Biophysical characterization of a transit peptide directing chloroplast protein import

Hsp90 is involved in the regulation of cytosolic precursor protein abundance in tomato

Cytosolic chaperones are involved in the regulation of cellular protein homeostasis in general. Members of the families of heat stress proteins 70 (Hsp70) and 90 (Hsp90) assist the transport of preproteins to organelles such as chloroplasts or mitochondria. In addition, Hsp70 was described to be involved in the degradation of chloroplast preproteins that accumulate in the cytosol. Because a similar function has not been established for Hsp90, we analyzed the influences of Hsp90 and Hsp70 on the protein abundance in the cellular context using an in vivo system based on mesophyll protoplasts. We observed a differential behavior of preproteins with respect to the cytosolic chaperone-dependent regulation. Some preproteins such as pOE33 show a high dependence on Hsp90, whereas the abundance of preproteins such as pSSU is more strongly dependent on Hsp70. The E3 ligase, C-terminus of Hsp70-interacting protein (Chip), appears to have a more general role in the control of cytosolic protein abundance. We discuss why the different reaction modes are comparable with the cytosolic unfolded protein response.

PredAlgo: a new subcellular localization prediction tool dedicated to green algae

The unicellular green alga Chlamydomonas reinhardtii is a prime model for deciphering processes occurring in the intracellular compartments of the photosynthetic cell. Organelle-specific proteomic studies have started to delineate its various subproteomes, but sequence-based prediction software is necessary to assign proteins subcellular localizations at whole genome scale. Unfortunately, existing tools are oriented toward land plants and tend to mispredict the localization of nuclear-encoded algal proteins, predicting many chloroplast proteins as mitochondrion targeted. We thus developed a new tool called PredAlgo that predicts intracellular localization of those proteins to one of three intracellular compartments in green algae: the mitochondrion, the chloroplast, and the secretory pathway. At its core, a neural network, trained using carefully curated sets of C. reinhardtii proteins, divides the N-terminal sequence into overlapping 19-residue windows and scores the probability that they belong to a cleavable targeting sequence for one of the aforementioned organelles. A targeting prediction is then deduced for the protein, and a likely cleavage site is predicted based on the shape of the scoring function along the N-terminal sequence. When assessed on an independent benchmarking set of C. reinhardtii sequences, PredAlgo showed a highly improved discrimination capacity between chloroplast- and mitochondrion-localized proteins. Its predictions matched well the results of chloroplast proteomics studies. When tested on other green algae, it gave good results with Chlorophyceae and Trebouxiophyceae but tended to underpredict mitochondrial proteins in Prasinophyceae. Approximately 18% of the nuclear-encoded C. reinhardtii proteome was predicted to be targeted to the chloroplast and 15% to the mitochondrion.

The invariant phenylalanine of precursor proteins discloses the importance of Omp85 for protein translocation into cyanelles

Background

Today it is widely accepted that plastids are of cyanobacterial origin. During their evolutionary integration into the metabolic and regulatory networks of the host cell the engulfed cyanobacteria lost their independency. This process was paralleled by a massive gene transfer from symbiont to the host nucleus challenging the development of a retrograde protein translocation system to ensure plastid functionality. Such a system includes specific targeting signals of the proteins needed for the function of the plastid and membrane-bound machineries performing the transfer of these proteins across the envelope membranes. At present, most information on protein translocation is obtained by the analysis of land plants. However, the analysis of protein import into the primitive plastids of glaucocystophyte algae, revealed distinct features placing this system as a tool to understand the evolutionary development of translocation systems. Here, bacterial outer membrane proteins of the Omp85 family have recently been discussed as evolutionary seeds for the development of translocation systems.

Results

To further explore the initial mode of protein translocation, the observed phenylalanine dependence for protein translocation into glaucophyte plastids was pursued in detail. We document that indeed the phenylalanine has an impact on both, lipid binding and binding to proteoliposomes hosting an Omp85 homologue. Comparison to established import experiments, however, unveiled a major importance of the phenylalanine for recognition by Omp85. This finding is placed into the context of the evolutionary development of the plastid translocon.

