Protein translocation at the envelope and thylakoid membranes of chloroplasts

Coherent X-Ray Diffraction Imaging of Chloroplasts from Cyanidioschyzon merolae by Using X-Ray Free Electron Laser

Coherent X-ray diffraction imaging (CXDI) is a lens-less technique for visualizing the structures of non-crystalline particles with the dimensions of submicrometer to micrometer at a resolution of several tens of nanometers. We conducted cryogenic CXDI experiments at 66 K to visualize the internal structures of frozen-hydrated chloroplasts of Cyanidioschyzon merolae using X-ray free electron laser (XFEL) as a coherent X-ray source. Chloroplast dispersed specimen disks at a number density of 7/(10×10 µm(2)) were flash-cooled with liquid ethane without staining, sectioning or chemical labeling. Chloroplasts are destroyed at atomic level immediately after the diffraction by XFEL pulses. Thus, diffraction patterns with a good signal-to-noise ratio from single chloroplasts were selected from many diffraction patterns collected through scanning specimen disks to provide fresh specimens into the irradiation area. The electron density maps of single chloroplasts projected along the direction of the incident X-ray beam were reconstructed by using the iterative phase-retrieval method and multivariate analyses. The electron density map at a resolution of 70 nm appeared as a C-shape. In addition, the fluorescence image of proteins stained with Flamingo™ dye also appeared as a C-shape as did the autofluorescence from Chl. The similar images suggest that the thylakoid membranes with an abundance of proteins distribute along the outer membranes of chloroplasts. To confirm the present results statistically, a number of projection structures must be accumulated through high-throughput data collection in the near future. Based on the results, we discuss the feasibility of XFEL-CXDI experiments in the structural analyses of cellular organelles.

Cytochrome c assembly

Cytochromes c are central proteins in energy transduction processes by virtue of their functions in electron transfer in respiration and photosynthesis. They have heme covalently attached to a characteristic CXXCH motif via protein-catalyzed post-translational modification reactions. Several systems with diverse constituent proteins have been identified in different organisms and are required to perform the heme attachment and associated functions. The necessary steps are translocation of the apocytochrome polypeptide to the site of heme attachment, transport and provision of heme to the appropriate compartment, reduction and chaperoning of the apocytochrome, and finally, formation of the thioether bonds between heme and two cysteines in the cytochrome. Here we summarize the established classical models for these processes and present recent progress in our understanding of the individual steps within the different cytochrome c biogenesis systems.

Plastocyanin transit peptide interacts with Potato virus X coat protein, while silencing of plastocyanin reduces coat protein accumulation in chloroplasts and symptom severity in host plants

Potato virus X coat protein (PVXCP) is, through communication with host proteins, involved in processes such as virus movement and symptom development. Here, we report that PVXCP also interacts with the precursor of plastocyanin, a protein involved in photosynthesis, both in vitro and in vivo. Yeast two-hybrid analysis indicated that PVXCP interacted with only the plastocyanin transit peptide. In subsequent bimolecular fluorescence complementation assays, both proteins were collocated within chloroplasts. Western blot analyses of chloroplast fractions showed that PVXCP could be detected in the envelope, stroma, and lumen fractions. Transmission electron microscopy demonstrated that grana were dilated in PVX-infected Nicotiana benthamiana. Furthermore, virus-induced gene silencing of plastocyanin by prior infection of N. benthamiana using a Tobacco rattle virus vector reduced the severity of symptoms that developed following subsequent PVX infection as well as the accumulation of PVXCP in isolated chloroplasts. However, PVXCP could not be detected in pea chloroplasts in an in vitro re-uptake assay using the plastocyanin precursor protein. Taken together, these data suggest that PVXCP interacts with the plastocyanin precursor protein and that silencing the expression of this protein leads to reduced PVXCP accumulation in chloroplasts and ameliorates symptom severity in host plants.

