Cell-free synthesis of the mitochondrial ADP/ATP carrier protein of Neurospora crassa

Mitochondrial membrane potential

The mitochondrial membrane potential (ΔΨm) generated by proton pumps (Complexes I, III and IV) is an essential component in the process of energy storage during oxidative phosphorylation. Together with the proton gradient (ΔpH), ΔΨm forms the transmembrane potential of hydrogen ions which is harnessed to make ATP. The levels of ΔΨm and ATP in the cell are kept relatively stable although there are limited fluctuations of both these factors that can occur reflecting normal physiological activity. However, sustained changes in both factors may be deleterious. A long-lasting drop or rise of ΔΨm vs normal levels may induce unwanted loss of cell viability and be a cause of various pathologies. Among other factors, ΔΨm plays a key role in mitochondrial homeostasis through selective elimination of dysfunctional mitochondria. It is also a driving force for transport of ions (other than H+) and proteins which are necessary for healthy mitochondrial functioning. We propose additional potential mechanisms for which ΔΨm is essential for maintenance of cellular health and viability and provide recommendations how to accurately measure ΔΨm in a cell and discuss potential sources of artifacts.

The role of Dermcidin isoform-2 in the occurrence and severity of Diabetes

Diabetes is now epidemic worldwide. Several hundred-million peoples are presently suffering from this disease with other secondary-disorders. Stress, hypertension, sedentary life-style, carbohydrate/lipid metabolic-disorders due to genetic or environmental factors attributes to type-1 and/or type-2 diabetes. Present investigation demonstrates that stress-induced protein dermcidin isoform-2 (DCN-2) which appears in the serum of diabetic-patients play a key-role in this disease pathogenesis/severity. DCN-2 suppresses insulin production-release from liver/pancreas. It also increases the insulin-resistance. Stress-induction at the onset/progression of this disease is noticed as the high-level of lipid peroxides/low-level of free-thiols in association with increase of inflammatory-markers c-reactive protein and TNF-α. DCN-2 induced decrease in the synthesis of glucose-activated nitric oxide synthase (GANOS) and lower production of NO in liver has been shown here where NO is demonstrated to lower the expression of glucose trabsporter-4 (GLUT-4) and its translocation on liver membrane surface. This finally impairs glucose transport to organs from the extracellular fluid. Low level of glucose uptake further decreases glucose-induced insulin synthesis. The central role of DCN-2 has been demonstrated in type-1/type-2 diabetic individuals, in rodent hepatocytes and pancreatic-cell, tissue-slices, in-vitro and in-vivo experimental model. It can be concluded that stress-induced decrease in insulin synthesis/function, glucose transport is an interactive consequence of oxidative threats and inflammatory events.

The control of hyperglycemia by a novel trypsin resistant oral insulin preparation in alloxan induced type I diabetic mice

A trypsin resistant oral insulin preparation was made by incubating insulin for 2 h at 23 °C with previously boiled cow milk at 100 °C that was coagulated with 0.6 M acetic acid. The precipitate was resuspended in the same volume of milk. The immunoblot analysis of the suspended proteins treated with 200 ng of trypsin/ml for 3 h demonstrated that the 80.1% of the insulin in the suspension survived the proteolytic degradation compared to 0% of the hormone survived in the control. The feeding of 0.4 ml (0.08 unit of insulin) of the resuspended proteins followed by 0.2 ml of the same protein to alloxan induced diabetic mice maximally decreased the blood glucose level from 508 ± 10 mg/dl to 130 ± 10 mg/dl in 7 h with simultaneous increase of the basal plasma concentration of insulin from 3 ± 1.1 μunits/ml to 18 ± 1.5 μunits/ml. In control experiment the absence of insulin in the identical milk suspension produced no hypoglycemic effect suggesting milk was not responsible for the hypoglycemic effect of milk-insulin complex. Coming out of insulin-casein complex from the intestinal gut to the circulation was spontaneous and facilitated diffusion transportation which was found from Gibbs free energy reaction.

The activation by glucose of liver membrane nitric oxide synthase in the synthesis and translocation of glucose transporter-4 in the production of insulin in the mice hepatocytes

Introduction

Glucose has been reported to have an essential role in the synthesis and secretion of insulin in hepatocytes. As the efflux of glucose is facilitated from the liver cells into the circulation, the mechanism of transportation of glucose into the hepatocytes for the synthesis of insulin was investigated.

Methods

Grated liver suspension (GLS) was prepared by grating intact liver from adult mice by using a grater. Nitric oxide (NO) was measured by methemoglobin method. Glucose transporter-4 (Glut-4) was measured by immunoblot technique using Glut-4 antibody.

Results

Incubation of GLS with different amounts of glucose resulted in the uptake of glucose by the suspension with increased NO synthesis due to the stimulation of a glucose activated nitric oxide synthase that was present in the liver membrane. The inhibition of glucose induced NO synthesis resulted in the inhibition of glucose uptake. Glucose at 0.02M that maximally increased NO synthesis in the hepatocytes led to the translocation and increased synthesis of Glut-4 by 3.3 fold over the control that was inhibited by the inhibition of NO synthesis. The glucose induced NO synthesis was also found to result in the synthesis of insulin, in the presence of glucose due to the expression of both proinsulin genes I and II in the liver cells.

Conclusion

It was concluded that glucose itself facilitated its own transportation in the liver cells both via Glut-4 and by the synthesis of NO which had an essential role for insulin synthesis in the presence of glucose in these cells.

Effect of progesterone receptor status on maspin synthesis via nitric oxide production in neutrophils in human breast cancer

BACKGROUND

Although progesterone receptor (PR) status, similarly to estrogen receptor status, is of prognostic importance in breast cancer, the involvement of the PR in breast cancer remains obscure. Studies were conducted to determine the function of the PR in neutrophils in the nitric oxide-induced synthesis of maspin, an anti-breast-cancer protein produced in nonmalignant mammary cells and in neutrophils in the circulation.

