondria by holocytochrome c synthase (HCCS), which is also necessary for the import of apocytochrome c (apocyt c). Thus, HCCS affects cellular levels of cyt c, impacting mitochondrial physiology and cell death. Here, we review the mechanisms of HCCS function and the roles of heme and residues in the CXXCH motif. Additionally, we consider concepts emerging within the two prokaryotic cytochrome c biogenesis pathways.

Cytochrome c biogenesis in mitochondria--Systems III and V

In c-type cytochromes, heme becomes covalently attached to the polypeptide chain by a reaction between the vinyl groups of the heme and cysteine thiols from the protein. There are two such cytochromes in mitochondria: cytochrome c and cytochrome c(1). The heme attachment is a post-translational modification that is catalysed by different biogenesis proteins in different organisms. Three types of biogenesis system are found or predicted in mitochondria: System I (the cytochrome c maturation system); System III (termed holocytochrome c synthase (HCCS) or heme lyase); and System V. This review focuses primarily on cytochrome c maturation in mitochondria containing HCCS (System III). It describes what is known about the enzymology and substrate specificity of HCCS; the role of HCCS in human disease; import of HCCS into mitochondria; import of apocytochromes c and c(1) into mitochondria and the close relationships with HCCS-dependent heme attachment; and the role of the fungal cytochrome c biogenesis accessory protein Cyc2. System V is also discussed; this is the postulated mitochondrial cytochrome c biogenesis system of trypanosomes and related organisms. No cytochrome c biogenesis proteins have been identified in the genomes of these organisms whose c-type cytochromes also have a unique mode of heme attachment.

Peptide 68-88 of apocytochrome c plays a crucial role in its insertion into membrane and binding to mitochondria

Apocytochrome c (Apocyt. c) is the precursor of cytochrome c. It is synthesized in the cytosol and posttranslationally imported into mitochondria. In order to determine the crucial sequence in apocyt. c translocation, deleted mutant and chemically synthesized peptides with different length were used. Obtained results showed that sequence 68-88 of apocyt. c plays a critical role in its insertion into membrane and binding to mitochondria.

Change of apocytochrome c translocation across membrane in consequence of hydrophobic segment deletion

Wild-type apocytochrome c and its hydrophobic segment deleted mutants, named delta28-39, delta72-86 and delta28-29/72-86 were constructed, expressed and highly purified respectively. Insertion ability into phospholipid monolayer, inducing leakage of entrapped fluorescent dye fluorescein sulfonate (FS) from liposomes, and translocation across model membrane system showed that the wild-type apoprotein and delta28-39 almost exhibited the same characteristics, while mutants with segment 72-86 deletion did not. Furthermore, CD spectra, intrinsic fluorescence emission spectra, and the accessibility of the protein to the fluorescence quenchers: KI, acrylamide and HB demonstrated that the segment 72-86 deletion has a significant effect on the conformational changes of apocytochrome c following its interaction with phospholipid. On the basis of these results it is postulated that the C-terminal hydrophobic segment 72-86 plays an important role in the translocation of apocytochrome c across membrane.

Mitochondrial dysfunction in cardiac disease: ischemia--reperfusion, aging, and heart failure

Mitochondria contribute to cardiac dysfunction and myocyte injury via a loss of metabolic capacity and by the production and release of toxic products. This article discusses aspects of mitochondrial structure and metabolism that are pertinent to the role of mitochondria in cardiac disease. Generalized mechanisms of mitochondrial-derived myocyte injury are also discussed, as are the strengths and weaknesses of experimental models used to study the contribution of mitochondria to cardiac injury. Finally, the involvement of mitochondria in the pathogenesis of specific cardiac disease states (ischemia, reperfusion, aging, ischemic preconditioning, and cardiomyopathy) is addressed.

Polymorphisms in the SOD2 and HLA-DRB1 genes are associated with nonfamilial idiopathic dilated cardiomyopathy in Japanese

To reveal genetic risk factors of nonfamilial idiopathic cardiomyopathy (IDC) in Japanese, polymorphisms in the SOD2 and HLA-DRB1 genes were investigated in 86 patients and 380 healthy controls. There was a significant excess of homozygotes for the V allele [Val versus Ala (A allele), a polymorphism in the leader peptide of manganese superoxide dismutase at position 16] of the SOD2 gene in the patients compared with the controls (87.2% versus 74.7%, odds ratio = 2.30, p = 0.013, pc < 0.03), and a significant increase in the frequency of HLA-DRB1*1401 in the patients was confirmed (14.0% vs 4.5%, odds ratio = 3.46, p = 0.001, pc < 0.03). A two-locus analysis suggested that these two genetic markers (SOD2-VV genotype and DRB1*1401) may play a synergistic role in controlling the susceptibility to nonfamilial IDC. In addition, processing efficiency of Val-type SOD2 leader peptide in the presence of mitochondria was siginificantly lower than that of the Ala-type by 11 +/- 4%, suggesting that this lower processing efficiency was in part an underlying mechanism of the association between the SOD2-VV genotype and nonfamilial IDC.

