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

Consequences of inner mitochondrial membrane protein misfolding

Proteins embedded in the inner mitochondrial membrane (IMM) perform essential cellular functions. Maintaining the folding state of these proteins is therefore of the utmost importance, and this is ensured by IMM chaperones and proteases that refold and degrade unassembled and misfolded proteins. However, the physiological consequences specific to IMM protein misfolding remain obscure because deletion of these chaperones/proteases (the typical experimental strategy) often affects many mitochondrial processes other than protein folding and turnover. Thus, novel experimental systems are needed to evaluate the direct effects of misfolded protein on the membrane. Such a system has been developed in recent years. Studies suggest that numerous pathogenic mutations in isoform 1 of adenine nucleotide translocase (Ant1) cause its misfolding on the IMM. In this review, we first discuss potential mechanisms by which dominant Ant1 mutations may cause disease, highlighting IMM protein misfolding, per se, as a likely pathological factor. Then we discuss the intramitochondrial effects of Ant1 misfolding such as IMM proteostatic stress, respiratory chain dysfunction, and mtDNA instability. Finally, we summarize the mounting evidence that IMM proteostatic stress can perturb mitochondrial protein import to cause the toxic accumulation of mitochondrial proteins in the cytosol: a cell stress mechanism termed mitochondrial Precursor Overaccumulation Stress (mPOS).

The N-end rule pathway and Ubr1 enforce protein compartmentalization via P2-encoded cellular location signals

The Arg/N-end rule pathway and Ubr1, a ubiquitin E3 ligase conserved from yeast to humans, is involved in the degradation of misfolded proteins in the cytosol. However, the root physiological purpose of this activity is not completely understood. Through a systematic examination of single-residue P2-position mutants of misfolded proteins, and global and targeted bioinformatic analyses of the Saccharomyces cerevisiae proteome, it was determined that Ubr1 preferentially targets mistranslocated secretory and mitochondrial proteins in the cytosol. Degradation by Ubr1 is dependent on the recognition of cellular location signals that are naturally embedded into the second amino acid residue of most proteins. This P2-encoded location signaling mechanism may shed light on how Ubr1 and the N-end rule pathway are involved in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. A corollary to this discovery is that the N-end rule pathway enforces the compartmentalization of secretory and mitochondrial proteins by degrading those that fail to reach their intended subcellular locations. The N-end rule pathway is therefore likely to have been critical to the evolution of endosymbiotic relationships that paved the way for advanced eukaryotic cellular life. This article has an associated First Person interview with the first author of the paper.

Both the autophagy and proteasomal pathways facilitate the Ubp3p-dependent depletion of a subset of translation and RNA turnover factors during nitrogen starvation in Saccharomyces cerevisiae

Protein turnover is an important regulatory mechanism that facilitates cellular adaptation to changing environmental conditions. Previous studies have shown that ribosome abundance is reduced during nitrogen starvation by a selective autophagy mechanism termed ribophagy, which is dependent upon the deubiquitinase Ubp3p. In this study, we asked whether the abundance of various translation and RNA turnover factors are reduced following the onset of nitrogen starvation in Saccharomyces cerevisiae. We found distinct differences in the abundance of the proteins tested following nitrogen starvation: (1) The level of some did not change; (2) others were reduced with kinetics similar to ribophagy, and (3) a few proteins were rapidly depleted. Furthermore, different pathways differentially degraded the various proteins upon nitrogen starvation. The translation factors eRF3 and eIF4GI, and the decapping enhancer Pat1p, required an intact autophagy pathway for their depletion. In contrast, the deadenylase subunit Pop2p and the decapping enzyme Dcp2p were rapidly depleted by a proteasome-dependent mechanism. The proteasome-dependent depletion of Dcp2p and Pop2p was also induced by rapamycin, suggesting that the TOR1 pathway influences this pathway. Like ribophagy, depletion of eIF4GI, eRF3, Dcp2p, and Pop2p was dependent upon Ubp3p to varying extents. Together, our results suggest that the autophagy and proteasomal pathways degrade distinct translation and RNA turnover factors in a Ubp3p-dependent manner during nitrogen starvation. While ribophagy is thought to mediate the reutilization of scarce resources during nutrient limitation, our results suggest that the selective degradation of specific proteins could also facilitate a broader reprogramming of the post-transcriptional control of gene expression.