METHODS

PR status was determined by immunohistochemistry. Maspin synthesis was determined by in-vitro translation of messenger RNA and quantified by enzyme-linked immunosorbent assay. Nitric oxide was determined by the methemoglobin method.

RESULTS

It was found that PR status in neutrophils was identical with that in malignant breast tissues. A Scatchard plot for progesterone binding to normal and PR-positive (PR+) neutrophils revealed that whereas normal neutrophils had 11.5 × 10(10) PR sites/cell with K d = 47.619 nM, PR+ neutrophils had 6.6 × 10(10) PR sites/cell with K d = 47.619 nM. The progesterone negative (PR-) neutrophils failed to bind to progesterone. Incubation of normal and PR+ neutrophils with 25 nM progesterone produced 1.317 μM NO and 2.329 nM maspin; the PR+ neutrophils produced 0.72 μM NO and 1.138 nM maspin. The PR- neutrophils failed to produce any NO or maspin in the presence of progesterone. Inhibition of progesterone-induced NO synthesis led to complete inhibition of maspin synthesis in all neutrophils.

CONCLUSION

These results suggest that estrogen and progesterone complement each other in NO-induced maspin synthesis, and do not necessarily antagonize in the synthesis of the anti-breast-cancer protein.

A Mitochondrial Odyssey

Good fortune let me be an innocent child during World War II, a hopeful adolescent with encouraging parents during the years of German recovery, and a self-determined adult in a period of peace, freedom, and wealth. My luck continued as a scientist who could entirely follow his fancy. My mind was always set on understanding how things are made. At a certain point, I found myself confronted with the question of how mitochondria and organelles, which cannot be formed de novo, are put together. Intracellular transport of proteins, their translocation across the mitochondrial membranes, and their folding and assembly were the processes that fascinated me. Now, after some 30 years, we have wonderful insights, unimagined views of a complex and at the same time simple machinery and its workings. We have glimpses of how orderly processes are established in the cell to assemble from single molecules our beautiful mitochondria that every day make some 50 kg of ATP for each of us. At the same time, we have learned amazing lessons from the tinkering of evolution that developed mitochondria from bacteria.

The Role of Neutrophil Estrogen Receptor Status on Maspin Synthesis via Nitric Oxide Production in Human Breast Cancer

Purpose

Estrogen, through its binding to nuclear estrogen receptor (ER), has been implicated in the development of human breast cancer. The presence or absence of ER in breast lesions has been used to classify breast cancer into ER+ or ER- type. Maspin, an anti-breast cancer protein produced in normal mammary cells, has also been reported to control the condition. Studies have been conducted to determine the role of ER+ and ER- status in neutrophils in the synthesis of maspin in human breast cancer.

Methods

Maspin presence was determined by enzyme linked immunosorbent assay, while nitric oxide (NO) level was determined using the methemoglobin method.

Results

Scatchard plots of the equilibrium binding of estrogen demonstrated the presence of 4.18×10⁷ receptors per normal neutrophil and 2.46×10⁷ receptors per ER+ neutrophil with a similar dissociation constant (0.926 nM). The ER- type showed nonspecific estrogen binding only. At 0.6 nM estrogen, NO synthesis was maximally increased to 1.829 and 0.887 µM NO/10⁹ cells at 4 hours in normal and ER+ neutrophils respectively, with synthesis of 2.383 and 1.422 nM maspin in normal and ER+ neutrophils respectively. Estrogen failed to produce these effects in ER- neutrophils.

Conclusion

ER status in neutrophils determined maspin synthesis in breast cancer through the stimulation of NO synthesis. Neutrophils with ER- status which do not produce any maspin when treated with estrogen, might imply a worse prognostic outcome in ER- breast cancer due to the lack of anti-breast cancer protein synthesis.

Selection of a DNA barcode for Nectriaceae from fungal whole-genomes

A DNA barcode is a short segment of sequence that is able to distinguish species. A barcode must ideally contain enough variation to distinguish every individual species and be easily obtained. Fungi of Nectriaceae are economically important and show high species diversity. To establish a standard DNA barcode for this group of fungi, the genomes of Neurospora crassa and 30 other filamentous fungi were compared. The expect value was treated as a criterion to recognize homologous sequences. Four candidate markers, Hsp90, AAC, CDC48, and EF3, were tested for their feasibility as barcodes in the identification of 34 well-established species belonging to 13 genera of Nectriaceae. Two hundred and fifteen sequences were analyzed. Intra- and inter-specific variations and the success rate of PCR amplification and sequencing were considered as important criteria for estimation of the candidate markers. Ultimately, the partial EF3 gene met the requirements for a good DNA barcode: No overlap was found between the intra- and inter-specific pairwise distances. The smallest inter-specific distance of EF3 gene was 3.19%, while the largest intra-specific distance was 1.79%. In addition, there was a high success rate in PCR and sequencing for this gene (96.3%). CDC48 showed sufficiently high sequence variation among species, but the PCR and sequencing success rate was 84% using a single pair of primers. Although the Hsp90 and AAC genes had higher PCR and sequencing success rates (96.3% and 97.5%, respectively), overlapping occurred between the intra- and inter-specific variations, which could lead to misidentification. Therefore, we propose the EF3 gene as a possible DNA barcode for the nectriaceous fungi.

The role of dermcidin isoform 2: a two-faceted atherosclerotic risk factor for coronary artery disease and the effect of acetyl salicylic Acid on it

Hypertension and diabetes mellitus are considered to be two major atherosclerotic risk factors for coronary artery disease (CAD). A stress-induced protein identified to be dermcidin isoform 2 of Mr. 11 kDa from blood plasma of hypertensive persons when injected (0.1 μM) in rabbits increased the systolic pressure by 77% and diastolic pressure by 45% over the controls within 2 h. Ingestion of acetyl salicylic acid (150 mg/70 kg) by these subjects reduced systolic (130 mm Hg) and diastolic pressures (80 mm Hg) with reduction of plasma dermcidin level to normal ranges (9 nM). The protein was found to be a potent activator of platelet cyclooxygenase and inhibited insulin synthesis. Aspirin was found to reduce hypertension by reduction of plasma dermcidin level, neutralized the effect of cyclooxygenase, and restored the pancreatic insulin synthesis through NO synthesis. These results indicated that dermcidin could be a novel atherosclerotic risk factor for its hypertensive and diabetogenic effects.

