An import signal in the cytosolic domain of the Neurospora mitochondrial outer membrane protein TOM22

TOM22 is an integral component of the preprotein translocase of the mitochondrial outer membrane (TOM complex). The protein is anchored to the lipid bilayer by a central trans-membrane segment, thereby exposing the amino-terminal domain to the cytosol and the carboxyl-terminal portion to the intermembrane space. Here, we describe the sequence requirements for the targeting and correct insertion of Neurospora TOM22 into the outer membrane. The orientation of the protein is not influenced by the charges flanking its trans-membrane segment, in contrast to observations regarding proteins of other membranes. In vitro import studies utilizing TOM22 preproteins harboring deletions or mutations in the cytosolic domain revealed that the combination of the trans-membrane segment and intermembrane space domain of TOM22 is not sufficient to direct import into the outer membrane. In contrast, a short segment of the cytosolic domain was found to be essential for the import and assembly of TOM22. This sequence, a novel internal import signal for the outer membrane, carries a net positive charge. A mutant TOM22 in which the charge of the import signal was altered to -1 was imported less efficiently than the wild-type protein. Our data indicate that TOM22 contains physically separate import and membrane anchor sequences.     

Mrs5p, an essential protein of the mitochondrial intermembrane space, affects protein import into yeast mitochondria

We have isolated a yeast nuclear gene that suppresses the previously described respiration-deficient mrs2-1 mutation when present on a multicopy plasmid. Elevated gene dosage of this new gene, termed MRS5, suppresses also the pet phenotype of a mitochondrial splicing-deficient group II intron mutation M1301. The MRS5 gene product, a 13-kDa protein of low abundance, shows no similarity to other known proteins and is associated with the inner mitochondrial membrane, protruding into the intermembrane space. MRS5 codes for an essential protein, as the disruption of this gene is lethal even during growth on fermentable carbon sources. Thus, the Mrs5 protein seems to be involved in mitochondrial key functions aside from oxidative energy conservation, which is dispensable in fermenting yeast cells. Depletion of Mrs5p in yeast cells causes accumulation of unprocessed precursors of the mitochondrial hsp60 protein and defects in all cytochrome complexes. These findings suggest an essential role of Mrs5p in mitochondrial biogenesis.     

GTP hydrolysis is essential for protein import into the mitochondrial matrix

Protein import into the innermost compartment of mitochondria (the matrix) requires a membrane potential (delta psi) across the inner membrane, as well as ATP-dependent interactions with chaperones in the matrix and cytosol. The role of nucleoside triphosphates other than ATP during import into the matrix, however, remains to be determined. Import of urea-denatured precursors does not require cytosolic chaperones. We have therefore used a purified and urea-denatured preprotein in our import assays to bypass the requirement of external ATP. Using this modified system, we demonstrate that GTP stimulates protein import into the matrix; the stimulatory effect is directly mediated by GTP hydrolysis and does not result from conversion of GTP to ATP. Both external GTP and matrix ATP are necessary; neither one can substitute for the other if efficient import is to be achieved. These results suggest a "push-pull" mechanism of import, which may be common to other post-translational translocation pathways.     

The protein import receptor MOM19 of yeast mitochondria

We have identified the protein import receptor MOM19 of Saccharomyces cerevisiae mitochondria. MOM19 is exposed on the outer membrane surface and present in the mitochondrial receptor complex. Antibodies raised against MOM19 strongly inhibited the import of preproteins into isolated yeast mitochondria. Fab fragments prepared from the antibodies showed the same inhibitory effect. By using mutant mitochondria, which lacked the second import receptor MOM72, we found that the import of preproteins via MOM19 did not require the presence of MOM72. We conclude that MOM19 is required for preprotein translocation across the yeast mitochondrial outer membrane and is able to function independently of the receptor MOM72.     

M-glycogenin, the protein moiety of Neurospora crassa proteoglycogen, is an auto- and transglucosylating enzyme

Neurospora crassa proteoglycogen was purified and its protein moiety, M-glycogenin, was released by amylolytic treatment. The released protein was capable of autoglucosylation from UDP-glucose forming glucosyl-alpha 1,4-glucosyl linkage. The kinetics of autoglucosylation suggested an intramolecular mechanism of reaction. M-glycogenin was also able to glucosylate dodecyl-beta-maltoside and autoglucosylate, simultaneously and independently. Both auto- and transglucosylation reactions were dependent on Mn2+. Thus, M-glycogenin, which has also been described as the constituent of Escherichia coli proteoglycogen (A. Goldraij and J. A. Curtino. 1993, Biochem. Mol. Biol. Int. 30, 453-458), is a glucosyltransferase that bears similar catalytic properties with mammalian glycogenin. This is the first report on the enzymatic character of the protein constituent of proteoglycogen in primitive organisms, which suggest that the mechanism for the de novo biosynthesis of glycogen was conserved over a very long period of evolution.     

