Photoaffinity Labeling of Mature and Precursor Forms of the Small Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase after Expression in Escherichia coli

Non-native, N-terminal Hsp70 molecular motor recognition elements in transit peptides support plastid protein translocation

Previously, we identified the N-terminal domain of transit peptides (TPs) as a major determinant for the translocation step in plastid protein import. Analysis of Arabidopsis TP dataset revealed that this domain has two overlapping characteristics, highly uncharged and Hsp70-interacting. To investigate these two properties, we replaced the N-terminal domains of the TP of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and its reverse peptide with a series of unrelated peptides whose affinities to the chloroplast stromal Hsp70 have been determined. Bioinformatic analysis indicated that eight out of nine peptides in this series are not similar to the TP N terminus. Using in vivo and in vitro protein import assays, the majority of the precursors containing Hsp70-binding elements were targeted to plastids, whereas none of the chimeric precursors lacking an N-terminal Hsp70-binding element were targeted to the plastids. Moreover, a pulse-chase assay showed that two chimeric precursors with the most uncharged peptides failed to translocate into the stroma. The ability of multiple unrelated Hsp70-binding elements to support protein import verified that the majority of TPs utilize an N-terminal Hsp70-binding domain during translocation and expand the mechanistic view of the import process. This work also indicates that synthetic biology may be utilized to create de novo TPs that exceed the targeting activity of naturally occurring sequences.

Cytosolic HSP90 cochaperones HOP and FKBP interact with freshly synthesized chloroplast preproteins of Arabidopsis

Most chloroplast and mitochondrial proteins are synthesized in the cytosol of the plant cell and have to be imported into the organelles post-translationally. Molecular chaperones play an important role in preventing protein aggregation of freshly translated preproteins and assist in maintaining the preproteins in an import competent state. Preproteins can associate with HSP70, HSP90, and 14-3-3 proteins in the cytosol. In this study, we analyzed a large set of wheat germ-translated chloroplast preproteins with respect to their chaperone binding. Our results demonstrate that the formation of distinct 14-3-3 or HSP90 containing preprotein complexes is a common feature in post-translational protein transport in addition to preproteins that seem to interact solely with HSP70. We were able to identify a diverse and extensive class of preproteins as HSP90 substrates, thus providing a tool for the investigation of HSP90 client protein association. The analyses of chimeric HSP90 and 14-3-3 binding preproteins with exchanged transit peptides indicate an involvement of both the transit peptide and the mature part of the proteins, in HSP90 binding. We identified two partner components of the HSP90 cycle, which were present in the preprotein containing high-molecular-weight complexes, the HSP70/HSP90 organizing protein HOP, as well as the immunophilin FKBP73. The results establish chloroplast preproteins as a general class of HSP90 client proteins in plants using HOP and FKBP as novel cochaperones.

Nano-scale characterization of the dynamics of the chloroplast Toc translocon

Translocons are macromolecular nano-scale machines that facilitate the selective translocation of proteins across membranes. Although common in function, different translocons have evolved diverse molecular mechanisms for protein translocation. Subcellular organelles of endosymbiotic origin such as the chloroplast and mitochondria had to evolve/acquire translocons capable of importing proteins whose genes were transferred to the host genome. These gene products are expressed on cytosolic ribosomes as precursor proteins and targeted back to the organelle by an N-terminal extension called the transit peptide or presequence. In chloroplasts the transit peptide is specifically recognized by the Translocon of the Outer Chloroplast membrane (Toc) which is composed of receptor GTPases that potentially function as gate-like switches, where GTP binding and hydrolysis somehow facilitate preprotein binding and translocation. Compared to other translocons, the dynamics of the Toc translocon are probably more complex and certainly less understood. We have developed biochemical/biophysical, imaging, and computational techniques to probe the dynamics of the Toc translocon at the nanoscale. In this chapter we provide detailed protocols for kinetic and binding analysis of precursor interactions in organeller, measurement of the activity and nucleotide binding of the Toc GTPases, native electrophoretic analysis of the assembly/organization of the Toc complex, visualization of the distribution and mobility of Toc apparatus on the surface of chloroplasts, and conclude with the identification and molecular modeling Toc75 POTRA domains. With these new methodologies we discuss future directions of the field.