Acetyl salicylic acid (aspirin) improves synthesis of maspin and lowers incidence of metastasis in breast cancer patients [corrected]

Maspin, a 42 kDa protein produced in normal breast cells, has been shown to inhibit the invasion and metastasis of breast cancer in an animal model. Ingestion of acetylsalicylic acid (aspirin) by breast cancer patients has been reported to restore the systemic synthesis of maspin through the stimulation of systemic nitric oxide production. Studies were carried out to determine the effect of aspirin on the incidence of breast cancer metastasis, which is reported to occur in 50% of patients who have previously received chemotherapy, radiation, and/or surgery. Thirty-five female patients (aged 41-65 years) with breast cancer who had previously received these therapies took one 75 mg/70 kg body weight enteric-coated aspirin tablet every 24 h, after an adequate meal, for 3 years. Their plasma nitric oxide and maspin levels were measured. The occurrence of metastasis was ascertained monthly by a qualified oncologist, and confirmed, if necessary, by biopsy. Daily ingestion of aspirin by participants resulted in an increase in maspin levels from 0.95 ± 0.04 to 4.63 ± 0.05 nM after 24 h. These levels were maintained for 3 years. These studies suggest that daily ingestion of aspirin might significantly reduce the incidence of breast cancer metastasis in patients who have previously received anticancer therapies.

The glucose-induced synthesis of insulin in liver

Pancreatic β cells, stimulated by glucose, are known to synthesize and secrete insulin. As liver diseases are reported to cause diabetes mellitus, studies were conducted to determine the possibility of glucose-induced insulin synthesis in the liver cells. The glucose-induced insulin synthesis was determined by in vitro translation of mRNA from the hepatocytes. The cDNA from mRNA was prepared and sequence analysis was performed. Incubation of hepatocytes from the liver of adult mice (n=10) with glucose (0.02 M) resulted in the insulin synthesis [0.03 (mean)±0.006 (S.D.) μunits/mg/h] compared to the pancreatic β cells [0.04±0.004 μunits/mg/h]. Immunohistochemical study also demonstrated the glucose-induced synthesis of insulin in liver cells. Incubation of the mice hepatocytes with glucose resulted in the synthesis of insulin mRNA. The purified mRNA which was used to prepare cDNA resulted in the formation of proinsulin I and proinsulin II genes corresponding to 182 and 188 base pairs, respectively. Sequence analysis of the cDNA indicated that proinsulin I as well as proinsulin II gene could be involved in the synthesis of insulin by hepatocytes. These results suggested that insulin synthesis in both hepatic and pancreatic cells could be involved in the control of diabetes mellitus.

Purification and mechanism of action of "cortexin," a novel antihypertensive protein hormone from kidney and its role in essential hypertension in men

Because kidney tissue damage is associated with both hypertension and impaired nitric oxide (NO) production, we investigated the possibility whether the kidney tissue contains any activator of endothelial NO synthase (eNOS) that could be important in essential hypertension. An activator protein of M(r) 43000 Da for eNOS from the goat kidney cortex homogenate was purified to homogeneity by chromatographic techniques. This activator trivially, called "cortexin," was determined by enzyme-linked immunosorbent assay using anticortexin antibody. NO was determined by the formation of methemoglobin. Injection of 0.5 nmol cortexin/kg body weight to rabbit pretreated with l-epinephrine that increased the systolic and diastolic pressures to 195 +/- 3.40 mm Hg and 98.14 +/- 6.64 mm Hg, respectively, reduced and kept the elevated pressures at normal ranges of 133.57 +/- 12.14 (systolic) and 51.03 +/- 3.21 (diastolic) for 45 hours with simultaneous increase of plasma NO level. The inhibition of cortexin-induced NO synthesis nullified the antihypertensive effect of cortexin. The plasma cortexin level in newly diagnosed persons with essential hypertension was 0 pmol/mL (median), which contrasted with 218.94 pmol cortexin/mL (median), in normotensive persons (P < .0005; n = 25). We concluded that the impaired production of cortexin in the cortex of kidney might lead to essential hypertension.

Systemic production of IFN-alpha by garlic (Allium sativum) in humans

The effect of foods on the production of interferon-alpha (IFN-alpha) is currently unknown. Garlic (Allium sativum) used as a folk medicine is reported to stimulate nitric oxide (NO) production. We investigated the systemic increase of NO due to the ingestion of garlic on the plasma IFN-alpha level in normal volunteers. Normal volunteers (10 groups, 10 in each group) ate 2 g fresh garlic, and plasma NO and IFN-alpha levels were determined after 2 and 4 h. The participants were also asked to eat garlic for various periods of time, and plasma NO and IFN-alpha were similarly assayed. Ingestion of 2 g fresh, but not boiled, garlic was found to increase the basal plasma level of NO from 2.7 +/- 0.1 microM to 8.76 +/- 0.21 microM at 2 and 4 h, respectively. The basal plasma IFN-alpha level increased from 9.51 +/- 0.26 nM to 46.3 +/- 1.2 nM in normal volunteers (n = 10) at the same time. The chronic eating of garlic was found to maintain IFN-alpha at high levels for at least 7 days. The exposure of neutrophils to garlic in vivo or in vitro, which also stimulated synthesis of NO in these cells, was found to stimulate IFN-alpha synthesis as measured by the stimulation of IFN-alpha mRNA synthesis. Thus, consumption of garlic resulted in stimulated synthesis of NO and, in turn, IFN-alpha in humans, which could be beneficial in viral or proliferative diseases.