Protein import into chloroplasts: an ever-evolving storyThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Chloroplasts are but one type of a diverse group of essential organelles that distinguish plant cells and house many critical biochemical pathways, including photosynthesis. The biogenesis of plastids is essential to plant growth and development and relies on the targeting and import of thousands of nuclear-encoded proteins from the cytoplasm. The import of the vast majority of these proteins is dependent on translocons located in the outer and inner envelope membranes of the chloroplast, termed the Toc and Tic complexes, respectively. The core components of the Toc and Tic complexes have been identified within the last 12 years; however, the precise functions of many components are still being elucidated, and new components are still being identified. In Arabidopsis thaliana (and other species), many of the components are encoded by more than one gene, and it appears that the isoforms differentially associate with structurally distinct import complexes. Furthermore, it appears that these complexes represent functionally distinct targeting pathways, and the regulation of import by these separate pathways may play a role in the differentiation and specific functions of distinct plastid types during plant growth and development. This review summarizes these recent discoveries and emphasizes the mechanisms of differential Toc complex assembly and substrate recognition.

Protein folding

The problem of protein folding is that how proteins acquire their native unique three-dimensional structure in the physiological milieu. To solve the problem, the following key questions should be answered: do proteins fold co- or post-translationally, i.e. during or after biosynthesis, what is the mechanism of protein folding, and what is the explanation for fast folding of proteins? The two first questions are discussed in the current review. The general lines are to show that the opinion, that proteins fold after they are synthesized is hardly substantiated and suitable for solving the problem of protein folding and why proteins should fold cotranslationally. A possible tentative model for the mechanism of protein folding is also suggested. To this end, a thorough analysis is made of the biosynthesis, delivery to the folding compartments, and the rates of the biosynthesis, translocation and folding of proteins. A cursory attention is assigned to the role of GroEL/ES-like chaperonins in protein folding.

Thylakoid targeting of Tat passenger proteins shows no delta pH dependence in vivo

The Tat pathway is a major route for protein export in prokaryotes and for protein targeting to thylakoids in chloroplasts. Based on in vitro studies, protein translocation through this pathway is thought to be strictly dependent on a transmembrane delta pH. In this paper, we assess the delta pH sensitivity of the Tat pathway in vivo. Using Chlamydomonas reinhardtii, we observed changes in the efficiency of thylakoid targeting in vivo by mutating the Tat signal of the Rieske protein. We then employed two endogenous pH probes located on the lumen side of the thylakoid membranes to estimate spectroscopically the delta pH in vivo. Using experimental conditions in which the trans-thylakoid delta pH was almost zero, we found no evidence for a delta pH dependence of the Tat pathway in vivo. We confirmed this observation in higher plants using attached barley leaves. We conclude that the Tat pathway does not require a delta pH under physiological conditions, but becomes delta pH sensitive when probed in vitro/in organello because of the loss of some critical intracellular factors.

Eukaryotic transmembrane solute transport systems

A comprehensive classification system for transmembrane molecular transporters has been proposed. This system is based on (i) mode of transport and energy-coupling mechanism, (ii) protein phylogenetic family, (iii) phylogenetic cluster, and (iv) substrate specificity. The proposed "Transport Commission" (TC) system is superficially similar to that implemented decades ago by the Enzyme Commission for enzymes, but it differs from the latter system in that it uses phylogenetic and functional data for classification purposes. Very few families of transporters include members that do not function exclusively in transport. Analyses reported reveal that channels, primary carriers, secondary carriers (uni-, sym-, and antiporters), and group translocators comprise distinct categories of transporters, and that transport mode and energy coupling are relatively immutable characteristics. By contrast, substrate specificity and polarity of transport are often readily mutable. Thus, with very few exceptions, a unified family of transporters includes members that function by a single transport mode and energy-coupling mechanism although a variety of substrates may be transported with either inwardly or outwardly directed polarity. The TC system allows cross-referencing according to substrates transported and protein sequence database accession numbers. Thus, familial assignments of newly sequenced transport proteins are facilitated. In this article I examine families of transporters that are eukaryotic specific. These families include (i) channel proteins, mostly from animals; (ii) facilitators and secondary active transport carriers; (iii) a few ATP-dependent primary active transporters; and (iv) transporters of unknown mode of action or energy-coupling mechanism. None of the several ATP-independent primary active transport energy-coupling mechanisms found in prokaryotes is represented within the eukaryotic-specific families. The analyses reported provide insight into transporter families that may have arisen in eukaryotes after the separation of eukaryotes from archaea and bacteria. On the basis of the reported analyses, it is suggested that the horizontal transfer of genes encoding transport proteins between eukaryotes and members of the other two domains of life occurred very infrequently during evolutionary history.