Cytochrome c heme lyase activity of yeast mitochondria

A highly efficient in vitro system was established for measuring by high performance liquid chromatography the formation of holocytochrome c by yeast mitochondria. Holocytochrome c formation required reducing agents, of which dithiothreitol was the most effective. With biosynthetically made, pure Drosophila melanogaster apocytochrome c and Saccharomyces cerevisiae mitochondria, the activity of cytochrome c heme lyase amounted to about 800 fmol min-1 mg-1 mitochondrial protein. The kinetics were typical Michaelis-Menten (Km approximately 1 nM), as were those of mitoplasts with broken outer membranes (Km approximately 3 nM). As tested with mitoplasts, holocytochromes c from a range of species were found to be competitive inhibitors of heme lyase at physiological concentrations, providing a mechanism for controlling this concentration in vivo. Apocytochrome c associated with yeast mitochondria in two phases of Kd approximately 2 x 10(-10) and 10(-8) M, respectively, whereas mitoplasts had lost the high affinity binding. A site-directed mutant of apocytochrome c (lysines 5, 7, and 8 replaced by glutamine, glutamic acid, and asparagine) was found to be converted to holocytochrome c (Km approximately 3.3 nM; maximal activity unchanged), even though the mutations completely eliminated the high affinity binding. Thus, the high affinity binding of apocytochrome c to mitochondria is not directly related to holocytochrome c formation.

Translocation of apocytochrome c across the outer membrane of mitochondria

Apocytochrome c follows a unique pathway into mitochondria. Import does not require the general protein translocation machinery, protease-sensitive components of the outer membrane, or a membrane potential across the inner membrane. We investigated the membrane binding and translocation steps of the import reaction using purified outer membrane vesicles (OMV) from Neurospora crassa mitochondria. OMV specifically bound, but did not import apocytochrome c. If, however, specific antibodies were enclosed inside OMV, apocytochrome c was accumulated in soluble form in the lumen. Import was reversible, since apocytochrome c became accessible to external protease after release from the antibodies. Thus, OMV are competent of translocating apocytochrome c into their lumen, but lack a binding partner which traps the apoprotein. In intact mitochondria, cytochrome c heme lyase (CCHL), a peripheral protein of the inner membrane, serves such a function by stably associating with apocytochrome c in a complex which is detectable by co-immunoprecipitation. We suggest a model for the import mechanism of apocytochrome c in which the apoprotein specifically associates with and reversibly passes across the outer membrane. Translocation is rendered unidirectional by stable association with CCHL which serves as a "trans side receptor." Finally, heme is attached by CCHL and the holoprotein folds into its native structure.

Characterization of apocytochrome C binding to human erythrocytes

The binding of 125I-labeled apocytochrome c to human erythrocytes was determined for free apocytochrome c concentrations at 10(-10)-10(-6) M. At about 2 x 10(-9) M, maximum cell association of apocytochrome c occurs at 50 mM NaCl and at 22 degrees C. Intact erythrocytes at 22 degrees C have three classes of apocytochrome c binding sites: one high-affinity noncooperative site (n1 = 728 per cell, Kd1 = 1.5 x 10(-9) M) and two positively cooperative sites (n2 = 3.7 x 10(4) per cell, Kd2 = 1.2 x 10(-7) M, alpha 2 = 2.0, and n3 = 2.5 x 10(5) per cell, Kd3 = 7.1 x 10(-7) M, alpha 3 = 12). Erythrocytes at 37 degrees C, and erythrocyte ghosts at 22 degrees C, also have three classes of apocytochrome c binding sites, and most sites are positively cooperative.