Conclusion

The phenylalanine in the N-terminal domain signs as a prerequisite for protein translocation across the outer membrane assisted by a "primitive" translocon. This amino acid appears to be optimized for specifically targeting the Omp85 protein without enforcing aggregation on the membrane surface. The phenylalanine has subsequently been lost in the transit sequence, but can be found at the C-terminal position of the translocating pore. Thereby, the current hypothesis of Omp85 being the prokaryotic contribution to the ancestral Toc translocon can be supported.

Processing of the dual targeted precursor protein of glutathione reductase in mitochondria and chloroplasts

Pea glutathione reductase (GR) is dually targeted to mitochondria and chloroplasts by means of an N-terminal signal peptide of 60 amino acid residues. After import, the signal peptide is cleaved off by the mitochondrial processing peptidase (MPP) in mitochondria and by the stromal processing peptidase (SPP) in chloroplasts. Here, we have investigated determinants for processing of the dual targeting signal peptide of GR by MPP and SPP to examine if there is separate or universal information recognised by both processing peptidases. Removal of 30 N-terminal amino acid residues of the signal peptide (GRDelta1-30) greatly stimulated processing activity by both MPP and SPP, whereas constructs with a deletion of an additional ten amino acid residues (GRDelta1-40) and deletion of 22 amino acid residues in the middle of the GR signal sequence (GRDelta30-52) could be cleaved by SPP but not by MPP. Numerous single mutations of amino acid residues in proximity of the cleavage site did not affect processing by SPP, whereas mutations within two amino acid residues on either side of the processing site had inhibitory effect on processing by MPP with a nearly complete inhibition for mutations at position -1. Mutation of positively charged residues in the C-terminal half of the GR targeting peptide inhibited processing by MPP but not by SPP. An inhibitory effect on SPP was detected only when double and triple mutations were introduced upstream of the cleavage site. These results indicate that: (i) recognition of processing site on a dual targeted GR precursor differs between MPP and SPP; (ii) the GR targeting signal has similar determinants for processing by MPP as signals targeting only to mitochondria; and (iii) processing by SPP shows a low level of sensitivity to single mutations on targeting peptide and likely involves recognition of the physiochemical properties of the sequence in the vicinity of cleavage rather than a requirement for specific amino acid residues.

Chloroplast quest: a journey from the cytosol into the chloroplast and beyond

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.

The N-terminal portion of the preToc75 transit peptide interacts with membrane lipids and inhibits binding and import of precursor proteins into isolated chloroplasts

Toc75 is an outer envelope membrane protein of chloroplasts. It is unusual among the outer membrane proteins in that its precursor form has a bipartite transit peptide. The N-terminal portion of the Toc75 transit peptide is sufficient to target the protein to the stromal space of chloroplasts. We prepared a 45 amino-acid peptide containing the stromal targeting domain of the Toc75 transit peptide in Escherichia coli, using the intein-mediated system, and purified it by reverse-phase HPLC. Its identity was confirmed by N-terminal amino-acid sequencing and matrix assisted laser desorption ionization mass spectrometry. In monolayer experiments, the peptide inserted into the chloroplastic membrane lipids sulfoquinovosyl diacylglycerol and phosphatidylglycerol and into a nonchloroplastic lipid phosphatidylethanolamine. However, it did not insert into other chloroplastic lipids, such as mono- and digalactosyl diacylglycerol, and phosphatidylcholine. Furthermore, the peptide significantly inhibited binding of radiolabeled precursors of Toc75 and the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase to intact chloroplasts as effectively as did a bacterially produced precursor of the small subunit of 1,5-bisphosphate carboxylase/oxygenase. The peptide also inhibited import of radiolabeled precursors into isolated chloroplasts, however, to a lesser extent than did nonlabeled precursor of the small subunit of 1,5-bisphosphate carboxylase/oxygenase.

