How finicky is mitochondrial protein import?

The human mitochondrial import receptor, hTom20p, prevents a cryptic matrix targeting sequence from gaining access to the protein translocation machinery

Yeast Mas70p and NADH cytochrome b5 reductase are bitopic integral proteins of the mitochondrial outer membrane and are inserted into the lipid-bilayer in an Nin-Ccyto orientation via an NH2-terminal signal-anchor sequence. The signal anchor of both proteins is comprised of a short, positively charged domain followed by the predicted transmembrane segment. The positively charged domain is capable of functioning independently as a matrix-targeting signal in yeast mitochondria in vitro but does not support import into mammalian mitochondria (rat or human). Rather, this domain represents a cryptic signal that can direct import into mammalian mitochondria only if proximal components of the outer membrane import machinery are removed. This can be accomplished either by treating the surface of the intact mitochondria with trypsin or by generating mitoplasts. The import receptor Tom20p (Mas20p/MOM19) is responsible for excluding the cryptic matrix-targeting signal from mammalian mitochondria since replacement of yeast Tom20p with the human receptor confers this property to the yeast organelle while at the same time maintaining import of other proteins. In addition to contributing to positive recognition of precursor proteins, therefore, the results suggest that hTom20p may also have the ability to screen potential matrix-targeting sequences and exclude certain proteins that would otherwise be recognized and imported by distal components of the outer and inner membrane protein-translocation machinery. These findings also indicate, however, that cryptic signals, if they exist within otherwise native precursor proteins, may remain topogenically silent until the precursor successfully clears hTom20p, at which time the activity of the cryptic signal is manifested and can contribute to subsequent translocation and sorting of the polypeptide.

Functional analysis of a recently originating, atypical presequence: mitochondrial import and processing of GUS fusion proteins in transgenic tobacco and yeast

A gene family of at least five members encodes the tobacco mitochondrial Rieske Fe-S protein (RISP). To determine whether all five RISPs are translocated to mitochondria, fusion proteins containing the putative presequences of tobacco RISPs and Escherichia coli beta-glucuronidase (GUS) were expressed in transgenic tobacco, and the resultant GUS proteins were localized by cell fractionation. The amino-terminal 75 and 71 residues of RISP2 and RISP3, respectively, directed GUS import into mitochondria, where fusion protein processing occurred. The amino-terminal sequence of RISP4, which contains an atypical mitochondrial presequence, can translocate the GUS protein specifically into tobacco mitochondria with apparently low efficiency. Consistent with the proposal of a conserved mechanism for protein import in plants and fungi, the tobacco RISP3 and RISP4 presequences can direct import and processing of a GUS fusion protein in yeast mitochondria. Plant presequences, however, direct mitochondrial import in yeast less efficiently than the yeast presequence, indicating subtle differences between the plant and yeast mitochondrial import machineries. Our studies show that import of RISP4 may not require positively charged amino acid residues and an amphipathic secondary structure; however, these structural properties may improve the efficiency of mitochondrial import.

Identification of the human mitochondrial protein import receptor, huMas20p. Complementation of delta mas20 in yeast

The human homolog of the S. cerevisiaelN. crassa mitochondrial protein import receptor, Mas20p/MOM19, has been identified and characterized. Sequence similarities between these three proteins is most pronounced within the NH2-terminal third of the molecules. However, the mammalian protein exhibits only weak homology to the tetratricopeptide repeat B domain that is found in Mas20p/MOM19. huMas20p is targeted and inserted into the outer membrane of isolated rat heart mitochondria, in the Nin-Ccyto orientation. Antibodies directed against the soluble portion of huMas20p inhibited in vitro mitochondrial import of a diverse set of precursor proteins (including inner membrane uncoupling protein), but failed to block import of a fusion protein bearing the signal-anchor sequence of Mas20p itself. Finally, expression of huMAS20 complemented the respiratory defect of delta mas20 yeast cells. Together, these results demonstrate that huMAS20p is a component of the mammalian import apparatus.