The gene for the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit relocated to the plastid genome of tobacco directs the synthesis of small subunits that assemble into Rubisco

To assess the extent to which a nuclear gene for a chloroplast protein retained the ability to be expressed in its presumed preendosymbiotic location, we relocated the RbcS gene for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to the tobacco plastid genome. Plastid RbcS transgenes, both with and without the transit presequence, were equipped with 3' hepta-histidine-encoding sequences and psbA promoter and terminator elements. Both transgenes were transcribed abundantly, and their products were translated into small subunit polypeptides that folded correctly and assembled into the Rubisco hexadecamer. When present, either the transit presequence was not translated or the transit peptide was cleaved completely. After assembly into Rubisco, transplastomic small subunits were relatively stable. The hepta-histidine sequence fused to the C terminus of a single small subunit was sufficient for isolation of the whole Rubisco hexadecamer by Ni(2)+ chelation. Small subunits produced by the plastid transgenes were not abundant, never exceeding approximately 1% of the total small subunits, and they differed from cytoplasmically synthesized small subunits in their N-terminal modifications. The scarcity of transplastomic small subunits might be caused by inefficient translation or assembly.

A mammalian cytochrome fused to a chloroplast transit peptide is a functional haemoprotein and is imported into isolated chloroplasts

The small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a major chloroplast stromal protein that is cytosolically synthesized as a precursor with an N-terminal extension, known as the transit sequence or transit peptide (Tp). The Tp is essential for the post-translational uptake of the precursor by the chloroplast. The Tp is thought to influence the conformation of the precursor protein and to facilitate polypeptide translocation across the chloroplast envelope barrier via a Tp-selective translocon. To address these issues we have devised a novel strategy to generate substrate amounts of a chloroplast targeting sequence as a fusion with the chromogenic globular domain of cytochrome b(5) (Cyt). The chimaeric protein is an ideal probe for investigating the conformation of a preprotein and events surrounding protein import into isolated chloroplasts. The Cyt of liver endoplasmic reticulum was fused at its N-terminus with the Tp of the small subunit of Rubisco of Pisum sativum (pea). To enhance its production by clearance from the cytoplasm of Escherichia coli, the chimaera was engineered by further N-terminal linkage of a prokaryotic secretory signal. Expression of this tripartite fusion resulted in mg quantities of the signal sequence-processed Tp-Cyt protein, which was eventually targeted to the membranes. The chromogenic nature of the chimaera and its localization to the bacterial membrane facilitated the biochemical isolation of the precursor in a soluble and functional form. The purified preprotein displayed spectral and enzymic properties that were indistinguishable from the native parental Cyt, implying an absence of observable influence of the Tp on the conformation of the haemoprotein. The chimaeric precursor was imported into the stroma of the isolated chloroplasts in a dose-dependent manner. Import was also strongly dependent upon exogenously supplied ATP. The stromally imported chimaeric precursor protein was processed to a size characteristic of Cyt.

A chloroplast envelope-transfer sequence functions as an export signal in Escherichia coli

The small subunit precursor of pea ribulose-1,5-bisphosphate carboxylase/oxygenase engineered with prokaryotic elements was expressed in Escherichia coli. This resulted in a dependable level of synthesis of the precursor protein in E. coli. The bacterially synthesised plant precursor protein was translocated from the cytoplasm and targeted to the outer membrane of the envelope zone. During the translocation step, a significant proportion of the precursor was processed to a soluble, mature SSU and found localised in the periplasm. The determined amino acid sequence of the isolated precursor showed that it had a deletion of an arginine residue at position -15 in the transit peptide. Expression of this transit peptide-appended mammalian cytochrome b(5) in E. coli displayed a targeting profile of the chromogenic chimera that was similar to that observed with the plant precursor protein.

14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants

Transit sequences of chloroplast-destined precursor proteins are phosphorylated on a serine or threonine residue. The amino acid motif around the phosphorylation site is related to the phosphopeptide binding motif for 14-3-3 proteins. Plant 14-3-3 proteins interact specifically with wheat germ lysate-synthesized chloroplast precursor proteins and require an intact phosphorylation motif within the transit sequence. Chloroplast precursor proteins do not interact with 14-3-3 when synthesized in the heterologous reticulocyte lysate. In contrast, a precursor protein destined for plant mitochondria was found to be associated with 14-3-3 proteins present in the reticulocyte lysate but not with 14-3-3 from wheat germ lysate. This indicates an unrecognized selectivity of 14-3-3 proteins for precursors from mitochondria and plastids in plants in comparison to fungi and animals. The heterooligomeric complex has an apparent size of 200 kD. In addition to the precursor protein, it contains 14-3-3 (probably as a dimer) and a heat shock protein Hsp70 isoform. Dissociation of the precursor complex requires ATP. Protein import experiments of precursor from the oligomeric complex into intact pea chloroplasts reveal three- to fourfold higher translocation rates compared with the free precursor, which is not complexed. We conclude that the 14-3-3-Hsp70-precursor protein complex is a bona fide intermediate in the in vivo protein import pathway in plants.