Characterization of a gene encoding tRNA nucleotidyltransferase from Candida glabrata

A gene encoding ATP (CTP):tRNA nucleotidyltransferase (EC2.7.7.25) was isolated from Candida (Torulopsis) glabrata by complementation in Saccharomyces cerevisiae. The predicted amino acid sequence of the protein revealed a large region with high sequence similarity to members of the Class II group of the nucleotidyltransferase superfamily and an N-terminal region characteristic of a mitochondrial targeting sequence. The essential role of the carboxylates within the conserved DXD and RRD motifs was confirmed by mutagenesis. C. glabrata strains bearing truncated CCA1 genes that lacked sequences encoding the putative mitochondrial targeting peptide were unable to grow on non-fermentable carbon sources but were able to grow on a fermentable carbon source. These results suggest that, as in S. cerevisiae, the C. glabrata CCA-adding enzyme is a sorting isozyme that functions in multiple cellular compartments. Mapping of the 5'-ends of primary transcripts of CCA1 revealed multiple transcription start sites located both upstream of and between two in-frame start codons. When the cells were cultured on a non-fermentable carbon source the longer transcripts appeared more abundant, suggesting that the choice of transcription start sites was influenced by carbon source. The shorter transcripts, which lacked sequences encoding the mitochondrial targeting information, were more predominant in cells grown on glucose. These observations suggest that expression of CCA-adding isozymes in C. glabrata may be regulated. The DNA sequence has been assigned GenBank Accession No. AF098803.

Transport of proteins into mitochondria

Most mitochondrial proteins are nuclear-encoded and synthesised as preproteins on polysomes in the cytosol. They must be targeted to and translocated into mitochondria. Newly synthesised preproteins interact with cytosolic factors until their recognition by receptors on the surface of mitochondria. Import into or across the outer membrane is mediated by a dynamic protein complex coined the translocase of the outer membrane (TOM). Preproteins that are imported into the matrix or inner membrane of mitochondria require the action of one of two translocation complexes of the inner membrane (TIMs). The import pathway of preproteins is predetermined by their intrinsic targeting and sorting signals. Energy input in the form of ATP and the electrical gradient across the inner membrane is required for protein translocation into mitochondria. Newly imported proteins may require molecular chaperones for their correct folding.

Loss of the major isoform of phosphoglucomutase results in altered calcium homeostasis in Saccharomyces cerevisiae

Phosphoglucomutase (PGM) is a key enzyme in glucose metabolism, where it catalyzes the interconversion of glucose 1-phosphate (Glc-1-P) and glucose 6-phosphate (Glc-6-P). In this study, we make the novel observation that PGM is also involved in the regulation of cellular Ca(2+) homeostasis in Saccharomyces cerevisiae. When a strain lacking the major isoform of PGM (pgm2Delta) was grown on media containing galactose as sole carbon source, its rate of Ca(2+) uptake was 5-fold higher than an isogenic wild-type strain. This increased rate of Ca(2+) uptake resulted in a 9-fold increase in the steady-state total cellular Ca(2+) level. The fraction of cellular Ca(2+) located in the exchangeable pool in the pgm2Delta strain was found to be as large as the exchangeable fraction observed in wild-type cells, suggesting that the depletion of Golgi Ca(2+) stores is not responsible for the increased rate of Ca(2+) uptake. We also found that growth of the pgm2Delta strain on galactose media is inhibited by 10 microM cyclosporin A, suggesting that activation of the calmodulin/calcineurin signaling pathway is required to activate the Ca(2+) transporters that sequester the increased cytosolic Ca(2+) load caused by this high rate of Ca(2+) uptake. We propose that these Ca(2+)-related alterations are attributable to a reduced metabolic flux between Glc-1-P and Glc-6-P due to a limitation of PGM enzymatic activity in the pgm2Delta strain. Consistent with this hypothesis, we found that this "metabolic bottleneck" resulted in an 8-fold increase in the Glc-1-P level compared with the wild-type strain, while the Glc-6-P and ATP levels were normal. These results suggest that Glc-1-P (or a related metabolite) may participate in the control of Ca(2+) uptake from the environment.