Cloning and characterization of a plastidal and a mitochondrial isoform of tobacco protoporphyrinogen IX oxidase

Protoporphyrinogen IX oxidase is the last enzyme in the common pathway of heme and chlorophyll synthesis and provides precursor for the mitochondrial and plastidic heme synthesis and the predominant chlorophyll synthesis in plastids. We cloned two different, full-length tobacco cDNA sequences by complementation of the protoporphyrin-IX-accumulating Escherichia coli hemG mutant from heme auxotrophy. The two sequences show similarity to the recently published Arabidopsis PPOX, Bacillus subtilis hemY, and to mammalian sequences encoding protoporphyrinogen IX oxidase. One cDNA sequence encodes a 548-amino acid residues protein with a putative transit sequence of 50 amino acid residues, and the second cDNA encodes a protein of 504 amino acid residues. Both deduced protein sequences share 27.2% identical amino acid residues. The first in vitro translated protoporphyrinogen IX oxidase could be translocated to plastids, and the approximately 53-kDa mature protein was detected in stroma and membrane fraction. The second enzyme was targeted to mitochondria without any detectable reduction in size. Localization of both enzymes in subcellular fractions was immunologically confirmed. Steady-state RNA analysis indicates an almost synchronous expression of both genes during tobacco plant development, greening of young seedlings, and diurnal and circadian growth. The mature plastidal and the mitochondrial isoenzyme were overexpressed in E. coli. Bacterial extracts containing the recombinant mitochondrial enzyme exhibit high protoporphyrinogen IX oxidase activity relative to control strains, whereas the plastidal enzyme could only be expressed as an inactive peptide. The data presented confirm a compartmentalized pathway of tetrapyrrole synthesis with protoporphyrinogen IX oxidase in plastids and mitochondria.

Import of carrier proteins into the mitochondrial inner membrane mediated by Tim22

Translocation of mitochondrial preproteins across the inner membrane is facilitated by the TIM machinery. Tim23 binds to matrix targeting signals and initiates membrane potential-dependent import. Tim23 and Tim17 are constituents of a translocation channel across the inner membrane. Tim44 is associated with this channel at the matrix side, and Tim44 recruits mitochondrial Hsp70 and its co-chaperone Mgel, which drive protein translocation into the matrix using ATP as an energy source. Tim22 is a new component of the import machinery of mitochondria, which shares sequence similarity with both Tim23 and Tim17. Here we report that Tim22 is required for the import of proteins of the mitochondrial ADP/ATP carrier (AAC) family into the inner membrane. Members of the yeast AAC family are synthesized without matrix targeting signals. Tim22 is in an assembly of high relative molecular mass that is distinct from the Tim23-Tim17 complex. Import of proteins of the AAC family is independent of Tim23, and import of matrix targeting signals containing preproteins is independent of Tim22.

The ADP/ATP translocator from potato has a long amino-terminal extension

The ADP/ATP translocator is an abundant protein of the mitochondrial inner membrane, which in fungi and mammals is synthesized without a presequence. Here we report that the translocator from potato has an amino-terminal extension which may function in mitochondrial targeting. Several cDNA clones encoding the nucleotide sequence of the ADP/ATP translocator have been isolated from potato leaf and tuber cDNA libraries constructed in lambda phages. Only one class of cDNA clones was found but possibly different translocator genes are expressed in other tissues. High levels of transcripts for the translocator are found in all tissues analysed. Sequence determination of the complete insert of one of the clones reveals a long open reading frame of 1158 bp encoding a protein of 386 amino acids corresponding to a calculated molecular weight of 42 kDa. In contrast, the ADP/ATP translocator proteins from fungi and mammals are significantly smaller. Comparison of the Neurospora translocator with the potato protein shows about 75% sequence homology, being confined to the region after amino acid 85 of the potato polypeptide. Antibodies directed against the fungal translocator recognize a protein of 30 kDa in the inner membrane of potato mitochondria, suggesting that the mature protein has a similar size as the translocators from fungi and mammals. Thus, the additional segment of the potato ADP/ATP translocator forms an amino-terminal extension which may be involved in the import of the protein into plant mitochondria.

Biogenesis of the mitochondrial phosphate carrier

The mitochondrial phosphate carrier (PiC) is a member of the family of inner-membrane carrier proteins which are generally synthesized without a cleavable presequence. Surprisingly, the cDNA sequences of bovine and rat PiC suggested the existence of an amino-terminal extension sequence in the precursor of PiC. By expressing PiC in vitro, we found that PiC is indeed synthesized as a larger precursor. This precursor was imported and proteolytically processed by mitochondria, whereby the correct amino-terminus of the mature protein was generated. Import of PiC showed the characteristics of mitochondrial protein uptake, such as dependence on ATP and a membrane potential and involvement of contact sites between mitochondrial outer and inner membranes. The precursor imported in vitro was correctly assembled into the functional form, demonstrating that the authentic import and assembly pathway of PiC was reconstituted when starting with the presequence-carrying precursor. These results are discussed in connection with the recently postulated role of PiC as an import receptor located in the outer membrane.

The precursor of mitochondrial aspartate aminotransferase is imported into mitochondria faster than the homologous cytosolic isoenzyme with the same presequence attached

Mitochondrial and cytosolic aspartate aminotransferase (AspAT) are homologous proteins with identically folded polypeptide chains. The cDNAs of the two isoenzymes of chicken were used to express the following proteins in yeast: the precursor of mitochondrial AspAT, mature mitochondrial AspAT, and two chimeric proteins in one of which (pc) the presequence of the precursor was attached to the entire cytosolic isoenzyme and in the other one (pmc) the N-terminal segment (amino acid residues -22 to 23) of the precursor was linked to the slightly truncated cytosolic isoenzyme (residues 34 to 412). All presequence containing proteins were imported into the mitochondria and processed to the mature form whereas mature mitochondrial AspAT remained in the cytosol. The rate of import of the authentic precursor was four times faster than that of the chimeric proteins pc and pmc, t1/2 for importation at 29 degrees C being 3, 13 and 14 min, respectively. Apparently, the mature moiety of the precursor of mitochondrial AspAT promotes importation.