Abietadiene synthase from grand fir (Abies grandis): characterization and mechanism of action of the "pseudomature" recombinant enzyme

The oleoresin secreted by grand fir (Abies grandis) is composed of resin acids derived largely from the abietane family of diterpene olefins as precursors which undergo subsequent oxidation of the C18-methyl group to a carboxyl function, for example, in the conversion of abieta-7,13-diene to abietic acid. A cDNA encoding abietadiene synthase has been isolated from grand fir and the heterologously expressed bifunctional enzyme shown to catalyze both the protonation-initiated cyclization of geranylgeranyl diphosphate to the intermediate (+)-copalyl diphosphate and the ionization-dependent cyclization of (+)-copalyl diphosphate, via a pimarenyl intermediate, to the olefin end products. Abietadiene synthase is translated as a preprotein bearing an N-terminal plastidial targeting sequence, and this form of the recombinant protein expressed in Escherichia coli proved to be unsuitable for detailed structure-function studies. Since the transit peptide-mature protein cleavage site could not be determined directly, a truncation series was constructed to delete the targeting sequence and prepare a "pseudomature" form of the enzyme that resembled the native abietadiene synthase in kinetic properties. Both the native synthase and the pseudomature synthase having 84 residues deleted from the preprotein converted geranylgeranyl diphosphate and the intermediate (+)-copalyl diphosphate to a nearly equal mixture of abietadiene, levopimaradiene, and neoabietadiene, as well as to three minor products, indicating that this single enzyme accounts for production of all of the resin acid precursors of grand fir. Kinetic evaluation of abietadiene synthase with geranylgeranyl diphosphate and (+)-copalyl diphosphate provided evidence for two functionally distinct active sites, the first for the cyclization of geranylgeranyl diphosphate to (+)-copalyl diphosphate and the second for the cyclization of (+)-copalyl diphosphate to diterpene end products, and demonstrated that the rate-limiting step of the coupled reaction sequence resides in the second cyclization process. The structural implications of these findings are discussed in the context of primary sequence elements considered to be responsible for binding the substrate and intermediate and for initiating the respective cyclization steps.

Heterologous expression and characterization of a "Pseudomature" form of taxadiene synthase involved in paclitaxel (Taxol) biosynthesis and evaluation of a potential intermediate and inhibitors of the multistep diterpene cyclization reaction

The diterpene cyclase taxadiene synthase from yew (Taxus) species transforms geranylgeranyl diphosphate to taxa-4(5),11(12)-diene as the first committed step in the biosynthesis of the anti-cancer drug Taxol. Taxadiene synthase is translated as a preprotein bearing an N-terminal targeting sequence for localization to and processing in the plastids. Overexpression of the full-length preprotein in Escherichia coli and purification are compromised by host codon usage, inclusion body formation, and association with host chaperones, and the preprotein is catalytically impaired. Since the transit peptide-mature enzyme cleavage site could not be determined directly, a series of N-terminally truncated enzymes was created by expression of the corresponding cDNAs from a suitable vector, and each was purified and kinetically evaluated. Deletion of up to 79 residues yielded functional protein; however, deletion of 93 or more amino acids resulted in complete elimination of activity, implying a structural or catalytic role for the amino terminus. The pseudomature form of taxadiene synthase having 60 amino acids deleted from the preprotein was found to be superior with respect to level of expression, ease of purification, solubility, stability, and catalytic activity with kinetics comparable to the native enzyme. In addition to the major product, taxa-4(5),11(12)-diene (94%), this enzyme produces a small amount of the isomeric taxa-4(20), 11(12)-diene ( approximately 5%), and a product tentatively identified as verticillene ( approximately 1%). Isotopically sensitive branching experiments utilizing (4R)-[4-(2)H(1)]geranylgeranyl diphosphate confirmed that the two taxadiene isomers, and a third (taxa-3(4),11(12)-diene), are derived from the same intermediate taxenyl C4-carbocation. These results, along with the failure of the enzyme to utilize 2, 7-cyclogeranylgeranyl diphosphate as an alternate substrate, indicate that the reaction proceeds by initial ionization of the diphosphate ester and macrocyclization to the verticillyl intermediate, followed by a secondary cyclization to the taxenyl cation and deprotonation (i.e., formation of the A-ring prior to B/C-ring closure). Two potential mechanism-based inhibitors were tested with recombinant taxadiene synthase but neither provided time-dependent inactivation nor afforded more than modest competitive inhibition.