Distribution of binding sequences for the mitochondrial import receptors Tom20, Tom22, and Tom70 in a presequence-carrying preprotein and a non-cleavable preprotein

Preproteins destined for mitochondria either are synthesized with amino-terminal signal sequences, termed presequences, or possess internal targeting information within the protein. The preprotein translocase of the outer mitochondrial membrane (designated Tom) contains specific import receptors. The cytosolic domains of three import receptors, Tom20, Tom22, and Tom70, have been shown to interact with preproteins. Little is known about the internal targeting information in preproteins and the distribution of binding sequences for the three import receptors. We have studied the binding of the purified cytosolic domains of Tom20, Tom22, and Tom70 to cellulose-bound peptide scans derived from a presequence-carrying cleavable preprotein, cytochrome c oxidase subunit IV, and a non-cleavable preprotein with internal targeting information, the phosphate carrier. All three receptor domains are able to bind efficiently to linear 13-mer peptides, yet with different specificity. Tom20 preferentially binds to presequence segments of subunit IV. Tom22 binds to segments corresponding to the carboxyl-terminal part of the presequence and the amino-terminal part of the mature protein. Tom70 does not bind efficiently to any region of subunit IV. In contrast, Tom70 and Tom20 bind to multiple segments within the phosphate carrier, yet the amino-terminal region is excluded. Both charged and uncharged peptides derived from the phosphate carrier show specific binding properties for Tom70 and Tom20, indicating that charge is not a critical determinant of internal targeting sequences. This feature contrasts with the crucial role of positively charged amino acids in presequences. Our results demonstrate that linear peptide segments of preproteins can serve as binding sites for all three receptors with differential specificity and imply different mechanisms for translocation of cleavable and non-cleavable preproteins.

In vivo mitochondrial import. A comparison of leader sequence charge and structural relationships with the in vitro model resulting in evidence for co-translational import

The positive charges and structural properties of the mitochondrial leader sequence of aldehyde dehydrogenase have been extensively studied in vitro. The results of these studies showed that increasing the helicity of this leader would compensate for reduced import from positive charge substitutions of arginine with glutamine or the insertion of negative charged residues made in the native leader. In this in vivo study, utilizing the green fluorescent protein (GFP) as a passenger protein, import results showed the opposite effect with respect to helicity, but the results from mutations made within the native leader sequence were consistent between the in vitro and in vivo experiments. Leader mutations that reduced the efficiency of import resulted in a cytosolic accumulation of a truncated GFP chimera that was fluorescent but devoid of a mitochondrial leader. The native leader efficiently imported before GFP could achieve a stable, import-incompetent structure, suggesting that import was coupled with translation. As a test for a co-translational mechanism, a chimera of GFP that contained the native leader of aldehyde dehydrogenase attached at the N terminus and a C-terminal endoplasmic reticulum targeting signal attached to the C terminus of GFP was constructed. This chimera was localized exclusively to mitochondria. The import result with the dual signal chimera provides support for a co-translational mitochondrial import pathway.

A regional net charge and structural compensation model to explain how negatively charged amino acids can be accepted within a mitochondrial leader sequence

Mitochondrial leader sequences have been found to be statistically enriched for positively charged residues, with only a few known leader sequences possessing negatively charged residues. Mutational studies that have introduced negatively charged residues into various leader sequences have shown a general, but not absolute, trend toward reduced import. The leader sequence of rat liver aldehyde dehydrogenase has been previously determined by NMR to form a helix-linker-helix structure. A negative charge introduced into this leader did not prevent import, provided that a net positive charge remained in the N-helical segment. When the net charge of the N-terminal helical segment was reduced to zero, import could be recovered by removing the linker, which resulted in a longer, more stable leader. This structural recovery of import was effective enough to compensate for a net charge of zero within the first 10 residues, even when a glutamate is the first charged side chain presented in the sequence.