Cytosolic L-alanine:4,5-dioxovalerate transaminase differs from the mitochondrial form

L-Alanine:4,5-dioxovalerate transaminase was detected in the kidney cytosolic fraction with a lower specific activity than the mitochondrial enzyme. The enzyme was purified from the cytosol to homogeneity with a yield of 32%, and comparative analysis with the mitochondrial form was performed. Both forms of the enzyme have identical pH and temperature optima and also share common antigenic determinants. However, differences in their molecular properties exist. The molecular mass of the native cytoplasmic enzyme is 260 kDa, whereas that of the mitochondrial enzyme is 210 kDa. In addition, the cytoplasmic L-alanine: 4,5-dioxovalerate transaminase had a homopolymeric subunit molecular mass of 67 kDa compared to a subunit molecular mass of 50 kDa for the mitochondrial L-alanine:4,5-dioxovalerate transaminase. This is the first report of two forms of L-alanine:4,5-dioxovalerate transaminase. The different responses of cytosolic and mitochondrial L-alanine:4,5-dioxovalerate transaminases to hemin supplementation both in vitro and in vivo was demonstrated. Maximum inhibition of mitochondrial L-alanine:4,5-dioxovalerate transaminase activity was demonstrated with hemin injected at a dose of 1.2 mg/kg body mass, whereas the same dose of hemin stimulated the cytosolic enzyme to 150% of the control. A one-dimensional peptide map of partially digested cytosolic and mitochondrial L-alanine:4,5-dioxovalerate transaminase shows that the two forms of the enzymes are structurally related. Partial digestion of the cytosolic form of the enzyme with papain generated a fragment of 50 kDa which was identical to that of the undigested mitochondrial form (50 kDa). Moreover, papain digestion resulted in a threefold increase in cytosolic enzyme activity over the native enzyme, and such enhancement was comparable to the activity of the mitochondrial form of the enzyme. Therefore, we conclude that the cytosolic form of L-alanine: 4,5-dioxovalerate transaminase is different from the mitochondrial enzyme. Furthermore, immunoblot analysis indicated that the mitochondrial enzyme has antigenic similarity to the cytosolic enzyme as well as to the papain-digested cytosolic enzyme 50-kDa fragment.

Mitochondrial protein import

A dynamic picture of the mitochondrial protein import pathway is emerging, with conformational alteration a critical feature both preceding and following membrane translocation. The mediators of these steps of conformational alteration, as well as steps of recognition, translocation, and proteolytic cleavage, appear to be proteins. Using powerful tools of genetics and biochemistry, in years to come it should be possible to determine the precise molecular function of these proteins in mediating these novel reactions.

Mitochondrial targeting of yeast apoiso-1-cytochrome c is mediated through functionally independent structural domains

An iso-1-cytochrome c-chloramphenicol acetyltransferase fusion protein (iso-1/CAT) was expressed in Saccharomyces cerevisiae and used to delineate two stages in the cytochrome c import pathway in vivo (S. H. Nye and R. C. Scarpulla, Mol. Cell. Biol. 10:5753-5762, 1990 [this issue]). Fusion proteins with the CAT reporter domain in its native conformation were arrested at the initial stage of mitochondrial membrane recognition and insertion. In contrast, those with a deletional disruption of the CAT moiety were relieved of this block and allowed to translocate to the intermembrane space, where they functioned in respiratory electron transfer. In the present study, iso-1/CAT was used to map structural determinants in apoiso-1-cytochrome c involved in the initial step of targeting to the mitochondrial membrane. Carboxy-terminal deletions revealed that one of these determinants consisted of the amino-terminal 68 residues. Deletion mutations either within or at the ends of this determinant destroyed mitochondrial targeting activity, suggesting that functionally important information spans the length of this fragment. Disruption of an alpha-helix near the amino terminus by a helix-breaking proline substitution for leucine 14 also eliminated the targeting activity of the 1 to 68 determinant, suggesting a contribution from this structure. A second, functionally independent targeting determinant was found in the carboxy half of the apoprotein between residues 68 and 85. This determinant coincided with a stretch of 11 residues that are invariant in nearly 100 eucaryotic cytochromes c. Therefore, in lieu of an amino-terminal presequence, apocytochrome c has redundant structural information located in both the amino and carboxy halves of the molecule that can function independently to specify mitochondrial targeting and membrane insertion in vivo.