Chloroplast precursor proteins compete to form early import intermediates in isolated pea chloroplasts

In order to ascertain whether there is one site for the import of precursor proteins into chloroplasts or whether different precursor proteins are imported via different import machineries, chloroplasts were incubated with large quantities of the precursor of the 33 kDa subunit of the oxygen-evolving complex (pOE33) or the precursor of the light-harvesting chlorophyll a/b-binding protein (pLHCP) and tested for their ability to import a wide range of other chloroplast precursor proteins. Both pOE33 and pLHCP competed for import into chloroplasts with precursors of the stromally-targeted small subunit of Rubisco (pSSu), ferredoxin NADP(+) reductase (pFNR) and porphobilinogen deaminase; the thylakoid membrane proteins LHCP and the Rieske iron-sulphur protein (pRieske protein); ferrochelatase and the gamma subunit of the ATP synthase (which are both associated with the thylakoid membrane); the thylakoid lumenal protein plastocyanin and the phosphate translocator, an integral membrane protein of the inner envelope. The concentrations of pOE33 or pLHCP required to cause half-maximal inhibition of import ranged between 0.2 and 4.9 microM. These results indicate that all of these proteins are imported into the chloroplast by a common import machinery. Incubation of chloroplasts with pOE33 inhibited the formation of early import intermediates of pSSu, pFNR and pRieske protein.

A second, substrate-dependent site of protein import into chloroplasts

Chloroplasts must import a large number of proteins from the cytosol. It generally is assumed that this import proceeds for all stromal and thylakoid proteins in an identical manner and is caused by the operation of two distinctive protein import machineries in the outer and inner plastid envelope, which form the general import site. Here we show that there is a second site of protein translocation into chloroplasts of barley, tobacco, Arabidopsis thaliana, and five other tested monocotyledonous and dicotyledonous plant species. This import site is specific for the cytosolic precursor of the NADPH:protochlorophyllide (Pchlide) oxidoreductase A, pPORA. It couples Pchlide synthesis to pPORA import and thereby reduces the actual level of free Pchlide, which, because of its photodynamic properties, would be destructive to the plastids. Consequently, photoprotection is conferred onto the plant.

Characterization of the early steps of OE17 precursor transport by the thylakoid DeltapH/Tat machinery

In order to probe the structure and protein translocation function of the thylakoid Tat machinery, a 25-residue C-terminal extension containing a 13-residue in vivo biotinylation tag and a 6x His tag was added to a mutant precursor of the 17-kDa subunit of the oxygen-evolving complex to form pOE17(C)-BioHis. When avidin was attached to biotinylated precursor in situ, the precursor-avidin complex was neither imported nor did it form a membrane-spanning translocation intermediate. It did, however, competitively inhibit the translocation of unbiotinylated precursor with an apparent KI unaffected by avidin. It is shown that the precursor protein achieves a stable folded structure upon dilution from urea, suggesting that the avidin-induced inhibition of transport results from a folding-induced proximity of N-terminal and C-terminal domains. It is further demonstrated that the majority of precursor rapidly binds to the thylakoid membrane, remaining import competent and yet undissociable by high salt or high pH treatment at ice temperature. The membrane binding event is unaffected by avidin. Import kinetics reveal that nonproton motive force-driven transport steps make up a major fraction of the transport time. These observations suggest that the N-terminal presequence on the avidin-bound precursor is available for membrane binding and initial recognition by the transport machinery, but the attached avidin signals the machinery that the precursor is an incorrectly configured substrate and thus import is aborted. Consequently, the DeltapH/Tat machinery's proofreading mechanism must operate after precursor recognition but before the committed step in transport.

A coil-helix instead of a helix-coil motif can be induced in a chloroplast transit peptide from Chlamydomonas reinhardtii

A synthetic peptide MQVTMKSSAVSGQRVGGARVATRSVRRAQLQV corresponding to the 32 amino acid chloroplast transit sequence of the ribulose bisphosphatase carboxylase/oxygenase activase preprotein from Chlamydomonas reinhardtii, required for translocation through the envelope of the chloroplast, has been characterized structurally using CD and NMR under the same experimental conditions as used previously for the 32 amino acid presequence of preferredoxin from the same organism [Lancelin, J.-M., Bally, I., Arlaud, G. J., Blackledge, M., Gans, P., Stein, M. & Jacquot, J.-P. (1994) FEBS Lett. 343, 261-266]. The peptide is found to undergo a conformational transition in aqueous 2,2,2-trifluoroethanol, characterized by three turns of amphiphilic alpha-helix in the C-terminal region preceded by a disordered coil in the N-terminal region. Compared with the preferredoxin transit peptide, the helical and coiled domains are arranged in the reverse order along the peptide sequence, but the positively charged groups are distributed analogously as well as the hydrophobic residues within the amphiphilic alpha-helix. It is proposed that such coil-helix or helix-coil motifs, occasionally repeated, could be an intrinsic structural feature of chloroplastic transit peptides, adapted to the proper translocase and possibly to each nuclear-encoded chloroplast preproteins. This feature may distinguish chloroplastic transit sequences from the other organelle-targeting peptides in the eukaryotic green alga C. reinhardtii, particularly the mitochondrial transit sequences.