Differential recognition of preproteins by the purified cytosolic domains of the mitochondrial import receptors Tom20, Tom22, and Tom70

The preprotein translocase of the outer mitochondrial membrane (Tom) is a multi-subunit complex required for specific recognition and membrane translocation of nuclear-encoded preproteins. We have expressed and purified the cytosolic domains of three postulated import receptors, Tom20, Tom22, and Tom70. Each receptor domain is able to bind mitochondrial preproteins but with different specificity. Tom20 binds both preproteins with N-terminal presequences and preproteins with internal targeting signals; the binding is enhanced by the addition of salt. Tom22 selectively recognizes presequence-carrying preproteins in a salt-sensitive manner. Tom70 preferentially binds preproteins with internal targeting information. A chemically synthesized presequence peptide competes with preproteins for binding to Tom20 and Tom22 but not to Tom70. We conclude that each of the three import receptors binds preproteins independently and by a different mechanism. Both Tom20 and Tom22 function as presequence receptors.

Metabolic effects of altering redundant targeting signals for yeast mitochondrial malate dehydrogenase

Eukaryotic cells contain highly homologous isozymes of malate dehydrogenase which catalyze the same reaction in different cellular compartments. To examine whether the metabolic functions of these isozymes are interchangeable, we have altered the cellular localization of mitochondrial malate dehydrogenase (MDH1) in yeast. Since a previous study showed that removal of the targeting presequence from MDH1 does not prevent mitochondrial import in vivo, we tested the role of a putative cryptic targeting sequence near the amino terminus of the mature polypeptide. Three residues in this region were changed to residues present in analogous positions in the other two yeast MDH isozymes. Alone, these replacements did not affect activity or localization of MDH1 but, in combination with deletion of the presequence, prevented mitochondrial import in vivo. Measurable levels of the resulting cytosolic form of MDH1 were low with expression from a centromere-based plasmid but were comparable to normal cellular levels with expression from a multicopy plasmid. The cytosolic form of MDH1 restored the ability of a deltaMDH1 disruption strain to grow on ethanol or acetate, suggesting that mitochondrial localization of MDH1 is not essential for its function in the TCA cycle. This TCA cycle function observed for the cytosolic form of MDH1 is unique to that isozyme since overexpression of MDH2 and of a cytosolic form of MDH3 in a deltaMDH1 strain failed to restore growth. Finally, only partial restoration of growth of a deltaMDH2 disruption mutant was attained with the cytosolic form of MDH1, suggesting that MDH2 may also have unique metabolic functions.

Isolation of differentially expressed cDNA clones from salt-adapted Aspergillus nidulans

Differentially expressed cDNA clones were isolated from salt-adapted Aspergillus nidulans (FGSC #359). Poly (A)+ RNA from adapted mycelia was used to construct a lambda Uni-ZAP cDNA library. The library was screened with mixed subtracted cDNA probes. Three-hundred and fifty-seven positive plaques were isolated in the primary screening. Sixty-two randomly selected plaques were purified and placed into eight different cross-hybridization groups. A representative cDNA from each group was used to study expression under unadapted, salt-adapted and salt-shock conditions. These clones, representing eight different genes, displayed enhanced expression under salt stress. Exploratory nucleotide sequencing was performed, and the predicted amino-acid sequence was compared with known gene sequences in the data-bank. Five of the cDNA clones were identified as a mitochondrial (mt) ATPase beta subunit, a mt ATPase subunit 9, a mt transport protein, a ubiquitin-extension protein and a ribosomal protein. Three cDNA clones could not be identified due to lack of adequate homology with known sequences. These results suggest that at least five genes with known function in cellular processes like ATP generation and protein synthesis, and three other genes of unknown identity, are greatly induced in salt-adapted conditions.

Truncated presequences of mitochondrial F1-ATPase beta subunit from Nicotiana plumbaginifolia transport CAT and GUS proteins into mitochondria of transgenic tobacco

The mitochondrial F1-ATPase beta subunit (ATPase-beta) of Nicotiana plumbaginifolia is nucleus-encoded as a precursor containing an NH2-terminal extension. By sequencing the mature N. tabacum ATPase-beta, we determined the length of the presequence, viz. 54 residues. To define the essential regions of this presequence, we produced a series of 3' deletions in the sequence coding for the 90 NH2-terminal residues of ATPase-beta. The truncated sequences were fused with the chloramphenicol acetyl transferase (cat) and beta-glucuronidase (gus) genes and introduced into tobacco plants. From the observed distribution of CAT and GUS activity in the plant cells, we conclude that the first 23 amino-acid residues of ATPase-beta remain capable of specifically targeting reporter proteins into mitochondria. Immunodetection in transgenic plants and in vitro import experiments with various CAT fusion proteins show that the precursors are processed at the expected cleavage site but also at a cryptic site located in the linker region between the presequence and the first methionine of native CAT.