Biogenesis of mitochondrial c-type cytochromes

Cytochromes c and c1 are essential components of the mitochondrial respiratory chain. In both cytochromes the heme group is covalently linked to the polypeptide chain via thioether bridges. The location of the two cytochromes is in the intermembrane space; cytochrome c is loosely attached to the surface of the inner mitochondrial membrane, whereas cytochrome c1 is firmly anchored to the inner membrane. Both cytochrome c and c1 are encoded by nuclear genes, translated on cytoplasmic ribosomes, and are transported into the mitochondria where they become covalently modified and assembled. Despite the many similarities, the import pathways of cytochrome c and c1 are drastically different. Cytochrome c1 is made as a precursor with a complex bipartite presequence. In a first step the precursor is directed across outer and inner membranes to the matrix compartment of the mitochondria where cleavage of the first part of the presequence takes place. In a following step the intermediate-size form is redirected across the inner membrane; heme addition then occurs on the surface of the inner membrane followed by the second processing reaction. The import pathway of cytochrome c is exceptional in practically all aspects, in comparison with the general import pathway into mitochondria. Cytochrome c is synthesized as apocytochrome c without any additional sequence. It is translocated selectively across the outer membrane. Addition of the heme group, catalyzed by cytochrome c heme lyase, is a requirement for transport. In summary, cytochrome c1 import appears to follow a "conservative pathway" reflecting features of cytochrome c1 sorting in prokaryotic cells. In contrast, cytochrome c has "invented" a rather unique pathway which is essentially "non-conservative."

Mitochondrial targeting of yeast apoiso-1-cytochrome c is mediated through functionally independent structural domains

An iso-1-cytochrome c-chloramphenicol acetyltransferase fusion protein (iso-1/CAT) was expressed in Saccharomyces cerevisiae and used to delineate two stages in the cytochrome c import pathway in vivo (S. H. Nye and R. C. Scarpulla, Mol. Cell. Biol. 10:5753-5762, 1990 [this issue]). Fusion proteins with the CAT reporter domain in its native conformation were arrested at the initial stage of mitochondrial membrane recognition and insertion. In contrast, those with a deletional disruption of the CAT moiety were relieved of this block and allowed to translocate to the intermembrane space, where they functioned in respiratory electron transfer. In the present study, iso-1/CAT was used to map structural determinants in apoiso-1-cytochrome c involved in the initial step of targeting to the mitochondrial membrane. Carboxy-terminal deletions revealed that one of these determinants consisted of the amino-terminal 68 residues. Deletion mutations either within or at the ends of this determinant destroyed mitochondrial targeting activity, suggesting that functionally important information spans the length of this fragment. Disruption of an alpha-helix near the amino terminus by a helix-breaking proline substitution for leucine 14 also eliminated the targeting activity of the 1 to 68 determinant, suggesting a contribution from this structure. A second, functionally independent targeting determinant was found in the carboxy half of the apoprotein between residues 68 and 85. This determinant coincided with a stretch of 11 residues that are invariant in nearly 100 eucaryotic cytochromes c. Therefore, in lieu of an amino-terminal presequence, apocytochrome c has redundant structural information located in both the amino and carboxy halves of the molecule that can function independently to specify mitochondrial targeting and membrane insertion in vivo.

Apo forms of cytochrome c550 and cytochrome cd1 are translocated to the periplasm of Paracoccus denitrificans in the absence of haem incorporation caused either mutation or inhibition of haem synthesis

An apo form of cytochrome c550 can be detected by immunoblotting cell-free extracts of a mutant of Paracoccus denitrificans that is deficient in c-type cytochromes. This apoprotein is found predominantly in the periplasm, the location of the holocytochrome in the wild-type organism, indicating that translocation of the polypeptide occurs in the absence of haem attachment. The polypeptide molecular weight, as judged by sodium dodecyl sulphate/polyacrylamide gel electrophoresis, is indistinguishable from that of the holoprotein and the chemically prepared apoprotein; this suggests that the N-terminal signal sequence is removed in the mutant as in the wild-type organism. In the presence of levulinic acid, an inhibitor of haem biosynthesis, apocytochrome c550 and aponitrite reductase (cytochrome cd1) accumulated in the periplasm of wild-type cells. Synthesis of these apoproteins was blocked by chloramphenicol. Thus in P. denitrificans the synthesis of these polypeptides is neither autoregulated nor regulated by the availability of haem. That the apoproteins appear in the periplasm argues against the possibility of polypeptide/haem co-transport from cytoplasm to periplasm. These observations are related to, and contrasted with, the biosynthesis of c-type cytochromes in eukaryotic cells.