MRS3 and MRS4, two suppressors of mtRNA splicing defects in yeast, are new members of the mitochondrial carrier family

When present in high copy number plasmids, the nuclear genes MRS3 and MRS4 from Saccharomyces cerevisiae can suppress the mitochondrial RNA splicing defects of several mit- intron mutations. Both genes code for closely related proteins of about Mr 32,000; they are 73% identical. Sequence comparisons indicate that MRS3 and MRS4 may be related to the family of mitochondrial carrier proteins. Support for this notion comes from a structural analysis of these proteins. Like the ADP/ATP carrier protein (AAC), the mitochondrial phosphate carrier protein (PiC) and the uncoupling protein (UCP), the two MRS proteins have a tripartite structure; each of the three repeats consists of two hydrophobic domains that are flanked by specific amino acid residues. The spacing of these specific residues is identical in all domains of all proteins of the family, whereas spacing between the hydrophobic domains is variable. Like the AAC protein, the MRS3 and MRS4 proteins are imported into mitochondria in vitro and without proteolytic cleavage of a presequence and they are located in the inner mitochondrial membrane. In vivo studies support this mitochondrial localization of the MRS proteins. Overexpression of the MRS3 and MRS4 proteins causes a temperature-dependent petite phenotype; this is consistent with a mitochondrial function of these proteins. Disruption of these genes affected neither mitochondrial functions nor cellular viability. Their products thus have no essential function for mitochondrial biogenesis or for whole yeast cells that could not be taken over by other gene products. The findings are discussed in relation to possible functions of the MRS proteins in mitochondrial solute translocation and RNA splicing.

Precursor proteins in transit through mitochondrial contact sites interact with hsp70 in the matrix

We previously reported that hsp70 in the mitochondrial matrix (mt-hsp 70 = Ssclp) is required for import of precursor proteins destined for the matrix or intermembrane space. Here we show that mt-hsp70 is also needed for the import of mitochondrial inner membrane proteins. In particular, the precursor of ADP/ATP carrier that is known not to interact with hsp60 on its assembly pathway requires functional mt-hsp70 for import, suggesting a general role of mt-hsp70 in membrane translocation of precursors. Moreover, a precursor arrested in contact sites was specifically co-precipitated with antibodies directed against mt-hsp70. We conclude that mt-hsp70 is directly involved in the translocation of many, if not all, precursor proteins that are transported across the inner membrane.

Isolation of cDNAs encoding subunit VIb of cytochrome c oxidase and steady-state levels of coxVIb mRNA in different tissues

A full-length cDNA clone specifying the nuclear-encoded subunit VIb of human cytochrome c oxidase (COX) was isolated from a human skeletal muscle cDNA expression library. This was done with antiserum directed against the group of subunits VIa, b and c of bovine heart COX. A potential ribosome-binding site was located immediately upstream from the initiation codon. The predicted amino acid sequence revealed 85% similarity with the corresponding subunit of bovine heart COX. Subunit VIb lacks a cleavable presequence for mitochondrial addressing. We assume that there are no tissue-specific isoforms of subunit VIb, since (i) in a Northern blot experiment a single hybridizing band of approx. 500 nucleotides was demonstrated in RNA from liver, skeletal muscle, MOLT-4 cells and fibroblasts and (ii) a full-length cDNA clone with an identical sequence was isolated from a human liver cDNA library. Steady-state levels of the coxVIb transcript were different in the tissues examined.

A mitochondrial import receptor for the ADP/ATP carrier

We have identified a mitochondrial outer membrane protein of 72 kd (MOM72) that exhibits the properties of an import receptor for the ADP/ATP carrier (AAC), the most abundant mitochondrial protein. Monospecific antibodies and Fab fragments against MOM72 selectively inhibit import of AAC at the level of specific binding to the mitochondria. AAC bound to the mitochondrial surface is coprecipitated with antibodies against MOM72 after lysis of mitochondria with detergent. MOM72 thus has a complementary function to that of MOM19, which acts as an import receptor for the majority of mitochondrial proteins studied so far but not for the AAC. The import pathway of the precursor of MOM72 appears to involve MOM19 as receptor.

Antibodies to ADP-ATP carrier--an autoantigen in myocarditis and dilated cardiomyopathy--impair cardiac function

The adenosine diphosphate (ADP)-adenosine triphosphate (ATP) carrier of the inner mitochondrial membrane is identified as an autoantigen in myocarditis and dilated cardiomyopathy. Sera of patients with these diseases contain autoantibodies to the ADP-ATP carrier capable of inhibiting nucleotide transport in vitro. Recently, an antibody-related infringement of energy metabolism was shown in intact perfused hearts isolated from guinea pigs immunized with the ADP-ATP carrier. A decreased cytosolic-mitochondrial difference of the phosphorylation potential of ATP was measured that originated from a reduction in mitochondrial-cytosolic nucleotide transport. Nonimmunized animals did not show these changes in energy metabolism, despite being in a comparable metabolic state and performing equal external heart work. To establish whether antibodies to the ADP-ATP carrier can also alter cardiac function, hemodynamic parameters of isolated hearts of guinea pigs that were preimmunized with the carrier protein were measured. Cardiac metabolism was stimulated by exposing the hearts to a high calcium concentration in conjunction with a maximum elevation of the afterload. Mean aortic pressure, stroke volume, stroke work, and external heart work were found to be lowered significantly (p less than 0.005). The external heart work of the immunized hearts reached only about 20% of the level performed by control hearts. Myocardial oxygen consumption was lowered 2.5-fold, whereas the extent of lactate production was found to be more than doubled. These results show a diminished cardiac performance of hearts from animals immunized with the ADP-ATP carrier. Our findings demonstrate that autoimmunity to the ADP-ATP carrier may contribute to the pathophysiology of dilated cardiomyopathy as a subsequent stage of myocarditis by causing an autoantibody-mediated reduction in cardiac function on the basis of an imbalance between energy delivery and demand.