Mitochondrial transport of mitoribosomal proteins, YmL8 and YmL20, in Saccharomyces cerevisiae

Two mitochondrial ribosomal (mitoribosomal) proteins, YmL8 and YmL20, of the yeast Saccharomyces cerevisiae and their derivatives were synthesized in vitro and their transport into isolated yeast mitochondria was examined. Of the two proteins, YmL20 possesses an N-terminal presequence of 18 amino acid residues, while YmL8 has no such presequence. Both proteins were found to be transported into isolated mitochondria in an energy-dependent manner. Furthermore, YmL20 protein without its N-terminal presequence was also transported, despite the fact that the presequence alone was capable of transporting a fused passenger protein, Chinese hamster dihydrofolate reductase (DHFR). Therefore, YmL20 protein appears to possess redundant transport signals in its structure. Similarly, YmL8 derivatives lacking either 40 or 86 amino acid residues from the N-terminus and/or 52 amino acid residues from the C-terminus were transported. In addition, the N-terminal segment of this protein was capable of transporting Chinese hamster DHFR into mitochondria, while its C-terminal segment was not. Thus, YmL8 protein also appears to possess two or more transport signals in its structure. Perhaps the presence of many basic amino acid residues in these proteins might, at least partly, contribute to their mitochondrial transport.

Rapid evolution of the plastid translational apparatus in a nonphotosynthetic plant: loss or accelerated sequence evolution of tRNA and ribosomal protein genes

The vestigial plastid genome of Epifagus virginiana (beechdrops), a nonphotosynthetic parasitic flowering plant, is functional but lacks six ribosomal protein and 13 tRNA genes found in the chloroplast DNAs of photosynthetic flowering plants. Import of nuclear gene products is hypothesized to compensate for many of these losses. Codon usage and amino acid usage patterns in Epifagus plastic genes have not been affected by the tRNA gene losses, though a small shift in the base composition of the whole genome (toward A+T-richness) is apparent. The ribosomal protein and tRNA genes that remain have had a high rate of molecular evolution, perhaps due to relaxation of constraints on the translational apparatus. Despite the compactness and extensive gene loss, one translational gene (infA, encoding initiation factor 1) that is a pseudogene in tobacco has been maintained intact in Epifagus.

Targeting of the master receptor MOM19 to mitochondria

The targeting of proteins to mitochondria involves the recognition of the precursor proteins by receptors on the mitochondrial surface followed by insertion of the precursors into the outer membrane at the general insertion site GIP. Most mitochondrial proteins analyzed so far use a mitochondrial outer membrane protein of 19 kilodaltons (MOM19) as an import receptor. The gene encoding MOM19 has now been isolated. The deduced amino acid sequence predicts that MOM19 is anchored in the outer membrane by an NH2-terminal hydrophobic sequence, while the rest of the protein forms a hydrophilic domain exposed to the cytosol. MOM19 was targeted to the mitochondria via a pathway that is independent of protease-accessible surface receptors and controlled by direct assembly of the MOM19 precursor with GIP.

Biosynthesis and assembly of alcohol oxidase, a peroxisomal matrix protein in methylotrophic yeasts: a review

Alcohol oxidase (AO) catalyses the first step of methanol metabolism in yeasts. In vivo the enzyme is compartmentalized in special cell compartments, called peroxisomes. The enzyme along with the organelles are induced during growth of methylotrophic yeasts on methanol as the sole carbon source. Like all other peroxisomal matrix proteins, AO is encoded by a nuclear gene. Expression of the protein is regulated by a repression/derepression mechanism, but also by induction. Inactive monomeric precursor protein is synthesized in the cytosol and subsequently imported post-translationally into peroxisomes without further processing. Assembly into the active homo-octameric enzyme and binding of the prosthetic group flavin adenine dinucleotide occurs inside the organelle. When enhanced concentration of octameric alcohol oxidase are present in the organelles, the enzyme may form a crystalloid. Oligomerization is not dependent on translocation of AO precursors into their target organelle since octameric, active AO is detected in the cytosol and nucleus of peroxisome-deficient mutants of Hansenula polymorpha: at high expression rates large cytosolic AO crystalloids are formed, which occasionally are also encountered inside the nucleus of such mutants. This paper summarizes recent findings and views on the mechanisms involved in synthesis, import, assembly and crystallization of this important peroxisomal enzyme.

Mitochondrial import receptors for precursor proteins

The specific targeting of precursor proteins synthesized in the cytosol to various cell organelles is a central aspect of intracellular protein traffic. Several hundred different proteins are imported from the cytosol into the mitochondria. Recent studies have identified the mitochondrial outer membrane proteins MOM19, MOM72, MOM38 (approximately ISP42) and p32 which have a role in initial steps of protein import. The first three components are present in a multi-subunit complex that catalyses recognition and membrane insertion of precursor proteins.