Expression of a plant protein by Neurospora crassa

Heterologous expression of plant genes may serve as an important alternative for producing plant proteins. We have investigated the ability of the fungus Neurospora crassa to secrete zeamatin, a protein produced by Zea mays. Zeamatin was induced after being fused to glucoamylase, an extracellular hydrolase produced by N. crassa. Glucoamylase induction and other culture parameters were monitored in untransformed N. crassa grown in shaken liquid culture. A DNA plasmid, pGEZ, was constructed by inserting zeamatin-encoding cDNA into an expression cassette containing the promoter, a truncated open reading frame, and the terminator sequence of the N. crassa glucoamylase gene. Zeamatin-encoding cDNA was modified at the N terminus to include a kex-2 protease site, allowing cleavage of the chimeric product in the secretory pathway. Strains containing the chimeric gene construct were grown in liquid culture and induced for glucoamylase and zeamatin production. Zeamatin antibody detected a protein in a Western blot of concentrated culture supernatants that comigrated with authentic zeamatin. Secreted zeamatin was active in inhibiting the growth of Candida albicans in an agar diffusion assay, indicating that zeamatin had been correctly synthesized, processed, and secreted by N. crassa.     

Tim9p, an essential partner subunit of Tim10p for the import of mitochondrial carrier proteins

Tim10p, a protein of the yeast mitochondrial intermembrane space, was shown previously to be essential for the import of multispanning carrier proteins from the cytoplasm into the inner membrane. We now identify Tim9p, another essential component of this import pathway. Most of Tim9p is associated with Tim10p in a soluble 70 kDa complex. Tim9p and Tim10p co-purify in successive chromatographic fractionations and co-immunoprecipitated with each other. Tim9p can be cross-linked to a partly translocated carrier protein. A small fraction of Tim9p is bound to the outer face of the inner membrane in a 300 kDa complex whose other subunits include Tim54p, Tim22p, Tim12p and Tim10p. The sequence of Tim9p is 25% identical to that of Tim10p and Tim12p. A Ser67-->Cys67 mutation in Tim9p suppresses the temperature-sensitive growth defect of tim10-1 and tim12-1 mutants. Tim9p is a new subunit of the TIM machinery that guides hydrophobic inner membrane proteins across the aqueous intermembrane space.     

Mitochondrial protein import: modification of sulfhydryl groups of the inner mitochondrial membrane import machinery in Solanum tuberosum inhibits protein import

Protein import into mitochondria involves several components of the mitochondrial outer and inner membranes as well as molecular chaperones located inside mitochondria. Here, we have investigated the effect of sulfhydryl group reagents on import of the in vitro transcribed/translated precursor of the F1 beta subunit of the ATP synthase (pF1 beta) into Solanum tuberosum mitochondria. We have used a reducing agent, dithiothreitol (DTT), a membrane-permeant alkylating agent, N-ethylmaleimide (NEM), a non-permeant alkylating agent, 3-(N-maleimidopropionyl)biocytin (MPB), an SH-group specific agent and cross-linker 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) as well as an oxidizing cross-linker, copper sulfate. DTT stimulated the mitochondrial protein import, whereas NEM, MPB, DTNB and Cu2+ were inhibitory. Inhibition by Cu2+ could be reversed by addition of DTT. The efficiency of inhibition was higher in energized mitochondrial than in non-energized. We have dissected the effect of the SH-group reagents on binding, unfolding and transport of the precursor into mitochondria. Our results demonstrated that the inhibitory effect of NEM, DTNB and Cu2+ on the efficiency of import was not due to the interaction of the SH-group reagents with import receptors. Modification of pF1 beta with NEM prior to the import resulted in stimulation of import, whereas DTNB and Cu2+ were inhibitory. NEM, MPB, DTNB and Cu2+ inhibited import of the NEM-modified pF1 beta into intact mitochondria. Import of pF1 beta through a receptor-independent bypass-route as well as import into mitoplasts were sensitive to DTT, NEM, MPB, DTNB and Cu2+ in a similar manner as import into mitochondria. As MPB does not cross the inner membrane, these results indicated that redox and conformational status of SH groups located on the outer surface of the inner mitochondrial membrane were essential for protein import.     