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.

Conformational analysis of a mitochondrial presequence derived from the F1-ATPase beta-subunit by CD and NMR spectroscopy

Previous studies on mitochondrial targeting presequences have indicated that formation of an amphiphillic helix may be required for efficient targeting of the precursor protein into mitochondria, but the structural details are not well understood. We have used CD and NMR spectroscopy to characterize in detail the structure of a synthetic peptide corresponding to the presequence for the beta-subunit of F1-ATPase, a mitochondrial matrix protein. Although this peptide is essentially unstructured in water, alpha-helix formation is induced when the peptide is placed in structure-promoting environments, such as SDS micelles or aqueous trifluoroethanol (TFE). In 50% TFE (by volume), the peptide is in dynamic equilibrium between random coil and alpha-helical conformations, with a significant population of alpha-helix throughout the entire peptide. The helix is somewhat more stable in the N-terminal part of the presequence (residues 4-10), and this result is consistent with the structure proposed previously for the presequence of another mitochondrial matrix protein, yeast cytochrome oxidase subunit IV. Addition of increasing amounts of TFE causes the alpha-helical content to increase even further, and the TFE titration data for the presequence peptide of the F1-ATPase beta-subunit are not consistent with a single, cooperative transition from random coil to alpha-helix. There is evidence that helix formation is initiated in two different regions of the peptide. This result helps to explain the redundancy of the targeting information contained in the presequence for the F1-ATPase beta-subunit.

Expression and function of heterologous forms of malate dehydrogenase in yeast

The structure of the tricarboxylic acid cycle enzyme malate dehydrogenase is highly conserved in various organisms. To test the extent of functional conservation, the rat mitochondrial enzyme and the enzyme from Escherichia coli were expressed in a strain of Saccharomyces cerevisiae containing a disruption of the chromosomal MDH1 gene encoding yeast mitochondrial malate dehydrogenase. The authentic precursor form of the rat enzyme, expressed using a yeast promoter and a multicopy plasmid, was found to be efficiently targeted to yeast mitochondria and processed to a mature active form in vivo. Mitochondrial levels of the polypeptide and malate dehydrogenase activity were found to be similar to those for MDH1 in wild-type yeast cells. Efficient expression of the E. coli mdh gene was obtained with multicopy plasmids carrying gene fusions encoding either a mature form of the procaryotic enzyme or a precursor form with the amino terminal mitochondrial targeting sequence from yeast MDH1. Very low levels of mitochondrial import and processing of the precursor form were obtained in vivo and activity could be demonstrated for only the expressed precursor fusion protein. Results of in vitro import experiments suggest that the percursor form of the E. coli protein associates with yeast mitochondria but is not efficiently internalized. Respiratory rates measured for isolated yeast mitochondria containing the mammalian or procaryotic enzyme were, respectively, 83 and 62% of normal, suggesting efficient delivery of NADH to the respiratory chain. However, expression of the heterologous enzymes did not result in full complementation of growth phenotypes associated with disruption of the yeast MDH1 gene.

Mitochondrial protein import: isolation and characterization of the Saccharomyces cerevisiae MFT1 gene

Mitochondrial targeting of an Atp2-LacZ fusion protein confers a respiration-defective phenotype on yeast cells. This effect has been utilized to select strains that grow on nonfermentable carbon sources, some of which have decreased levels of hybrid protein localized to the organelle. Many of the mutants obtained were also temperature-sensitive for growth on all media. The recessive mft (mitochondrial fusion targeting) mutants have been assigned to three complementation groups. MFT1 was cloned and sequenced: it encodes a 255 amino acid protein that is highly basic and has no predicted membrane-spanning domains or organelle-targeting sequences. The MFT1 gene is 91% identical to an open reading frame 3' of the SIR3 gene. Evidence is presented that these two closely related genes could represent a recent gene duplication.