Membrane insertion and lateral diffusion of fluorescence-labelled cytochrome c oxidase subunit IV signal peptide in charged and uncharged phospholipid bilayers

The synthetic 25-residue signal peptide of cytochrome c oxidase subunit IV was labelled with the fluorophor 7-nitrobenz-2-oxa-1,3-diazole (NBD) at its single cysteine residue. Addition of small unilamellar vesicles of 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC) to the labelled peptide resulted in a shift of the NBD excitation and emission spectra to shorter wavelengths. Binding of the peptide to the vesicles was measured by the increase in the fluorescence emission yield. A surface partition constant of (3.9 +/- 0.5) x 10(3) M-1 was derived from these titrations. When the membrane contained, in addition to POPC, negatively charged 1-palmitoyl 2-oleoyl phosphatidylglycerol (POPG), the NBD fluorescence spectra were further shifted to shorter wavelengths and exhibited increased quantum yields. The apparent partition constants were increased to 10(4)-10(5) M-1 for vesicles with 20 or 100 mol% POPG. Lateral diffusion of the peptide was measured by fluorescence recovery after photobleaching in multibilayers of POPC, POPG, POPC/POPG (4:1) and 1,2-dimyristoyl phosphatidylcholine. The lateral diffusion coefficients of the peptide in bilayers of POPC (8 x 10(-8) cm2/s at 21 degrees C) were 1.5-1.6-fold greater than those of NBD-labelled phospholipids (5 x 10(-8) cm2/s at 21 degrees C), but 1.5-1.8-fold smaller (3 x 10(-8) cm2/s in 20% POPG and at 21 degrees C) than the lipid diffusion coefficients in the negatively charged bilayers. It is concluded that the signal peptide associates with phospholipid bilayers in two different forms, which depend on the lipid charge. The experiments with POPC bilayers are well explained by a model in which the peptide partitions into the region of the phospholipid head-groups and diffuses along the membrane/water interface. If POPG is present in the membrane, electrostatic attractions between the basic residues of the peptide and the acidic lipid head-groups result in a deeper penetration of the bilayer. For this case, two models that are both consistent with the experimental data are discussed, in which the peptide either forms an oligomer of three to six partially helical membrane-spanning monomers, or inserts into the bilayer with its amphiphilic helical segment aligned parallel to the plane of the membrane and located near the head-group and outer hydrocarbon region of the bilayer.

Mitochondrial protein import

Most polypeptides of mitochondria are imported from the cytosol. Precursor proteins contain targeting and sorting information, often in the form of amino-terminal presequences. Precursors first bind to receptors in the outer membrane. Two putative import receptors have been identified: a 19-kilodalton protein (MOM19) in Neurospora mitochondria, and a 70-kilodalton protein (MAS70) in yeast. Some precursors integrate directly into the outer membrane, but the majority are translocated through one or both membranes. This process requires an electrochemical potential across the inner membrane. Import appears to occur through a hydrophilic pore, although the inner and outer membranes may contain functionally separate translocation machineries. In yeast, a 42-kilodalton protein (ISP42) probably forms part of the outer membrane channel. After import, precursors interact with "chaperonin" ATPases in the matrix. Presequences then are removed by the matrix protease. Finally, some proteins are retranslocated across the inner membrane to the intermembrane space.

Import of ADP/ATP carrier into mitochondria: two receptors act in parallel

We have identified the yeast homologue of Neurospora crassa MOM72, the mitochondrial import receptor for the ADP/ATP carrier (AAC), by functional studies and by cDNA sequencing. Mitochondria of a yeast mutant in which the gene for MOM72 was disrupted were impaired in specific binding and import of AAC. Unexpectedly, we found a residual, yet significant import of AAC into mitochondria lacking MOM72 that occurred via the receptor MOM19. We conclude that both MOM72 and MOM19 can direct AAC into mitochondria, albeit with different efficiency. Moreover, the precursor of MOM72 apparently does not require a positively charged sequence at the extreme amino terminus for targeting to mitochondria.

Overexpression of a leaderless form of yeast cytochrome c oxidase subunit Va circumvents the requirement for a leader peptide in mitochondrial import

Subunit Va of Saccharomyces cerevisiae cytochrome c oxidase is a nucleus-encoded mitochondrial protein that is derived from a precursor with a 20-residue leader peptide. We previously reported that this leader peptide is required for import of subunit Va into mitochondria in vivo (S. M. Glaser, C. E. Trueblood, L. K. Dircks, R. O. Poyton, and M. G. Cumsky, J. Cell. Biochem. 36:275-278, 1988). Here we show that overproduction of a leaderless form of subunit Va circumvents the leader peptide requirement for import into mitochondria in vivo.