Purification and characterization of the catalytic subunit of protein phosphatase 1 from Neurospora crassa

Protein phosphorylation is a universal regulatory mechanism in eukaryotic cells. The phosphorylation state of proteins is affected by the antagonistic activities of protein kinases and phosphatases. Protein phosphatases (PPs) can be classified as serine/threonine and tyrosine specific phosphatases. Ser/Thr phosphatases are divided into four subclasses (PP1, PP2A, PP2B, PP2C) on the basis of their substrate specificity, metal ion dependence and inhibitor sensitivity. We were able to detect the activities of all four Ser/Thr protein phosphatases in the mycelial extract of Neurospora crassa. The catalytic subunit of PP1 was purified 1500-fold with a yield of 1.3% using ammonium sulfate-ethanol precipitation, DEAE-Sephacel, heparin-Sepharose and MonoQ FPLC chromatography. The protein product was nearly homogenous, as judged by SDS-polyacrylamide gel electrophoresis. The most important properties of the enzyme were the following: /1/ its molecular mass proved to be 35 kD, /2/ it was completely inhibited by inhibitor-2, microcystin and okadaic acid, /3/ it was bound to heparin-Sepharose, and /4/ its specific activity was 2000 mU/mg. These biochemical properties are very similar to those of the homologous enzyme from rabbit muscle and indicate a high level of conservation of PP1 structure during evolution.     

A component of the chloroplast protein import apparatus functions in bacteria

Toc36 is a family of 44-kDa envelope polypeptides previously identified as components of the chloroplast protein import apparatus by virtue of their close physical proximity to translocating proteins. An indication of their function thus remains at large. A heterologous in vivo approach for studying the function of Toc36 was developed in this study by introducing a member of Toc36 into E. coli to assess its effect on bacterial protein translocation. The presence of Toc36 enhances the translocation of two bacterial periplasmic proteins in a manner resembling the chloroplast system. Translocation of the two bacterial periplasmic proteins was less sensitive to sodium azide, resembling more the azide-insensitive nature of the chloroplast protein import process. Mutated Toc36 proteins were not capable of causing the same effect as that observed for unaltered Toc36. Toc36 was also capable of complementing bacterial strains with temperature-sensitive secA mutations that affected protein translocation. The combined results provide evidence that Toc36 plays a central role in the chloroplast protein translocation process.     

Active unfolding of precursor proteins during mitochondrial protein import

Precursor proteins made in the cytoplasm must be in an unfolded conformation during import into mitochondria. Some precursor proteins have tightly folded domains but are imported faster than they unfold spontaneously, implying that mitochondria can unfold proteins. We measured the import rates of artificial precursors containing presequences of varying length fused to either mouse dihydrofolate reductase or bacterial barnase, and found that unfolding of a precursor at the mitochondrial surface is dramatically accelerated when its presequence is long enough to span both membranes and to interact with mhsp70 in the mitochondrial matrix. If the presequence is too short, import is slow but can be strongly accelerated by urea-induced unfolding, suggesting that import of these 'short' precursors is limited by spontaneous unfolding at the mitochondrial surface. With precursors that have sufficiently long presequences, unfolding by the inner membrane import machinery can be orders of magnitude faster than spontaneous unfolding, suggesting that mhsp70 can act as an ATP-driven force-generating motor during protein import.     

Functional analysis of different regions of the positive-acting CYS3 regulatory protein of Neurospora crassa