A chloroplast processing enzyme functions as the general stromal processing peptidase

A highly specific stromal processing activity is thought to cleave a large diversity of precursors targeted to the chloroplast, removing an N-terminal transit peptide. The identity of this key component of the import machinery has not been unequivocally established. We have previously characterized a chloroplast processing enzyme (CPE) that cleaves the precursor of the light-harvesting chlorophyll a/b binding protein of photosystem II (LHCPII). Here we report the overexpression of active CPE in Escherichia coli. Examination of the recombinant enzyme in vitro revealed that it cleaves not only preLHCPII, but also the precursors for an array of proteins essential for different reactions and destined for different compartments of the organelle. CPE also processes its own precursor in trans. Neither the recombinant CPE nor the native CPE of chloroplasts process a preLHCPII mutant with an altered cleavage site demonstrating that both forms of the enzyme are sensitive to the same structural modification of the substrate. The transit peptide of the precursor of ferredoxin is released by a single cleavage event and found intact after processing by recombinant CPE and a chloroplast extract as well. These results provide the first direct demonstration that CPE is the general stromal processing peptidase that acts as an endopeptidase. Significantly, recombinant CPE cleaves in the absence of other chloroplast proteins, and this activity depends on metal cations, such as zinc.

Transit peptide mutations that impair in vitro and in vivo chloroplast protein import do not affect accumulation of the gamma-subunit of chloroplast ATPase

We have begun to take a genetic approach to study chloroplast protein import in Chlamydomonas reinhardtii by creating deletions in the transit peptide of the gamma-subunit of chloroplast ATPase-coupling factor 1 (CF1-gamma, encoded by AtpC) and testing their effects in vivo by transforming the altered genes into an atpC mutant, and in vitro by importing mutant precursors into isolated C. reinhardtii chloroplasts. Deletions that removed 20 or 23 amino acid residues from the center of the transit peptide reduced in vitro import to an undetectable level but did not affect CF1-gamma accumulation in vivo. The CF1-gamma transit peptide does have an in vivo stroma-targeting function, since chimeric genes in which the stroma-targeting domain of the plastocyanin transit peptide was replaced by the AtpC transit peptide-coding region allowed plastocyanin to accumulate in vivo. To determine whether the transit peptide deletions were impaired in in vivo stroma targeting, mutant and wild-type AtpC transit peptide-coding regions were fused to the bacterial ble gene, which confers bleomycin resistance. Although 25% of the wild-type fusion protein was associated with chloroplasts, proteins with transit peptide deletions remained almost entirely cytosolic. These results suggest that even severely impaired in vivo chloroplast protein import probably does not limit the accumulation of CF1-gamma.

Identification of a Role for an Azide-Sensitive Factor in the Thylakoid Transport of the 17-Kilodalton Subunit of the Photosynthetic Oxygen-Evolving Complex

We have examined the transport of the precursor of the 17-kD subunit of the photosynthetic O2-evolving complex (OE17) in intact chloroplasts in the presence of inhibitors that block two protein-translocation pathways in the thylakoid membrane. This precursor uses the transmembrane pH gradient-dependent pathway into the thylakoid lumen, and its transport across the thylakoid membrane is thought to be independent of ATP and the chloroplast SecA homolog, cpSecA. We unexpectedly found that azide, widely considered to be an inhibitor of cpSecA, had a profound effect on the targeting of the photosynthetic OE17 to the thylakoid lumen. By itself, azide caused a significant fraction of mature OE17 to accumulate in the stroma of intact chloroplasts. When added in conjunction with the protonophore nigericin, azide caused the maturation of a fraction of the stromal intermediate form of OE17, and this mature protein was found only in the stroma. Our data suggest that OE17 may use the sec-dependent pathway, especially when the transmembrane pH gradient-dependent pathway is inhibited. Under certain conditions, OE17 may be inserted across the thylakoid membrane far enough to allow removal of the transit peptide, but then may slip back out of the translocation machinery into the stromal compartment.