Regulation of lamp2a levels in the lysosomal membrane

The selective degradation of cytosolic proteins in lysosomes by chaperone-mediated autophagy depends, at least in part, on the levels of a substrate receptor at the lysosomal membrane. We have previously identified this receptor as the lysosome-associated membrane protein type 2a (lamp2a) and showed that levels of lamp2a at the lysosomal membrane directly correlate with the activity of the proteolytic pathway. Here we show that levels of lamp2a at the lysosomal membrane are mainly controlled by changes in its half-life and its distribution between the lysosomal membrane and the matrix. The lysosomal degradation of lamp2a requires the combined action of at least two different proteolytic activities at the lysosomal membrane. Lamp2a is released from the membrane by the action of these proteases, and then the truncated lamp2a is rapidly degraded within the lysosomal matrix. Membrane degradation of lamp2a is a regulated process that is inhibited in the presence of substrates for chaperone-mediated autophagy and under conditions that activate that type of autophagy. Uptake of substrate proteins also results in transport of some intact lamp2a from the lysosomal membrane into the matrix. This fraction of lamp2a can be reinserted back into the lysosomal membrane. The traffic of lamp2a through the lysosomal matrix is not mediated by vesicles, and lamp2a reinsertion requires the lysosomal membrane potential and protein components of the lysosomal membrane. The distribution of lamp2a between the lysosomal membrane and matrix is a dynamic process that contributes to the regulation of lysosomal membrane levels of lamp2a and consequently to the activity of the chaperone-mediated autophagic pathway.

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.

Isolation and characterisation of the cDNA encoding a glycosylated accessory protein of pea chloroplast DNA polymerase

The cDNA encoding p43, a DNA binding protein from pea chloroplasts (ct) that binds to cognate DNA polymerase and stimulates the polymerase activity, has been cloned and characterised. The characteristic sequence motifs of hydroxyproline-rich glyco-proteins (HRGP) are present in the cDNA corres-ponding to the N-terminal domain of the mature p43. The protein was found to be highly O-arabinosylated. Chemically deglycosylated p43 (i.e. p29) retains its binding to both DNA and pea ct-DNA polymerase but fails to stimulate the DNA polymerase activity. The mature p43 is synthesised as a pre-p43 protein containing a 59 amino acid long transit peptide which undergoes stromal cleavage as evidenced from the post-translational in vitro import of the precursor protein into the isolated intact pea chloroplasts. Surprisingly, p43 is found only in pea chloroplasts. The unique features present in the cloned cDNA indicate that p43 is a novel member of the HRGP family of proteins. Besides p43, no other DNA-polymerase accessory protein with O-glycosylation has been reported yet.

The isolated complex of the translocase of the outer membrane of mitochondria. Characterization of the cation-selective and voltage-gated preprotein-conducting pore

The complex of the translocase mitochondrial outer membrane (TOM), mediates recognition, unfolding, and translocation of preproteins. We have used a combination of biochemical and electrophysiological methods to study the properties of the preprotein-conducting pore of the purified TOM complex. The pore is cation-selective and voltage-gated. It shows three main conductance levels with characteristic slow and fast kinetics transitions to states of lower conductance following application of transmembrane voltages. These electrical properties distinguish it from the mitochondrial voltage-dependent anion channel (porin) and are identical to those of the previously described peptide-sensitive channel. Binding of antibodies to the C terminus of Tom40 on the intermembrane space side of the outer membrane modifies the channel properties and allows determination of the orientation of the channel within the lipid bilayer. Mitochondrial presequence peptides specifically interact with the pore and decrease the ion flow through the channel in a voltage-dependent manner. We propose that the presequence-induced closures of the pore are related to structural alterations of the TOM complex observed during the various stages of preprotein movement across the mitochondrial outer membrane.