14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants

Transit sequences of chloroplast-destined precursor proteins are phosphorylated on a serine or threonine residue. The amino acid motif around the phosphorylation site is related to the phosphopeptide binding motif for 14-3-3 proteins. Plant 14-3-3 proteins interact specifically with wheat germ lysate-synthesized chloroplast precursor proteins and require an intact phosphorylation motif within the transit sequence. Chloroplast precursor proteins do not interact with 14-3-3 when synthesized in the heterologous reticulocyte lysate. In contrast, a precursor protein destined for plant mitochondria was found to be associated with 14-3-3 proteins present in the reticulocyte lysate but not with 14-3-3 from wheat germ lysate. This indicates an unrecognized selectivity of 14-3-3 proteins for precursors from mitochondria and plastids in plants in comparison to fungi and animals. The heterooligomeric complex has an apparent size of 200 kD. In addition to the precursor protein, it contains 14-3-3 (probably as a dimer) and a heat shock protein Hsp70 isoform. Dissociation of the precursor complex requires ATP. Protein import experiments of precursor from the oligomeric complex into intact pea chloroplasts reveal three- to fourfold higher translocation rates compared with the free precursor, which is not complexed. We conclude that the 14-3-3-Hsp70-precursor protein complex is a bona fide intermediate in the in vivo protein import pathway in plants.

The C terminus of a chloroplast precursor modulates its interaction with the translocation apparatus and PIRAC

The import of proteins into chloroplasts involves a cleavable, N-terminal targeting sequence known as the transit peptide. Although the transit peptide is both necessary and sufficient to direct precursor import into chloroplasts, the mature domain of some precursors has been shown to modulate targeting and translocation efficiency. To test the influence of the mature domain of the small subunit of Rubisco during import in vitro, the precursor (prSSU), the mature domain (mSSU), the transit peptide (SS-tp), and three C-terminal deletion mutants (Delta52, Delta67, and Delta74) of prSSU were expressed and purified from Escherichia coli. Activity was then evaluated by competitive import of (35)S-prSSU. Both IC(50) and K(i) values consistently suggest that removal of C-terminal prSSU sequences inhibits its interaction with the translocation apparatus. Non-competitive import studies demonstrated that prSSU and Delta52 were properly processed and accumulated within the chloroplast, whereas Delta67 and Delta74 were rapidly degraded via a plastid-localized protease. The ability of prSSU-derived proteins to induce inactivation of the protein-import-related anion channel was also evaluated. Although the C-terminal deletion mutants were less effective at inducing channel closure upon import, they did not effect the mean duration of channel closure. Possible mechanisms by which C-terminal residues of prSSU modulate chloroplast targeting are discussed.

Positive charges determine the topology and functionality of the transmembrane domain in the chloroplastic outer envelope protein Toc34

The chloroplastic outer envelope protein Toc34 is inserted into the membrane by a COOH-terminal membrane anchor domain in the orientation Ncyto-Cin. The insertion is independent of ATP and a cleavable transit sequence. The cytosolic domain of Toc34 does not influence the insertion process and can be replaced by a different hydrophilic reporter peptide. Inversion of the COOH-terminal, 45-residue segment, including the membrane anchor domain (Toc34Cinv), resulted in an inverted topology of the protein, i.e., Nin-Ccyto. A mutual exchange of the charged amino acid residues NH2- and COOH-proximal of the hydrophobic alpha-helix indicates that a double-positive charge at the cytosolic side of the transmembrane alpha-helix is the sole determinant for its topology. When the inverted COOH-terminal segment was fused to the chloroplastic precursor of the ribulose-1,5-bisphosphate carboxylase small subunit (pS34Cinv), it engaged the transit sequence-dependent import pathway. The inverted peptide domain of Toc34 functions as a stop transfer signal and is released out of the outer envelope protein translocation machinery into the lipid phase. Simultaneously, the NH2-terminal part of the hybrid precursor remained engaged in the inner envelope protein translocon, which could be reversed by the removal of ATP, demonstrating that only an energy-dependent force but no further ionic interactions kept the precursor in the import machinery.