Overexpression of a leaderless form of yeast cytochrome c oxidase subunit Va circumvents the requirement for a leader peptide in mitochondrial import

Subunit Va of Saccharomyces cerevisiae cytochrome c oxidase is a nucleus-encoded mitochondrial protein that is derived from a precursor with a 20-residue leader peptide. We previously reported that this leader peptide is required for import of subunit Va into mitochondria in vivo (S. M. Glaser, C. E. Trueblood, L. K. Dircks, R. O. Poyton, and M. G. Cumsky, J. Cell. Biochem. 36:275-278, 1988). Here we show that overproduction of a leaderless form of subunit Va circumvents the leader peptide requirement for import into mitochondria in vivo.

Outer membrane proteins of Pseudomonas

In this review, we describe the outer membrane proteins of Pseudomonas aeruginosa and related strains from the Pseudomonas fluorescens rRNA homology group of the Pseudomonadaceae, with emphasis on the physiological function and biochemical characteristics of these proteins. The use of opr (for outer membrane protein) is proposed as the genetic designation for the P. aeruginosa outer membrane proteins and letters are assigned, in conjunction with this designation, to known outer membrane proteins. Proteins whose primary functions involve pore formation, transport of specific substrates, cell structure determination and membrane stabilization are discussed. The conservation of selected proteins in the above Pseudomonas species is also examined.

Apocytochrome c: an exceptional mitochondrial precursor protein using an exceptional import pathway

The cytochrome c import pathway differs markedly from the general route taken by the majority of other imported proteins, which is characterized by the import involvement of namely, surface receptors, the general insertion protein (GIP), contact sites and by the requirement of a membrane potential (delta psi). Unique features of both the cytochrome c precursor (apocytochrome c) and of the mechanism that transports it into mitochondria, have contributed to the evolution of a distinct import pathway that is not shared by any other mitochondrial protein analysed thus far. The cytochrome c pathway is particularly unique because i) apocytochrome c appears to have spontaneous membrane insertion-activity; ii) cytochrome c heme lyase seems to act as a specific binding site in lieu of a surface receptor and; iii) covalent heme addition and the associated refolding of the polypeptide appears to provide the free energy for the translocation of the cytochrome c polypeptide across the outer mitochondrial membrane.

Early steps in mitochondrial protein import: receptor functions can be substituted by the membrane insertion activity of apocytochrome c

The process of insertion of precursor proteins into mitochondrial membranes was investigated using a hybrid protein (pSc1-c) that contains dual targeting information and, at the same time, membrane insertion activity. pSc1-c is composed of the matrix-targeting domain of the cytochrome c1 presequence joined to the amino terminus of apocytochrome c. It can be selectively imported along either a cytochrome c1 route into the mitochondrial matrix or via the cytochrome c route into the intermembrane space. In contrast to cytochrome c1, pSc1-c does not require the receptor system/GIP for entry into the matrix. The apocytochrome c in the pSc1-c fusion protein appears to exert its membrane insertion activity in such a manner that the matrix-targeting sequence gains direct access to the membrane potential-dependent step. These results attribute an essential function to the receptor system in facilitating the initial insertion of precursors into the mitochondrial membranes.

Amino-terminal extension generated from an upstream AUG codon increases the efficiency of mitochondrial import of yeast N2,N2-dimethylguanosine-specific tRNA methyltransferases

Fusions between the TRM1 gene of Saccharomyces cerevisiae and COXIV or DHFR were made to examine the mitochondrial targeting signals of N2,N2-dimethylguanosine-specific tRNA methyltransferase [tRNA (m2(2)G)dimethyltransferase]. This enzyme is responsible for the modification of both mitochondrial and cytoplasmic tRNAs. We have previously shown that two forms of the enzyme are translated from two in-frame ATGs in this gene, that they differ by a 16-amino-acid amino-terminal extension, and that both the long and short forms are imported into mitochondria. Results of studies to test the ability of various TRM1 sequences to serve as surrogate mitochondrial targeting signals for passenger protein import in vitro and in vivo showed that the most efficient signal derived from tRNA (m2(2)G)dimethyltransferase included a combination of sequences from both the amino-terminal extension and the amino terminus of the shorter form of the enzyme. The amino-terminal extension itself did not serve as an independent mitochondrial targeting signal, whereas the amino terminus of the shorter form of tRNA (m2(2)G)dimethyltransferase did function in this regard, albeit inefficiently. We analyzed the first 48 amino acids of tRNA (m2(2)G)dimethyltransferase for elements of primary and secondary structure shared with other known mitochondrial targeting signals. The results lead us to propose that the most efficient signal spans the area around the second ATG of TRM1 and is consistent with the idea that there is a mitochondrial targeting signal present at the amino terminus of the shorter form of the enzyme and that the amino-terminal extension augments this signal by extending it to form a larger, more efficient mitochondrial targeting signal.