Alterations in the Chlamydomonas plastocyanin transit peptide have distinct effects on in vitro import and in vivo protein accumulation

Nucleus-encoded chloroplast proteins that reside in the thylakoid lumen are synthesized as precursors with bipartite transit peptides that contain information for uptake and intra-chloroplast localization. We have begun to apply the superb molecular and genetic attributes of Chlamydomonas to study chloroplast protein import by creating a series of deletions in the transit peptide of plastocyanin and determining their effects on translocation into isolated Chlamydomonas chloroplasts. Most N-terminal mutations dramatically inhibited in vitro import, whereas replacement with a transit peptide from the gamma-subunit of chloroplast ATPase restored uptake. Thus, the N-terminal region has stroma-targeting function. Deletions within the C-terminal portion of the transit peptide resulted in the appearance of import intermediates, suggesting that this region is required for lumen translocation and processing. Thus, despite its short length and predicted structural differences, the Chlamydomonas plastocyanin transit peptide has functional domains similar to those of vascular plants. Similar mutations have been analyzed in vivo by transforming altered genes into a mutant defective at the plastocyanin locus (K. L. Kindle, manuscript in preparation). Most mutations affected in vitro import more severely than plastocyanin accumulation in vivo. One exception was a deletion that removed residues 2-8, which nearly eliminated in vivo accumulation but had a modest effect in vitro. We suggest that this mutant precursor may not compete successfully with other proteins for the translocation pathway in vivo. Apparently, in vivo and in vitro analyses reveal different aspects of chloroplast protein biogenesis.

Common principles of protein translocation across membranes

Most major systems that transport proteins across a membrane share the following features: an amino-terminal transient signal sequence on the transported protein, a targeting system on the cis side of the membrane, a hetero-oligomeric transmembrane channel that is gated both across and within the plane of the membrane, a peripherally attached protein translocation motor that is powered by the hydrolysis of nucleoside triphosphate, and a protein folding system on the trans side of the membrane. These transport systems are divided into two families: export systems that export proteins out of the cytosol, and import systems that transport proteins into cytosol-like compartments.

Ricin A chain fused to a chloroplast-targeting signal is unfolded on the chloroplast surface prior to import across the envelope membranes

The initial stages of chloroplast protein import involve the binding of precursor proteins to surface-bound receptors prior to translocation across the envelope membranes in a partially folded conformation. We have analyzed the unfolding process by examining the conformation of a construct, comprising the presequence of a chloroplast protein linked to ricin A chain, before and after binding to the chloroplast surface. We show that the presequence is highly susceptible to proteolysis in solution, probably reflecting a lack of tertiary structure, whereas the A chain passenger protein is resistant to extremely high concentrations of protease, unless deliberately unfolded using denaturant. The A chain moiety is furthermore active, indicating that the presence of the presequence does not prevent formation of a tightly folded, native state. In contrast, receptor-bound p33KRA (fusion protein comprising the 33-kDa presequence plus 22 residues of mature protein, linked to the A chain of ricin) is quantitatively digested by protease concentrations that have little effect on the A chain in solution. We conclude that protein unfolding can take place on the chloroplast surface in the absence of translocation and without the aid of soluble factors.

A new chloroplast protein import intermediate reveals distinct translocation machineries in the two envelope membranes: energetics and mechanistic implications

Chloroplast protein import presents a complex membrane traversal problem: precursor proteins must cross two envelope membranes to reach the stromal compartment. This work characterizes a new chloroplast protein import intermediate which has completely traversed the outer envelope membrane but has not yet reached the stroma. The existence of this intermediate demonstrates that distinct protein transport machineries are present in both envelope membranes, and that they are able to operate independently of one another under certain conditions. Energetic characterization of this pathway led to the identification of three independent energy-requiring steps: binding of the precursor to the outer envelope membrane, outer membrane transport, and inner membrane transport. Localization of the sites of energy utilization for each of these steps, as well as their respective nucleotide specificities, suggest that three different ATPases mediate chloroplast envelope transport.