Protein sorting to mitochondria: evolutionary conservations of folding and assembly

According to the endosymbiont hypothesis, mitochondria have lost the autonomy of their prokaryotic ancestors. They have to import most of their proteins from the cytosol because the mitochondrial genome codes for only a small percentage of the polypeptides that reside in the organelle. Recent findings show that the sorting of proteins into the mitochondrial subcompartments and their folding and assembly follow principles already developed in prokaryotes. The components involved may have structural and functional equivalents in bacteria.

Antibodies to the ADP/ATP carrier, an autoantigen in myocarditis and dilated cardiomyopathy, penetrate into myocardial cells and disturb energy metabolism in vivo

We identified the ADP/ATP carrier, located within the inner mitochondrial membrane, to be an organ- and conformation-specific autoantigen in myocarditis and dilated cardiomyopathy. We also showed that autoantibodies to the ADP/ATP carrier inhibit the nucleotide transport in vitro. Specific binding of the autoantibodies to the carrier was demonstrated by radioimmunoassay and the immunoblot technique; the inhibition of the nucleotide transport was determined by the inhibitor stop method. To establish if these autoantibodies might also affect cardiac energy metabolism in vivo, we measured whether they are capable of penetrating into myocytes and whether subcellular ATP/ADP ratios and phosphorylation potentials of ATP change in hearts of guinea pigs that have been immunized with the isolated ADP/ATP carrier. An intracellular deposition of autoantibodies was observed by direct immunofluorescence and by immunoperoxidase staining on cryosections of the myocardial tissue of animals immunized with the ADP/ATP carrier. Furthermore, binding of autoantibodies to mitochondrial membrane structures was shown by immunoelectron-microscopic methods. The cytosolic and intramitochondrial distribution of adenine nucleotides in stimulated, isolated perfused hearts of guinea pigs immunized with the ADP/ATP carrier was measured by nonaqueous fractionation. Compared with controls performing equal external heart work, the cytosolic ATP decreased in the immunized animals, whereas the mitochondrial ATP increased strongly; ADP concentrations showed an opposite change. Thus, a resultant cytosolic decrease and a marked mitochondrial increase of the ATP/ADP ratio was established. As a consequence, the cytosolic-mitochondrial phosphorylation potential of ATP was diminished. These findings demonstrate that antibodies against intracellular antigens are able to penetrate into living cells, and that autoimmunity to the ADP/ATP carrier may contribute to the pathophysiology of myocarditis and dilated cardiomyopathy by causing an autoantibody-mediated imbalance between intracellular energy delivery and demand.

Mitochondrial protein import

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

Molecular cloning of a cDNA for rat liver monoamine oxidase B

The cDNA for rat monoamine oxidase B mRNA was isolated from liver cDNA library in lambda gt11 using specific antibody and oligonucleotide probes derived from FAD-containing peptide of the enzyme. The primary structure of the protein, deduced from the nucleotide sequence, consisted of 520 amino acid residues and its molecular weight was calculated to be 58.4 kD which is in good agreement with that of the in vitro-synthesized peptide. FAD-binding site is located in the carboxy-terminal region. There is no typical structural feature common to the targeting signals for mitochondria, the periodic distribution of basic amino acids spaced by several uncharged residues, at its amino-terminal region. This region has an uninterrupted stretch of 14 hydrophobic residues.

Cloning and sequencing of a cDNA for human mitochondrial ubiquinone-binding protein of complex III

The ubiquinone-binding protein (QP-C) is a nuclear-encoded component of ubiquinol-cytochrome c oxidoreductase in the mitochondrial respiratory chain and plays an important role in electron transfer as a ubiquinone-QP-C complex. We obtained a partial cDNA for rat liver QP-C by screening a lambda gt11 rat liver cDNA library using antiserum directed against bovine heart QP-C. Using this cDNA as a probe, a cDNA clone was isolated from a human fibroblast cDNA library by colony hybridization. The total length of the cloned cDNA was 518 base pairs with an open reading frame of 333 base pairs. The 111-amino acid sequence deduced from the nucleotide sequence of the cDNA is 85% homologous to that of bovine QP-C and contains only a single additional amino-terminal methionine. This implies that the human QP-C is synthesized without a presequence which is required for import of most nuclear-encoded mitochondrial proteins into mitochondria.

Identification of peroxisomal targeting signals located at the carboxy terminus of four peroxisomal proteins

As part of an effort to understand how proteins are imported into the peroxisome, we have sought to identify the peroxisomal targeting signals in four unrelated peroxisomal proteins: human catalase, rat hydratase:dehydrogenase, pig D-amino acid oxidase, and rat acyl-CoA oxidase. Using gene fusion experiments, we have identified a region of each protein that can direct heterologous proteins to peroxisomes. In each case, the peroxisomal targeting signal is contained at or near the carboxy terminus of the protein. For catalase, the peroxisomal targeting signal is located within the COOH-terminal 27 amino acids of the protein. For hydratase:dehydrogenase, D-amino acid oxidase, and acyl-CoA oxidase, the targeting signals are located within the carboxy-terminal 15, 14, and 15 amino acids, respectively. A tripeptide of the sequence Ser-Lys/His-Leu is present in each of these targeting signals as well as in the peroxisomal targeting signal identified in firefly luciferase (Gould, S.J., G.-A. Keller, and S. Subramani. 1987. J. Cell Biol. 105:2923-2931). When the peroxisomal targeting signal of the hydratase:dehydrogenase is mutated so that the Ser-Lys-Leu tripeptide is converted to Ser-Asn-Leu, it can no longer direct proteins to peroxisomes. We suggest that this tripeptide is an essential element of at least one class of peroxisomal targeting signals.