Truncation of limonene synthase preprotein provides a fully active 'pseudomature' form of this monoterpene cyclase and reveals the function of the amino-terminal arginine pair

The monoterpene cyclase limonene synthase transforms geranyl diphosphate to a monocyclic olefin and constitutes the simplest model for terpenoid cyclase catalysis. (-)-4S-Limonene synthase preprotein from spearmint bears a long plastidial targeting sequence. Difficulty expressing the full-length preprotein in Escherichia coli is encountered because of host codon usage, inclusion body formation, and the tight association of bacterial chaperones with the transit peptide. The purified preprotein is also kinetically impaired relative to the mixture of N-blocked native proteins produced in vivo by proteolytic processing in plastids. Therefore, the targeting sequence, that precedes a tandem pair of arginines (R58R59) which is highly conserved in the monoterpene synthases, was removed. Expression of this truncated protein, from a vector that encodes a tRNA for two rare arginine codons (pSBET), affords a soluble, tractable 'pseudomature' form of the enzyme that is catalytically more efficient than the native species. Truncation up to and including R58, or substitution of R59, yields enzymes that are incapable of converting the natural substrate geranyl diphosphate, via the enzymatically formed tertiary allylic isomer 3S-linalyl diphosphate, to (-)-limonene. However, these enzymes are able to cyclize exogenously supplied 3S-linalyl diphosphate to the olefinic product. This result indicates a role for the tandem arginines in the unique diphosphate migration step accompanying formation of the intermediate 3S-linalyl diphosphate and preceding the final cyclization reaction catalyzed by the monoterpene synthases. The structural basis for this coupled isomerization-cyclization reaction sequence can be inferred by homology modeling of (-)-4S-limonene synthase based on the three-dimensional structure of the sesquiterpene cyclase epi-aristolochene synthase [Starks, C. M., Back, K., Chappell, J., and Noel, J. P. (1997) Science 277, 1815-1820].

A cell-free translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa

A translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa reproduces in vitro translocation and processing of a secretory protein. The translation extract was isolated from the wall-less variant by gently lysing cells by a freeze-thaw procedure. This method yielded more extract then the method developed previously (R. Addison, J. Biol. Chem. 262: 17031, 1987) as well as reducing microsomal contamination. The microsomal fraction was isolated from lysed cells using a series of discontinuous sucrose gradients. The resultant microsomes were less inhibitory to translation of various transcripts and consisted of a more homogenous mixture of vesicles then microsomes prepared previously. Polyclonal antibodies directed against a polypeptide of approximately 75 kDa from the microsomes were used in indirect-immunofluorescence microscopy. The resultant fluorescent pattern shows a network of tubulo-reticular structures in a juxtanuclear region, which is the pattern expected of the rough endoplasmic reticulum.

Tip loci: six Chlamydomonas nuclear suppressors that permit the translocation of proteins with mutant thylakoid signal sequences

Mutations within the signal sequence of cytochrome f (cytf) in Chlamydomonas inhibit thylakoid membrane protein translocation and render cells nonphotosynthetic. Twenty-seven suppressors of the mutant signal sequences were selected for their ability to restore photoautotrophic growth and these describe six nuclear loci named tip1 through 6 for thylakoid insertion protein. The tip mutations restore the translocation of cytf and are not allele specific, as they suppress a number of different cytf signal sequence mutations. Tip5 and 2 may act early in cytf translocation, while Tip1, 3, 4, and 6 are engaged later. The tip mutations have no phenotype in the absence of a signal sequence mutation and there is genetic interaction between tip4, and tip5 suggesting an interaction of their encoded proteins. As there is overlap in the energetic, biochemical and genetic requirements for the translocation of nuclear and chloroplast-encoded thylakoid proteins, the tip mutations likely identify components of a general thylakoid protein translocation apparatus.