Reversible import of apocytochrome c into mitochondria

35S-labeled Drosophila melanogaster apocytochrome c was made by in vitro transcription/translation of the gene and purified to the monomeric, fully reduced form. It was found that in the presence of a wheat germ extract factor there was a high-affinity phase of the uptake into mouse liver mitochondria at 10-300 pM apocytochrome c, and a lower-affinity phase through 4000 pM. Without the factor, the high-affinity phase was absent. The stimulatory effect of the factor could not be elicited with various reductants, such as NADH, FMN, and ferrous protoheme IX. Conversely, when mitochondria loaded with apocytochrome c were resuspended in fresh medium, the protein readily reequilibrated. Successive washings depleted greater than 95% of the associated apoprotein but removed no holoprotein. Proteases (proteinase K or trypsin) added to a suspension of mitochondria loaded with apoprotein digested an amount of apoprotein similar to that which would have been dissociated during the same time, as measured by successive washings in the absence of protease. Mitochondria loaded with apoprotein and similarly treated with protease continued exporting the apoprotein, even after the protease was inhibited and removed, suggesting that most of the apoprotein associated with the organelle was in a protease-resistant compartment. Apocytochrome c mutants in which serines or alanines replaced cysteines 14 and 17, which bind the prosthetic group, behaved like the cysteine-containing protein, indicating that the covalent attachment of the heme is unrelated to the translocation of the apoprotein.

Artificial red cells. A link between the membrane skeleton and RES detectability?

Factors governing nonspecific reticuloendothelial system (RES)-detectability are largely unknown. Will a liposome that mimics the lipid composition of the outer leaflet of the erythrocyte membrane be invisible to the RES? On both experimental and theoretical grounds we believe the answer is no, in part because 1) sorption of proteins is believed to be important in determining RES uptake, 2) a membrane skeleton is apparently necessary to inhibit protein sorption into erythrocyte membranes and 3) Neohemocytes (a liposome encapsulated hemoglobin product) currently lack a membrane skeleton. Neohemocytes with erythrocyte outer leaflet lipid composition do have extended circulation half-times, but these are at least two orders of magnitude shorter than the circulation half-times of erythrocytes. How might a membrane skeleton modulate RES-detectability? Can avoidance of opsonization result in part from the properties of the membrane skeleton? If so, then how? To explore and quantify such questions we have developed a theoretical, statistical-thermodynamic model of protein binding into membranes. It predicts that the membrane area available for rapid lateral diffusion is critically important in controlling the amount of sorbed protein per unit area, and that a membrane skeleton can reduce a protein's sorption by several orders of magnitude. Based on theoretical results, we offer a speculative model for the detection of non-self lipid bilayers by the RES.

In vitro import of cytochrome c peroxidase into the intermembrane space: release of the processed form by intact mitochondria

Cytochrome c peroxidase (CCP) is a nuclearly encoded hemoprotein located in the intermembrane space (IMS) of Saccharomyces cerevisiae mitochondria. Wild-type preCCP synthesized in rabbit reticulocyte lysates, however, was inefficiently translocated into isolated mitochondria and was inherently resistant to externally added proteases. To test whether premature heme addition to the apoprecursor was responsible for the protease resistance and the inability to import preCCP, site-directed mutagenesis was used to replace the axial heme ligand (His175) involved in forming a pseudo-covalent link between the heme iron and CCP. Mutant proteins containing Leu, Arg, Met, or Pro at residue 175 of mature CCP were sensitive to proteolysis and were imported into isolated mitochondria as judged by proteolytic processing of the precursor. The inhibition of wild-type CCP translocation across the outer membrane may result from the inability of the heme-containing protein to unfold during the translocation process. Although the protease responsible for cleaving preCCP to its mature form is believed to be located in the IMS, most of the processed CCP was located in the supernatant rather than the mitochondrial pellet. Since the outer membranes were shown to be intact, the anomalous localization indicated that preCCP may not have been completely translocated into the IMS before proteolytic processing or that conformationally labile proteins may not be retained by the outer membrane. Proteolytic maturation of preCCP also occurred in the presence of valinomycin, suggesting that the precursor may be completely or partially translocated across the outer mitochondrial membrane independent of a potential across the inner mitochondrial membrane.

Import of cytochrome c into mitochondria: reduction of heme, mediated by NADH and flavin nucleotides, is obligatory for its covalent linkage to apocytochrome c

The covalent attachment of heme to apocytochrome c, and therefore the import of cytochrome c into mitochondria, is dependent on both NADH plus a cytosolic cofactor that has been identified to be FMN or FAD. NADH in concert with flavin nucleotides mediates the reduction of heme. Heme in the reduced state is a prerequisite for its covalent attachment to apocytochrome c by the enzyme cytochrome c heme lyase and thus for subsequent translocation of cytochrome c across the outer mitochondrial membrane during import.