Topogenesis of mitochondrial inner membrane uncoupling protein. Rerouting transmembrane segments to the soluble matrix compartment

Brown adipose tissue uncoupling protein (UCP), an integral polytopic protein of the mitochondrial inner membrane, is composed of at least six transmembrane segments whose net hydrophobic character derives from paired amphiphilic helices. The protein is synthesized in the cytoplasm as a polypeptide (307 amino acids) lacking a cleavable targeting (signal) peptide. Deletion mutagenesis and fusion protein constructions revealed the existence of at least two import signals: one lying between UCP precursor amino acids 13-105 and the other downstream of position 101. The former resulted in both targeting and membrane insertion of a fusion protein, whereas the latter targeted UCP 102-307 into the organelle but failed to result in membrane insertion. When a strong matrix-targeting signal derived from precarbamoyl phosphate synthetase was fused to UCP amino acids 169-307 or 52-307 (containing three and five transmembrane domains, respectively), the fusion proteins were efficiently imported to the soluble matrix compartment where correct signal cleavage took place. We suggest that assembly of UCP into the inner membrane follows a coordinate insertion pathway for integration and may use more than one signal sequence to achieve this. In this respect, it might share certain mechanistic features with the insertion of polytopic proteins into the endoplasmic reticulum. The data also suggest, however, that integration of the amino-terminal third of UCP into the inner membrane may be required to help or enhance insertion of the remaining UCP transmembrane domains.

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.

The yeast F1-ATPase beta subunit precursor contains functionally redundant mitochondrial protein import information

The NH2 terminus of the yeast F1-ATPase beta subunit precursor directs the import of this protein into mitochondria. To define the functionally important components of this import signal, oligonucleotide-directed mutagenesis was used to introduce a series of deletion and missense mutations into the gene encoding the F1-beta subunit precursor. Among these mutations were three nonoverlapping deletions, two within the 19-amino-acid presequence (delta 5-12 and delta 16-19) and one within the mature protein (delta 28-34). Characterization of the mitochondrial import properties of various mutant F1-beta subunit proteins containing different combinations of these deletions showed that import was blocked only when all three deletions were combined. Mutant proteins containing all possible single and pairwise combinations of these deletions were found to retain the ability to direct mitochondrial import of the F1-beta subunit. These data suggest that the F1-beta subunit contains redundant import information at its NH2 terminus. In fact, we found that deletion of the entire F1-beta subunit presequence did not prevent import, indicating that a functional mitochondrial import signal is present near the NH2 terminus of the mature protein. Furthermore, by analyzing mitochondrial import of the various mutant proteins in [rho-] yeast, we obtained evidence that different segments of the F1-beta subunit import signal may act in an additive or cooperative manner to optimize the import properties of this protein.

cDNA sequence of a human skeletal muscle ADP/ATP translocator: lack of a leader peptide, divergence from a fibroblast translocator cDNA, and coevolution with mitochondrial DNA genes

We have characterized a 1400-nucleotide cDNA for the human skeletal muscle ADP/ATP translocator. The deduced amino acid sequence is 94% homologous to the beef heart ADP/ATP translocator protein and contains only a single additional amino-terminal methionine. This implies that the human translocator lacks an amino-terminal targeting peptide, a conclusion substantiated by measuring the molecular weight of the protein synthesized in vitro. A 1400-nucleotide transcript encoding the skeletal muscle translocator was detected on blots of total RNA from human heart, kidney, skeletal muscle, and HeLa cells by hybridization with oligonucleotide probes homologous to the coding region and 3' noncoding region of the cDNA. However, the level of this mRNA varied substantially among tissues. Comparison of our skeletal muscle translocator sequence with that of a recently published human fibroblast translocator cognate revealed that the two proteins are 88% identical and diverged about 275 million years ago. Hence, tissues vary both in the level of expression of individual translocator genes and in differential expression of cognate translocator genes. Comparison of the base substitution rates of the ADP/ATP translocator and the oxidative phosphorylation genes encoded by mitochondrial DNA revealed that the mitochondrial DNA genes fix 10 times more synonymous substitutions and 12 times more replacement substitutions; yet, these nuclear and cytoplasmic respiration genes experience comparable evolutionary constraints. This suggests that the mitochondrial DNA genes are highly prone to deleterious mutations.

The yeast F1-ATPase beta subunit precursor contains functionally redundant mitochondrial protein import information

The NH2 terminus of the yeast F1-ATPase beta subunit precursor directs the import of this protein into mitochondria. To define the functionally important components of this import signal, oligonucleotide-directed mutagenesis was used to introduce a series of deletion and missense mutations into the gene encoding the F1-beta subunit precursor. Among these mutations were three nonoverlapping deletions, two within the 19-amino-acid presequence (delta 5-12 and delta 16-19) and one within the mature protein (delta 28-34). Characterization of the mitochondrial import properties of various mutant F1-beta subunit proteins containing different combinations of these deletions showed that import was blocked only when all three deletions were combined. Mutant proteins containing all possible single and pairwise combinations of these deletions were found to retain the ability to direct mitochondrial import of the F1-beta subunit. These data suggest that the F1-beta subunit contains redundant import information at its NH2 terminus. In fact, we found that deletion of the entire F1-beta subunit presequence did not prevent import, indicating that a functional mitochondrial import signal is present near the NH2 terminus of the mature protein. Furthermore, by analyzing mitochondrial import of the various mutant proteins in [rho-] yeast, we obtained evidence that different segments of the F1-beta subunit import signal may act in an additive or cooperative manner to optimize the import properties of this protein.