In the filamentous fungus Neurospora crassa during conditions of sulfur limitation, CYS3, a major positive-acting regulatory protein, turns on the expression of an entire set of genes which encode permeases and enzymes involved in the acquisition of sulfur from environmental sources. CYS3 functions as a homodimeric protein and possesses a b-Zip domain that confers sequence-specific DNA binding. Expression of various hybrid GAL4-CYS3 fusion proteins in yeast was used to detect regions involved in gene activation. An amino-terminal serine/threonine-rich domain of CYS3 alone strongly activated expression of beta-galactosidase, the yeast reporter. Moreover, mutant CYS3 proteins with amino-acid substitutions in this region that showed increased expression in Neurospora also displayed an enhanced activation potential in yeast. The cys-3 gene of the exotic N. crassa Mauriceville strain and of N. intermedia were cloned and demonstrated to be functional for gene activation and for sulfur-mediated regulation by complementation of a loss-of-function cys-3 mutation. The amino-terminal serine/threonine-rich region is highly conserved in these two CYS3 proteins, in agreement with the possibility that it serves as the activation domain. Surprisingly, an extended promoter region of the cys-3 gene in the Mauriceville strain and in N. intermedia was very well conserved with that of the standard N. crassa gene, including the presence of three CYS3-binding sites possibly involved in autogenous control. Results are presented which indicate that synthesis of the CYS3 regulatory protein is highly regulated and can be detected in the nucleus of cells subjected to sulfur de-repression, but is not found in the nucleus or the cytoplasm of S-repressed cells. The amino-acid substitutions of the CYS3 protein present in a temperature-sensitive cys-3 mutant and in a second-site revertant of a cys-3 null mutation are presented and are shown to affect their DNA-binding activities.     

Genetic analysis of signal transduction through light-induced protein phosphorylation in Neurospora crassa perithecia

We have previously demonstrated that blue light induces the phosphorylation of a 15-kDa protein in crude membrane fractions of Neurospora crassa mycelia. Here we report the isolation and characterization of a mutant (psp; phosphorylation of small proteins that is completely defective for phosphorylation of that protein, as assayed in both crude membrane and soluble fractions. This mutation defines a unique locus that maps to linkage group VR between al-3 and his-6. To elucidate the photobiological significance of the phosphorylation of the protein, we analyzed known photobiological phenomena and discovered that the positioning of beaks on the perithecia, defined as perithecial polarity, was light-dependent in the wild type. In the psp mutant, beaks were phototropic as in the wild type, but their position was random. In a wc-l mutant, however, beaks were positioned at random and were not phototropic. Thus light-induced perithecial polarity and phototropism of perithecial beaks are controlled differently. A psp; wc-l double mutant showed the same phenotype as that of wc-l with respect to these two photomorphogenetic characters. These results indicate that the wc-l gene is epistatic to psp in the light-signal transduction pathway that controls both phototropism and perithecial polarity.     

Determination of the transmembrane topology of yeast Sec61p, an essential component of the endoplasmic reticulum translocation complex

Sec61p is a highly conserved integral membrane protein that plays a role in the formation of a protein-conducting channel required for the translocation of polypeptides into, and across, the membrane of the endoplasmic reticulum. As a major step toward elucidating the structure of the endoplasmic reticulum translocation apparatus, we have determined the transmembrane topology of Sec61p using a combination of C-terminal reporter-domain fusions and the in situ digestion of specifically inserted factor Xa protease cleavage sites. Our data indicate the presence of 10 transmembrane domains, including several with surprisingly limited hydrophobicity. Furthermore, we provide evidence for complex intramolecular interactions in which these weakly hydrophobic domains require C-terminal sequences for their correct topogenesis. The incorporation of sequences with limited hydrophobicity into the bilayer may play a vital role in the formation of an aqueous membrane channel required for the translocation of hydrophilic polypeptide chains.     

Isolation and characterization of a copper containing protein from blue cell walls of Neurospora crassa

Culturing Neurospora crassa in presence of toxic amounts of copper (0.63 mM) resulted in blue coloured mycelia and cell walls. Significant amounts (approximately 45%) of total mycelial copper were associated with cell wall isolates under conditions of copper toxicity. Hence, such blue cell walls were analysed to identify specific ligands involved in copper binding. While decuprification of the blue cell walls with 8-hydroxy quinoline (8 HQ) did not alter their copper binding abilities, similar treatment with EDTA (10 mM) decreased such abilities indicating that EDTA treatment lead to loss of copper binding ligands from cell walls. Treatment of blue cell walls with 8 HQ followed by EDTA resulted in the solubilization of a copper binding protein (relative MW approximately 14 kDa) which was associated with phosphate and carbohydrate moieties. On amino acid analysis, this protein was found to be devoid of free thiol groupings but enriched in acidic and basic amino acids, distinguishing it from classical intracellular metal binding proteins such as metallo-thioneins and phytochelatins that are inducively synthesized under conditions of metal toxicity. The biological significance of the isolated wall-bound copper binding protein, which appears to be a normal constituent of cell walls, is discussed in relation to cytoplasmic metal binding proteins and mechanism(s) adapted by fungi in countering metal toxicity.     