Inhibition of PhoE translocation across Escherichia coli inner-membrane vesicles by synthetic signal peptides suggests an important role of acidic phospholipids in protein translocation

To obtain insight into the mechanism of precursor protein translocation across membranes, the effect of synthetic signal peptides and other relevant (poly)peptides on in vitro PhoE translocation was studied. The PhoE signal peptide, associated with inner membrane vesicles, caused a concentration-dependent inhibition of PhoE translocation, as a result of a specific interaction with the membrane. Using a PhoE signal peptide analog and PhoE signal peptide fragments, it was demonstrated that the hydrophobic part of the peptide caused the inhibitory effect, while the basic amino terminus is most likely important for an optimal interaction with the membrane. A quantitative analysis of our data and the known preferential interaction of synthetic signal peptides with acidic phospholipids in model membranes strongly suggest the involvement of negatively charged phospholipids in the inhibitory interaction of the synthetic PhoE signal peptide with the inner membrane. The important role of acidic phospholipids in protein translocation was further confirmed by the observation that other (poly)peptides, known to have both a high affinity for acidic lipids and hydrophobic interactions with model membranes, also caused strong inhibition of PhoE translocation. The implication of these results with respect to the role of signal peptides in protein translocation is indicated.

Posttranscriptional regulation of cytochrome c expression during the developmental cycle of Trypanosoma brucei

We examined the expression of a nucleus-encoded mitochondrial protein, cytochrome c, during the life cycle of Trypanosoma brucei. The bloodstream forms of T. brucei, the long slender and short stumpy trypanosomes, have inactive mitochondria with no detectable cytochrome-mediated respiration. The insect form of T. brucei, the procyclic trypanosomes, has fully functional mitochondria. Cytochrome c is spectrally undetectable in the bloodstream forms of trypanosomes, but during differentiation to the procyclic form, spectrally detected holo-cytochrome c accumulates rapidly. We have purified T. brucei cytochrome c and raised antibodies that react to both holo- and apo-cytochrome c. In addition, we isolated a partial cDNA to trypanosome cytochrome c. An examination of protein expression and steady-state mRNA levels in T. brucei indicated that bloodstream trypanosomes did not express cytochrome c but maintained significant steady-state levels of cytochrome c mRNA. The results suggest that in T. brucei, cytochrome c is developmentally regulated by a posttranscriptional mechanism which prevents either translation or accumulation of cytochrome c in the bloodstream trypanosomes.

Import pathways of precursor proteins into mitochondria: multiple receptor sites are followed by a common membrane insertion site

The precursor of porin, a mitochondrial outer membrane protein, competes for the import of precursors destined for the three other mitochondrial compartments, including the Fe/S protein of the bc1-complex (intermembrane space), the ADP/ATP carrier (inner membrane), subunit 9 of the F0-ATPase (inner membrane), and subunit beta of the F1-ATPase (matrix). Competition occurs at the level of a common site at which precursors are inserted into the outer membrane. Protease-sensitive binding sites, which act before the common insertion site, appear to be responsible for the specificity and selectivity of mitochondrial protein uptake. We suggest that distinct receptor proteins on the mitochondrial surface specifically recognize precursor proteins and transfer them to a general insertion protein component (GIP) in the outer membrane. Beyond GIP, the import pathways diverge, either to the outer membrane or to translocation contact-sites, and then subsequently to the other mitochondrial compartments.

cDNA and deduced amino acid sequences of cytochrome c from Chlamydomonas reinhardtii: unexpected functional and phylogenetic implications

We have isolated complementary DNA (cDNA) clones for apocytochrome c from the green alga Chlamydomonas reinhardtii and shown that they are encoded by a single nuclear gene termed cyc. Cyc mRNA levels are found to depend primarily on the presence of acetate as a reduced carbon source in the culture medium. The deduced amino acid sequence shows that, apart from the probable removal of the initiating methionine, C. reinhardtii apocytochrome c is synthesized in its mature form. Its structure is generally similar to that of cytochromes c from higher plants. Several punctual deviations from the general pattern of cytochrome c sequences that is found in other organisms have interesting structural and functional implications. These include, in particular, valines 19 and 39, asparagine 78, and alanine 83. A phylogenetic tree was constructed by the matrix method from cytochrome c data for a representative range of species. The results suggest that C. reinhardtii diverged from higher plants approximately 700-750 million years ago; they also are not easy to reconcile with the current attribution of Chlamydomonas reinhardtii and Enteromorpha intestinalis to a unique phylum, because these two species probably diverged from one another at about the same time as they diverged from the line leading to higher plants.