Mitochondrial porin of Neurospora crassa: cDNA cloning, in vitro expression and import into mitochondria

cDNA encoding porin of Neurospora crassa, the major protein component of the outer mitochondrial membrane, was isolated and the nucleotide sequence was determined. The deduced protein sequence consists of 283 amino acids (29,979 daltons) and shows sequence homology of around 43% to yeast porin; however, no significant homology to bacterial porins was apparent. According to secondary structure predictions, mitochondrial porin consists mainly of membrane-spanning sided beta-sheets. Porin was efficiently synthesized in vitro from the cDNA; this allowed us to study in detail its import into mitochondria. Thereby, three characteristics of import were defined: (i) import depended on the presence of nucleoside triphosphates; (ii) involvement of a proteinaceous receptor-like component on the surface of the mitochondria was demonstrated; (iii) insertion into the outer membrane was resolved into at least two distinct steps: specific binding to high-affinity sites and subsequent assembly to the mature form.

Import of honeybee prepromelittin into the endoplasmic reticulum: structural basis for independence of SRP and docking protein

Honeybee prepromelittin is correctly processed and imported by dog pancreas microsomes. Insertion of prepromelittin into microsomal membranes, as assayed by signal sequence removal, does not depend on signal recognition particle (SRP) and docking protein. We addressed the question as to how prepromelittin bypasses the SRP/docking protein system. Hybrid proteins between prepromelittin, or carboxy-terminally truncated derivatives, and the cytoplasmic protein dihydrofolate reductase from mouse were constructed. These hybrid proteins were analysed for membrane insertion and sequestration into microsomes. The results suggest the following: (i) The signal sequence of prepromelittin is capable of interacting with the SRP/docking protein system, but this interaction is not mandatory for membrane insertion; this is related to the small size of prepromelittin. (ii) In prepromelittin a cluster of negatively charged amino acids must be balanced by a cluster of positively charged amino acids in order to allow membrane insertion. (iii) In general, a signal sequence can be sufficient to mediate membrane insertion independently of SRP and docking protein in the case of short precursor proteins; however, the presence and distribution of charged amino acids within the mature part of these precursors can play distinct roles.

Characterization of translocation contact sites involved in the import of mitochondrial proteins

Import of proteins into the mitochondrial matrix requires translocation across two membranes. Translocational intermediates of mitochondrial proteins, which span the outer and inner membrane simultaneously and thus suggest that translocation occurs in one step, have recently been described (Schleyer, M., and W. Neupert, 1985, Cell, 43:339-350). In this study we present evidence that distinct membrane areas are involved in the translocation process. Mitochondria that had lost most of their outer membrane by digitonin treatment (mitoplasts) still had the ability to import proteins. Import depended on proteinaceous structures of the residual outer membrane and on a factor that is located between the outer and inner membranes and that could be extracted with detergent plus salt. Translocational intermediates, which had been preformed before fractionation, remained with the mitoplasts under conditions where most of the outer membrane was subsequently removed. Submitochondrial vesicles were isolated in which translocational intermediates were enriched. Immunocytochemical studies also suggested that the translocational intermediates are located in areas where outer and inner membranes are in close proximity. We conclude that the membrane-potential-dependent import of precursor proteins involves translocation contact sites where the two membranes are closely apposed and are linked in a stable manner.

Construction of cDNA libraries by blunt-end ligation: high-frequency cloning of long cDNAs from filamentous fungi

A simplified cDNA synthesis and cloning method, suitable for efficient generation of cDNA libraries at frequencies up to 10(6) clones/micrograms mRNA, is described. Routine synthesis of transcripts of well over 4 kb is facilitated by the use of high-quality RNA template isolated from materials rich in RNases. Laborious cloning steps, like tailing or addition of linkers, can be omitted by the use of efficient blunt-end ligation to plasmid vectors, and rapid verification as well as characterization of the clones is possible by double-stranded plasmid sequencing. Using this method we have constructed several cDNA libraries of different filamentous fungi and show here the synthesis and cloning of cDNA copies larger than 1.8 kb corresponding to three Trichoderma reesei cellulases.

Mitochondrial protein import: nucleoside triphosphates are involved in conferring import-competence to precursors

The role of nucleoside triphosphates (NTPs) in mitochondrial protein import was investigated with the precursors of N. crassa ADP/ATP carrier, F1-ATPase subunit beta, F0-ATPase subunit 9, and fusion proteins between subunit 9 and mouse dihydrofolate reductase. NTPs were necessary for the initial interaction of precursors with the mitochondria and for the completion of translocation of precursors from the mitochondrial surface into the mitochondria. Higher levels of NTPs were required for the latter reactions as compared with the early stages of import. Import of precursors having identical presequences but different mature protein parts required different levels of NTPs. The sensitivity of precursors in reticulocyte lysate to proteases was decreased by removal of NTPs and increased by their readdition. We suggest that the hydrolysis of NTPs is involved in modulating the folding state of precursors in the cytosol, thereby conferring import competence.

Solute carriers involved in energy transfer of mitochondria form a homologous protein family

The sequences of three mitochondrial carriers involved in energy transfer, the ADP/ATP carrier, phosphate carrier and uncoupling carrier, are analyzed. Similarly to what has been previously reported for the ADP/ATP carrier and the uncoupling protein, now also the phosphate carrier is found to have a tripartite structure comprising three similar repeats of approx. 100 residues each. The three sequences show a fair overall homology with each other. More significant homologies are found by comparing the repeats within and between the carriers in a scheme where the sequences are spliced into repeats, which are arranged for maximum homology by allowing possible insertions or deletions. A striking conservation of critical residues, glycine, proline, of charged and of aromatic residues is found throughout all nine repeats. This is indicative of a similar structural principle in the repeats. Hydropathy profiles of the three proteins and a search for amphipathic alpha-spans reveal six membrane-spanning segments for each carrier, providing further support for the basic structural identity of the repeats. The proposed folding pattern of the carriers in the membrane is exemplified with the phosphate carrier. A possible tertiary arrangement of the repeats and the membrane-spanning helices is shown. The emergence of a mitochondrial carrier family by triplication and by divergent evolution from a common gene of about 100 residues is discussed.