Assembly of the chlorophyll-protein complexes

The biogenesis of photosynthetic complexes in plants and algae is a multi-step process that involves intricate coordination of steps in two intracellular compartments, the chloroplast and the cytoplasm. The process initiates with the transcription and translation of the various polypeptide subunits. The nuclear-encoded Chl-binding proteins are translated on cytoplasmic ribosomes as precursors that have a transit (leader) sequence at their amino-terminus. The precursors are post-translationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylakoid membrane, and bound to their co-factors (and pigments) and with other subunits to form an active complex. The order and mechanisms by which these events occur, are currently being discovered. Electrostatic interactions, the 'positive inside rule', interhelix interactions, interactions with lipids and chaperone proteins affect the insertion and stabilization of the Chl-proteins in the thylakoids. This review describes the events occurring during the integration and organization of the Chl-proteins.

A monomeric, tightly folded stromal intermediate on the delta pH-dependent thylakoidal protein transport pathway

Two distinct mechanisms have been previously identified for the transport of proteins across the chloroplast thylakoid membrane, one of which is unusual in that neither soluble factors nor ATP are required; the system requires only the transthylakoidal delta pH. We have examined this mechanism by studying the properties of one of its substrates: the extrinsic 23-kDa protein (23K) of photosystem II. Previous work has shown that this protein can be transported into isolated thylakoids as the full-length precursor protein; we show that the stromal import intermediate form of this protein is similarly translocation-competent. Gel filtration tests indicate that the stromal intermediate is probably monomeric. Protease sensitivity tests on both the initial in vitro translation product and the stromal import intermediate show that the presequence is highly susceptible to digestion whereas the mature protein is resistant to high concentrations of trypsin. The mature protein becomes very sensitive to digestion if unfolded in urea, or after heating, and we therefore propose that the natural substrate for this translocation system consists of a relatively unfolded presequence together with a tightly folded passenger protein. The ability of thylakoids to import pre-23K is destroyed by prior treatment of the thylakoids with low concentrations of trypsin, demonstrating the involvement of surface-exposed proteins in the import process. However, we can find no evidence for the binding of pre-23K or i23K to the thylakoid surface, and we therefore propose that the initial interaction of these substrates with the thylakoidal translocase is weak, reversible, and probably delta pH-independent. In the second phase of the translocation mechanism, the delta pH drives either the translocation and unfolding of proteins, or the translocation of a fully folded protein.

Subcellular distribution of serine acetyltransferase from Pisum sativum and characterization of an Arabidopsis thaliana putative cytosolic isoform

The intracellular compartmentation of serine acetyltransferase, a key enzyme in the L-cysteine biosynthesis pathway, has been investigated in pea (Pisum sativum) leaves, by isolation of organelles and fractionation of protoplasts. Enzyme activity was mainly located in mitochondria (approximately 76% of total cellular activity). Significant activity was also identified in both the cytosol (14% of total activity) and chloroplasts (10% of total activity). Three enzyme forms were separated by anion-exchange chromatography, and each form was found to be specific for a given intracellular compartment. To obtain cDNA encoding the isoforms, functional complementation experiments were performed using an Arabidopsis thaliana expression library and an Escherichia coli mutant devoid of serine acetyltransferase activity. This strategy allowed isolation of three distinct cDNAs encoding serine acetyltransferase isoforms, as confirmed by enzyme activity measurements, genomic hybridizations, and nucleotide sequencing. The cDNA and related gene for one of the three isoforms have been characterized. The predicted amino acid sequence shows that it encodes a polypeptide of M(r) 34,330 exhibiting 41% amino acid identity with the E. coli serine acetyltransferase. Since none of the general features of transit peptides could be observed in the N-terminal region of this isoform, we assume that it is a cytosolic form.

Efficient but aberrant cleavage of mitochondrial precursor proteins by the chloroplast stromal processing peptidase

Cytosol-synthesized chloroplast and mitochondrial precursor proteins are proteolytically processed after import by highly specific, metal-dependent soluble enzymes: the stromal processing peptidase (SPP) and the matrix processing peptidase (MPP), respectively. We have used in vitro processing assays to compare the reaction specificities of highly purified preparations of pea SPP and Neurospora crassa MPP, both of which are unable to cleave a variety of 'foreign' proteins. We show that SPP can cleave all five mitochondrial precursor proteins tested, namely cyclophilin, the beta subunit of the F1-ATPase complex, the Rieske FeS protein, the alpha-MPP subunit and cytochrome b2. In contrast, MPP is unable to cleave any chloroplast precursor proteins tested. Several of the mitochondrial precursor proteins are cleaved more efficiently by SPP than are many authentic chloroplast precursor proteins but, in each case, cleavage takes place at a site or sites which are N-terminal to the authentic MPP site; pre-cyclophilin is cleaved 5 residues upstream of the MPP site and the precursor of the beta subunit of the F1-ATPase complex is cleaved at sites 5 and 12 residues upstream. We discuss the implications of these data for the SPP reaction mechanism.