Posttranscriptional regulation of cytochrome c expression during the developmental cycle of Trypanosoma brucei

We examined the expression of a nucleus-encoded mitochondrial protein, cytochrome c, during the life cycle of Trypanosoma brucei. The bloodstream forms of T. brucei, the long slender and short stumpy trypanosomes, have inactive mitochondria with no detectable cytochrome-mediated respiration. The insect form of T. brucei, the procyclic trypanosomes, has fully functional mitochondria. Cytochrome c is spectrally undetectable in the bloodstream forms of trypanosomes, but during differentiation to the procyclic form, spectrally detected holo-cytochrome c accumulates rapidly. We have purified T. brucei cytochrome c and raised antibodies that react to both holo- and apo-cytochrome c. In addition, we isolated a partial cDNA to trypanosome cytochrome c. An examination of protein expression and steady-state mRNA levels in T. brucei indicated that bloodstream trypanosomes did not express cytochrome c but maintained significant steady-state levels of cytochrome c mRNA. The results suggest that in T. brucei, cytochrome c is developmentally regulated by a posttranscriptional mechanism which prevents either translation or accumulation of cytochrome c in the bloodstream trypanosomes.

Import of proteins into mitochondria: a multi-step process

Translocation of precursor proteins from the cytosol into mitochondria is a multi-step process. The generation of translocation intermediates, i.e. the reversible accumulation of precursors at distinct stages of their import pathway into mitochondria ('translocation arrest'), has allowed the experimental characterization of distinct functional steps of protein import. These steps include: ATP-dependent unfolding of precursors; specific recognition of precursors by distinct receptors on the mitochondrial surface; interaction of precursors; specific recognition of precursors by distinct receptors on the mitochondrial surface; interaction of precursors with a general insertion protein ('GIP') in the outer mitochondrial membrane; membrane-potential-dependent translocation into the inner membrane at contact sites between both membranes; proteolytic processing of precursors; and intramitochondrial sorting of precursors via the matrix space ('conservative sorting'). The functional characteristics unveiled by studying mitochondrial protein import appear to be of general interest for investigations on intracellular protein sorting.

Yeast iso-1-cytochrome c: genetic analysis of structural requirements

We describe the use of classical and molecular genetic techniques to investigate the folding, stability, and enzymatic requirements of iso-1-cytochrome c from the yeast Saccharomyces cerevisiae. Interpretation of the defects associated with an extensive series of altered forms of iso-1-cytochrome c was facilitated by the recently resolved three dimensional structure of iso-1-cytochrome c [(1987) J. Mol. Biol. 199, 295-314], and by comparison with the phylogenetic series of eukaryotic cytochromes c. Residue replacements that abolish iso-1-cytochrome c function appear to do so by affecting either heme attachment or protein stability; no replacements that abolish electron transfer function without affecting protein structure were uncovered. Most nonfunctional forms retained at least partial covalent attachment to the heme moiety; heme attachment was abolished only by replacements of Cys19 and Cys22, which are required for thioether linkage, and His23, a heme ligand. Replacements were uncovered that retain function at varying levels, including replacements at evolutionarily conserved positions, some of which were structurally and functionally indistinguishable from wild type iso-1-cytochrome c.

Mitochondrial protein import: differential recognition of various transport intermediates by antibodies

The precursors of the mitochondrial proteins ADP/ATP carrier (AAC) and F1-ATPase subunit beta (F1 beta) were accumulated at the stages of binding to receptor sites on the mitochondrial outer membrane, or in contact sites between outer and inner membranes. Specific antibodies raised against the mature proteins were added to the isolated mitochondria and efficiently bound to these translocation intermediates. Further movement of the precursors to consecutive steps along their import pathway was thereby inhibited. Controls showed that precursor proteins which were inserted into or translocated across the outer membrane were not recognized by the antibodies unless the mitochondrial membranes were disrupted. We conclude that the trapped translocation intermediates have antigenic sites exposed to the outside of the outer membrane.

Mitochondrial precursor proteins are imported through a hydrophilic membrane environment

We have analyzed how translocation intermediates of imported mitochondrial precursor proteins, which span contact sites, interact with the mitochondrial membranes. F1-ATPase subunit beta (F1 beta) was trapped at contact sites by importing it into Neurospora mitochondria in the presence of low levels of nucleoside triphosphates. This F1 beta translocation intermediate could be extracted from the membranes by treatment with protein denaturants such as alkaline pH or urea. By performing import at low temperatures, the ADP/ATP carrier was accumulated in contact sites of Neurospora mitochondria and cytochrome b2 in contact sites of yeast mitochondria. These translocation intermediates were also extractable from the membranes at alkaline pH. Thus, translocation of precursor proteins across mitochondrial membranes seems to occur through an environment which is accessible to aqueous perturbants. We propose that proteinaceous structures are essential components of a translocation apparatus present in contact sites.