Molecular mechanisms of cytochrome c biogenesis: three distinct systems

The past 10 years have heralded remarkable progress in the understanding of the biogenesis of c-type cytochromes. The hallmark of c-type cytochrome synthesis is the covalent ligation of haem vinyl groups to two cysteinyl residues of the apocytochrome (at a Cys-Xxx-Yyy-Cys-His signature motif). From genetic, genomic and biochemical studies, it is clear that three distinct systems have evolved in nature to assemble this ancient protein. In this review, common principles of assembly for all systems and the molecular mechanisms predicted for each system are summarized. Prokaryotes, plant mitochondria and chloroplasts use either system I or II, which are each predicted to use dedicated mechanisms for haem delivery, apocytochrome ushering and thioreduction. Accessory proteins of systems I and II co-ordinate the positioning of these two substrates at the membrane surface for covalent ligation. The third system has evolved specifically in mitochondria of fungi, invertebrates and vertebrates. For system III, a pivotal role is played by an enzyme called cytochrome c haem lyase (CCHL) in the mitochondrial intermembrane space.

The hydrophilic domain of Tic110, an inner envelope membrane component of the chloroplastic protein translocation apparatus, faces the stromal compartment

It has previously been found that Tic110, an integral protein of the chloroplast inner envelope membrane, is a component of the chloroplastic protein import apparatus. However, conflicting reports exist concerning the topology of this protein within the inner envelope membrane. In this report, we provide evidence that indicates that the large (>90-kDa) hydrophilic domain of Tic110 is localized within the chloroplast stroma. Trypsin, a protease that cannot penetrate the permeability barrier of the inner envelope membrane, degrades neither Tic110 nor other proteins exposed to the stromal compartment but is able to digest proteins exposed to the intermembrane space between the two envelope membranes. Previous reports indicating that trypsin is able to degrade Tic110 were influenced by incomplete quenching of protease activity. When trypsin is not sufficiently quenched, it is able to digest Tic110, but only after chloroplasts have been ruptured. It is therefore necessary to employ adequate quenching protocols, such as the one reported here, whenever trypsin is utilized as an analytical tool. Based on a stromal localization for the majority of Tic110, we propose that this protein may be involved in the recruitment of stromal factors, possibly molecular chaperones, to the translocation apparatus during protein import.

PROTEIN TARGETING TO THE THYLAKOID MEMBRANE

▪ Abstract  The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.

Evidence for a loop mechanism of protein transport by the thylakoid Delta pH pathway

The thylakoid Delta pH pathway is a protein transport system with unprecedented characteristics. To investigate its mechanism, the topology of precursor insertion was determined. A fusion protein comprising a large polypeptide domain fused to the amino terminus of pOE17 (a Delta pH pathway precursor) was efficiently processed by thylakoid membranes. The amino terminus, including the targeting peptide, remained on the cis side of the membrane. Mature OE17 was transported to the lumen. These experiments demonstrate that Delta pH directed precursors enter the thylakoid membrane in a loop, implying that the Delta pH pathway has evolved from an export-type protein translocation system.

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.

Mitochondrial protein import. Tom40 plays a major role in targeting and translocation of preproteins by forming a specific binding site for the presequence

During preprotein transport across the mitochondrial outer membrane, the N-terminal presequence initially binds to a surface-exposed site, termed cis site, of the protein translocation complex of this membrane (the TOM complex). The presequence then moves into the translocation pore and becomes exposed at the intermembrane space side. Membrane passage is driven by specific interaction of the presequence with the trans site. We have used chemical cross-linking to identify components in the vicinity of the translocating presequence. Preproteins bound to the surface-exposed cis site can be cross-linked via their N-terminal presequence to Tom20 and Tom22, demonstrating their direct association with this part of the preprotein. In addition, the presequence establishes an early contact to Tom40, a membrane-embedded protein of the TOM complex. Upon further entry of the preprotein into the translocation pore, the presequence loses its contact with Tom20/Tom22, but remains in firm association with Tom40. Our study suggests that Tom40 plays an important function in guiding the presequence of a preprotein across the mitochondrial outer membrane. We propose that Tom40 forms a major part of the trans presequence binding site.