The secondary structure of the ferredoxin transit sequence is modulated by its interaction with negatively charged lipids

Import of proteins into chloroplasts depends on an N-terminal transit sequence. Transit sequences contain little primary sequence similarity and therefore recognition of these sequences is thought to involve specific folding. To assess the conformational flexibility of the transit sequence, we studied the transit peptide of preferredoxin (trfd) by circular dichroism. In buffer, trfd is in a random coil conformation. A large increase in alpha-helix was induced in the presence of micelles or vesicles formed by anionic lipids. Less pronounced changes in secondary structure were induced by zwitterionic detergents but no changes were observed in the presence of neutral detergents or vesicles composed of phosphatidylcholine.

Intracellular translocation of a 28 kDa GTP-binding protein during osmotic shock-induced cell volume regulation in Dunaliella salina

The primary aim of this study was to determine if small GTP-binding proteins play a role in the conspicuous and much-examined volume control process in Dunaliella salina. We confirmed the previous identification by Rodriguez et al. (Rodriguez Rosales, M.P., Herrin, D.L. and Thompson, G.A., Jr. (1992) Plant Physiol. 98, 446-451) of small GTP-binding proteins in the green alga Dunaliella salina and revealed the presence of at least five such proteins, having molecular masses of approx. 21, 28, 28.5, 29 and 30 kDa. These proteins were concentrated largely in the endoplasmic reticulum (ER) and in an intermediate density organelle fraction (GA) containing mainly Golgi vesicles, mitochondria and flagella. The chloroplast fraction and plasma membrane contained the 28-kDa GTP-binding protein exclusively, while the cytosol contained both the 28-kDa component and small amounts of a 21-kDa GTP-binding protein. Immunodetection analysis showed that the D. salina 28-kDa protein cross-reacted strongly with a polyclonal antibody raised against a Volvox carteri yptV1 type GTP-binding protein. This antibody was utilized for quantitative GTP-binding protein measurements as described below. Certain anti-GTP-binding protein antibodies derived from non-plant sources, namely, monoclonal antibodies raised against yeast and mouse ypt1 GTP-binding proteins, cross-reacted not only with the D. salina 28-kDa protein but also the 29-kDa component. The 30-kDa GTP-binding protein of D. salina did not bind the antibodies mentioned above but did cross-react with an anti-yeast ypt1 polyclonal antibody. None of the D. salina GTP-binding proteins reacted positively with polyclonal antibodies raised against SEC4, rab1 or rab6 proteins. When D. salina cells were subjected to hypoosmotic swelling by abruptly reducing the NaCl concentration of their medium from 1.7 M to 0.85 M, the increase in cell surface area was accompanied by a substantial translocation of the 28-kDa GTP-binding protein from the ER and GA fractions to the plasma membrane, chloroplast and cytosolic fractions, as determined by quantitative [32P]GTP binding and [125I]antibody binding on nitrocellulose blots. This translocation increased the content of the 28-kDa component in the plasma membrane, chloroplast and cytosol by 3-4-fold. No net movement of the 30-kDa GTP-binding protein from either the ER or GA fractions was observed following hypoosmotic shock.(ABSTRACT TRUNCATED AT 400 WORDS)

Import of a mitochondrial presequence into protein-free phospholipid vesicles

A synthetic mitochondrial presequence has been shown to translocate across pure phospholipid bilayers. The presequence was fluorescently labeled so that its association with membranes could be monitored spectroscopically. In the presence of large unilamellar vesicles, the presequence showed time- and potential-dependent protection from reaction with added trypsin and dithionite. The protection was rapidly reversed by treatment of the vesicles with detergent. If the vesicles contained trypsin, the added presequence became sensitive to digestion by the protease. The results show that a mitochondrial presequence can translocate across phospholipid bilayers that lack a hydrophilic translocation pore.