Integration of porin synthesized in vitro into outer mitochondrial membranes

Porin, an intrinsic protein of outer mitochondrial membranes of rat liver, was synthesized in vitro in a cell-free in a cell-free translation system with rat liver RNA. The apparent molecular mass of porin synthesized in vitro was the same as that of its mature form (34 kDa). This porin was post-translationally integrated into the outer membrane of rat liver mitochondria when the cell-free translation products were incubated with mitochondria at 30 degrees C even in the presence of a protonophore (carbonyl cyanide m-chlorophenylhydrazone). Therefore, the integration of porin seemed to proceed energy-independently as reported by Freitag et al. [(1982) Eur. J. Biochem. 126, 197-202]. Its integration seemed, however, to require the participation of the inner membrane, since porin was not integrated when isolated outer mitochondrial membranes alone were incubated with the translation products. Porin in the cell-free translation products bound to the outside of the outer mitochondrial membrane when incubated with intact mitochondria at 0 degrees C for 5 min. When the incubation period at 0 degrees C was prolonged to 60 min, this porin was found in the inner membrane fraction, which contained monoamine oxidase, suggesting that porin might bind to a specific site on the outer membrane in contact or fused with the inner membrane (a so-called OM-IM site). This porin bound to the OM-IM site was integrated into the outer membrane when the membrane fraction was incubated at 30 degrees C for 60 min. These observations suggest that porin bound to the outside of the outer mitochondrial membrane is integrated into the outer membrane at the OM-IM site by some temperature-dependent process(es).

Evidence that a Chloroplast Surface Protein Is Associated with a Specific Binding Site for the Precursor to the Small Subunit of Ribulose-1,5-Bisphosphate Carboxylase

Most chloroplast proteins are encoded by nuclear genes and synthesized in the cytoplasm as higher molecular weight precursors. These precursors are imported posttranslationally into the chloroplasts, where they are proteolytically processed, and sorted to their proper locations. The first step of this import process is thought to be the binding of precursors to putative receptors on the outer envelope membrane of chloroplasts. We have investigated the interaction of the precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase with its putative receptor by using a heterobifunctional, photoactivatable cross-linker. The resulting cross-linked conjugate has a molecular weight of 86,000, and is present on the surface of chloroplasts as determined by its sensitivity to digestion with protease. Control experiments demonstrated that the label in the conjugate is derived from small subunit precursor and that the conjugate is formed only when modified precursor is reacted in the presence of chloroplasts. Based on these results, we postulate that a protein on the surface of chloroplasts is part of the receptor which interacts with the small subunit precursor.

Carboxyl-terminal sequences influence the import of mitochondrial protein precursors in vivo

The large subunit of carbamoyl phosphate synthase A [carbon-dioxide: L-glutamine amido-ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.5.5] from Neurospora crassa is encoded by a nuclear gene but is localized in the mitochondrial matrix. We have utilized N. crassa strains that produce both normal and carboxyl-terminal-truncated forms of carbamoyl phosphate synthase A to ask whether the carboxyl terminus affects import of the carbamoyl phosphate synthase A precursor. We found that carboxyl-terminal-truncated precursors were directed to mitochondria but that they were imported less efficiently than full-length proteins that were synthesized in the same cytoplasm. Our results suggest that effective import of proteins into mitochondria requires appropriate combinations of targeting sequences and three-dimensional structure.

The ornithine transcarbamylase leader peptide directs mitochondrial import through both its midportion structure and net positive charge

The cytoplasmically synthesized precursor of the mitochondrial matrix enzyme, ornithine transcarbamylase (OTC), is targeted to mitochondria by its NH2-terminal leader peptide. We previously established through mutational analysis that the midportion of the OTC leader peptide is functionally required. In this article, we report that study of additional OTC precursors, altered in either a site-directed or random manner, reveals that (a) the midportion, but not the NH2-terminal half, is sufficient by itself to direct import, (b) the functional structure in the midportion is unlikely to be an amphiphilic alpha-helix, (c) the four arginines in the leader peptide contribute collectively to import function by conferring net positive charge, and (d) surprisingly, proteolytic processing of the leader peptide does not require the presence of a specific primary structure at the site of cleavage, in order to produce the mature OTC subunit.