Impairment of calcineurin function in Neurospora crassa reveals its essential role in hyphal growth, morphology and maintenance of the apical Ca2+ gradient

The function of Neurospora crassa calcineurin was investigated in N. crassa strains transformed with a construct that provides for the inducible expression of antisense RNA for the catalytic subunit of calcineurin (cna-1). Induction of antisense RNA expression was associated with reduced levels of cna-1 mRNA and of immunodetectable CNA1 protein and decreased calcineurin enzyme activity, indicating that a conditional reduction of the target function had been achieved in antisense transformants with multiple construct integrations. Induction conditions caused growth arrest which indicated that the cna-1 gene is essential for growth of N. crassa. Growth arrest was preceded by an increase in hyphal branching, changes in hyphal morphology and concomitant loss of the distinctive tip-high Ca2+ gradient typical for growing wild-type hyphae. This demonstrates a novel and specific role for calcineurin in the precise regulation of apical growth, a common form of cellular proliferation. In vitro inhibition of N. crassa calcineurin by the complex of cyclosporin A (CsA) and cyclophilin20, and increased sensitivity of the induced transformants to the calcineurin-specific drugs CsA and FK506 imply that the drugs act in N. crassa, as in T-cells and Saccharomyces cerevisiae, by inactivating calcineurin. The finding that exposure of growing wild-type mycelium to these drugs leads to a phenotype very similar to that of the cna-1 antisense mutants is consistent with this idea.     

Identification of the protein import components of the rat mitochondrial inner membrane, rTIM17, rTIM23, and rTIM44

A large number of pharmaceutically active compounds have a high affinity to acidic phospholipids; good examples are the cationic compounds lidocaine, propranolol, and gentamycin. These drugs influenced the lipid dynamics of liposomes composed of phosphatidylcholine and the acidic phosphatidylglycerol, as judged by the excimer/monomer emission intensity ratio for a pyrene-labeled phospholipid analog, as well as by polarization of DPH fluorescence. When the mole fraction X of PG (XPG) was 0.20, lidocaine increased membrane fluidity. The opposite was true for propranolol, which caused the formation of pyrene lipid-enriched microdomains. Gentamycin had no apparent effect. At XPG = 1.00, all these drugs rigidified membrane. Subsequently, we investigated the detachment of a cationic peripheral membrane protein, cytochrome c (cyt c), by these compounds from liposomes. This was accomplished by monitoring resonance energy transfer from a pyrene-labeled phospholipid to the heme of cyt c. The efficiency of the above compounds to dissociate cyt c varied considerably. In brief, significantly lower concentrations of gentamycin than propranolol or lidocaine were required for half-maximal dissociation of cyt c from liposomes, although the final extent of protein detachment by gentamycin was less complete. ATP augmented the dissociation of cyt c from membranes by lidocaine and propranolol. Stopped-flow measurements also revealed that the half-times differed for the release of cyt c from the membranes. Our results are likely to reflect differences in the contributions of the electrostatic interactions and hydrophobicity to the drug/lipid interaction and comply with two different acidic phospholipid binding sites in cyt c.     

Antioxidant function of the mitochondrial protein SP-22 in the cardiovascular system

The mitochondrial protein SP-22 has recently been reported to be a member of the thioredoxin-dependent peroxide reductase family, suggesting that it may be one of the antioxidant systems in mitochondria, which are the major site of reactive oxygen intermediate generation. The aim of this study was to examine whether SP-22 is involved in mitochondrial antioxidant mechanisms and whether its expression is induced by oxidative stresses, particularly those in mitochondria. The expression of SP-22 protein was enhanced by about 1.5-4.6-fold when bovine aortic endothelial cells (BAEC) were exposed to various oxidative stresses, including mitochondrial respiratory inhibitors which increased the superoxide generation in BAEC mitochondria. The expression of SP-22 mRNA increased 2.0-3.5-fold with a peak at 3-6 h after exposure to Fe2+/dithiothreitol or a respiratory inhibitor, antimycin A. BAEC with an increased level of SP-22 protein caused by pretreatment with mild oxidative stress became tolerant to subsequent intense oxidative stress. On the other hand, BAEC that had been depleted of SP-22 with an antisense oligodeoxynucleotide against SP-22 mRNA became more labile to oxidative stress than control BAEC. The induction of SP-22 protein by oxidative stress in vivo was demonstrated in an experimental model of myocardial infarction in rat heart. These findings indicate that SP-22 functions as an antioxidant in mitochondria of the cardiovascular system.