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.

In vitro and in vivo studies on the mitochondrial import of CBS1, a translational activator of cytochrome b in yeast

Translation of mitochondrial cytochrome b mRNA in yeast is activated by the product of the nuclear gene CBS1. CBS1 encodes a 27 kDa precursor protein, which is cleaved to a 24 kDa mature protein during the import into isolated mitochondria. The sequences required for mitochondrial import reside in the amino-terminal end of the CBS1 precursor. Deletion of the 76 amino-terminal amino acids renders the protein incompetent for mitochondrial import in vitro and non-functional in vivo. When present on a high copy number plasmid and under the control of a strong yeast promoter, biological function can be restored by this truncated derivative. This observation indicates that the CBS1 protein devoid of mitochondrial targeting sequences can enter mitochondria in vivo, possibly due to a bypass of the mitochondrial import system.

The amyloplast-targeting transit peptide of the waxy protein of maize also mediates protein transport in vitro into chloroplasts

The transit peptide of the waxy protein of maize which in the maize plant targets this protein only into amyloplasts was used for in vitro protein transport experiments with isolated amyloplasts from maize and chloroplasts from maize, pea and potato. In the presence of both intact and disrupted amyloplasts an artificial preprotein (TP30), consisting of the waxy transit peptide plus the first 34 amino acids of the mature waxy protein fused in-frame to the beta-glucuronidase of Escherichia coli, is processed to the size expected when the transit peptide is cleaved off. The chloroplasts studied show in vitro import and correct processing of both TP30 and the authentic waxy protein, but not of the beta-glucuronidase without the waxy transit peptide. The in vitro import of TP30 into chloroplasts is almost as efficient as that of the precursor of the small subunit of pea ribulose-1,5-bisphosphate carboxylase, a nuclear-encoded chloroplast protein, whereas the waxy protein accumulates to a lesser extent in the chloroplasts. Since the amino-terminal transit peptides of TP30 and the waxy precursor are the same, this difference must be due to the mature part of the waxy protein. One possible explanation is the observed instability of the waxy protein in the presence of chloroplasts.

Amino-terminal extension generated from an upstream AUG codon increases the efficiency of mitochondrial import of yeast N2,N2-dimethylguanosine-specific tRNA methyltransferases

Fusions between the TRM1 gene of Saccharomyces cerevisiae and COXIV or DHFR were made to examine the mitochondrial targeting signals of N2,N2-dimethylguanosine-specific tRNA methyltransferase [tRNA (m2(2)G)dimethyltransferase]. This enzyme is responsible for the modification of both mitochondrial and cytoplasmic tRNAs. We have previously shown that two forms of the enzyme are translated from two in-frame ATGs in this gene, that they differ by a 16-amino-acid amino-terminal extension, and that both the long and short forms are imported into mitochondria. Results of studies to test the ability of various TRM1 sequences to serve as surrogate mitochondrial targeting signals for passenger protein import in vitro and in vivo showed that the most efficient signal derived from tRNA (m2(2)G)dimethyltransferase included a combination of sequences from both the amino-terminal extension and the amino terminus of the shorter form of the enzyme. The amino-terminal extension itself did not serve as an independent mitochondrial targeting signal, whereas the amino terminus of the shorter form of tRNA (m2(2)G)dimethyltransferase did function in this regard, albeit inefficiently. We analyzed the first 48 amino acids of tRNA (m2(2)G)dimethyltransferase for elements of primary and secondary structure shared with other known mitochondrial targeting signals. The results lead us to propose that the most efficient signal spans the area around the second ATG of TRM1 and is consistent with the idea that there is a mitochondrial targeting signal present at the amino terminus of the shorter form of the enzyme and that the amino-terminal extension augments this signal by extending it to form a larger, more efficient mitochondrial targeting signal.

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.