Mitochondrial protein import

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

Certain N-terminal peptides inhibit uptake of mature aspartate aminotransferase by isolated mitochondria

To gain insight into the uptake of mature aspartate aminotransferase by isolated mitochondria, the capability of certain cyanogen bromide peptides from mature beef heart mitochondrial aspartate aminotransferase to inhibit enzyme uptake was kinetically tested. N-terminal peptides (1-9 and 10-31) proved to inhibit the rate of aspartate aminotransferase uptake respectively in purely competitive and non-competitive ways, whereas other peptides distal from the N-terminus (203-217, 321-327 and 328-353) were found to be completely ineffective.

Mutant alcohol dehydrogenase (ADH III) presequences that affect both in vitro mitochondrial import and in vitro processing by the matrix protease

Point mutations in the presequence of the mitochondrial alcohol dehydrogerase isoenzyme (ADH III) have been shown to affect either the import of the precursor protein into yeast mitochondria in vivo or its processing within the organelle. In the present work, the behavior of these mutants during in vitro import into isolated mitochondria was investigated. All point mutants tested were imported with a slower initial rate than that of the wild-type precursor. This defect was corrected when the precursors were treated with urea prior to import. Once imported, the extent of processing to the mature form of mutant precursors varied greatly and correlated well with the defects observed in vivo. This result was not affected by prior urea treatment. When matrix extracts enriched for the processing protease were used, this defect was shown to be due to failure of the protease to efficiently recognize or cleave the presequence, rather than to a lack of access to the precursor. The rate of import of two ADH III precursors bearing internal deletions in the leader sequence was similar to those of the point mutants, whereas a deletion leading to the removal of the 15 amino-terminal amino acids was poorly imported. The mature amino terminus of wild-type ADH III was determined to be Gln-25. Mutant m01 (Ser-26 to Phe), which reduced the efficiency of cleavage in vitro by 80%, was cleaved at the correct site.

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.

Nuclear protein transport

The nucleus, like all organelles, is composed of a unique set of proteins. This article discusses the possible mechanisms for localization of only certain proteins to the nucleus, transport of proteins across the nuclear envelope, and retention of proteins in the nuclear interior. In addition, nuclear protein transport is compared with transport of proteins into the endoplasmic reticulum and the mitochondria.

The presequence of yeast 5-aminolevulinate synthase is not required for targeting to mitochondria

A truncated form of the yeast mitochondrial 5-aminolevulinate (ALA) synthase was constructed by deletion of the first 75 amino acid residues of its precursor form. This truncated ALA synthase which lost its entire presequence and 40 residues of the mature part possesses a new amino terminus quite different from a typical mitochondrial presequence. This modified protein expressed in vivo is found entirely located within mitochondria. Although it was now unable to reach the matrix space, it was internalized as shown by its resistance to protease in isolated mitochondria. Pulse-chase radiolabeling in the presence of an uncoupler suggests that a membrane potential is not required for the targeting of this truncated ALA synthase. Thus, the amino-terminal signal, if indispensable as a matrix targeting signal, could be replaced by an internal sequence or a particular folding for recognition by the import machinery.

Recent developments in chloroplast protein transport

Most proteins located in chloroplasts are encoded by nuclear genes, synthesized in the cytoplasm, and transported into the organelle. The study of protein uptake by chloroplasts has greatly expanded over the past few years. The increased activity in this field is due, in part, to the application of recombinant DNA methodology to the analysis of protein translocation. Added interest has also been gained by the realization that the transport mechanisms that mediate protein uptake by chloroplasts, mitochondria and the endoplasmic reticulum display certain characteristics in common. These include amino terminal sequences that target proteins to particular organelles, a transport process that is mechanistically independent from the events of translation, and an ATP-requiring transport step that is thought to involve partial unfolding of the protein to be translocated. In this review we examine recent studies on the binding of precursors to the chloroplast surface, the energy-dependent uptake of proteins into the stroma, and the targeting of proteins to the thylakoid lumen. These aspects of protein transport into chloroplasts are discussed in the context of recent studies on protein uptake by mitochondria.