Reconstitution of a chloroplast protein import channel

The chloroplastic outer envelope protein OEP75 with a molecular weight of 75 kDa probably forms the central pore of the protein import machinery of the outer chloroplastic membrane. Patch-clamp analysis shows that heterologously expressed, purified and reconstituted OEP75 constitutes a voltage-gated ion channel with a unit conductance of Lambda = 145pS. Activation of the OEP75 channel in vitro is completely dependent on the magnitude and direction of the voltage gradient. Therefore, movements of protein charges of parts of OEP75 in the membrane electric field are required either for pore formation or its opening. In the presence of precursor protein from only one side of the bilayer, strong flickering and partial closing of the channel was observed, indicating a specific interaction of the precursor with OEP75. The comparatively low ionic conductance of OEP75 is compatible with a rather narrow aqueous pore (dporeapproximately equal to 8-9 A). Provided that protein and ion translocation occur through the same pore, this implies that the environment of the polypeptide during the transit is mainly hydrophilic and that protein translocation requires almost complete unfolding of the precursor.

A nuclear-coded chloroplastic inner envelope membrane protein uses a soluble sorting intermediate upon import into the organelle

The chloroplastic inner envelope protein of 110 kD (IEP110) is part of the protein import machinery in the pea. Different hybrid proteins were constructed to assess the import and sorting pathway of IEP110. The IEP110 precursor (pIEP110) uses the general import pathway into chloroplasts, as shown by the mutual exchange of presequences with the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSSU). Sorting information to the chloroplastic inner envelope is contained in an NH2-proximal part of mature IEP110 (110N). The NH2-terminus serves to anchor the protein into the membrane. Large COOH-terminal portions of this protein (80-90 kD) are exposed to the intermembrane space in situ. Successful sorting and integration of IEP110 and the derived constructs into the inner envelope are demonstrated by the inaccessability of processed mature protein to the protease thermolysin but accessibility to trypsin, i.e., the imported protein is exposed to the intermembrane space. A hybrid protein consisting of the transit sequence of SSU, the NH2-proximal part of mature IEP110, and mature SSU (tpSSU-110N-mSSU) is completely imported into the chloroplast stroma, from which it can be recovered as soluble, terminally processed 110NmSSU. The soluble 110N-mSSU then enters a reexport pathway, which results not only in the insertion of 110N-mSSU into the inner envelope membrane, but also in the extrusion of large portions of the protein into the intermembrane space. We conclude that chloroplasts possess a protein reexport machinery for IEPs in which soluble stromal components interact with a membrane-localized translocation machinery.

Identification of a translocation intermediate occupying functional protein import sites in the chloroplastic envelope membrane

We used complexes of avidin and biotinylated precursors to generate translocation intermediates that occupy functional transport sites and thereby block the transport of other precursor proteins into pea chloroplasts. Cysteine residues of purified precursor to the small subunit of rubisco (prSS) were modified with the biotinylation reagent biotin-1-biotinamido-4-[-4'-(maleimidomethyl)-cyclohexane-ca rboxamido ]butane. Chemically biotinylated prSS was readily imported into chloroplasts. The addition of avidin, however, resulted in the formation of an avidin-biotinylated precursor complex that could not be imported into chloroplasts even when precursors had already engaged the transport apparatus before avidin was added. On fractionation, the avidin-biotinylated precursor complex associated with envelope membranes. Titration of transport sites with avidin-biotinylated precursor complexes revealed that saturation was reached at 2,000 molecules/chloroplast. Even with less than saturating levels of complexes, a sufficient number of translocation sites could be occupied with avidin-precursor complexes so that the import rate of freshly added radiolabeled prSS was reduced by 35%. From these observations we conclude that the trapped intermediates were blocking functional translocation sites. These biotinylated translocation intermediates should be useful in future efforts to purify and characterize the chloroplastic protein import machinery.

Altered acyl chain length specificity of Rhizopus delemar lipase through mutagenesis and molecular modeling

The acyl binding site of Rhizopus delemar prolipase and mature lipase was altered through site-directed mutagenesis to improve lipase specificity for short- or medium-chain length fatty acids. Computer-generated structural models of R. delemar lipase were used in mutant protein design and in the interpretation of the catalytic properties of the resulting recombinant enzymes. Molecular dynamics simulations of the double mutant, val209trp + phe112trp, predicted that the introduction of trp112 and trp209 in the acyl binding groove would sterically hinder the docking of fatty acids longer than butyric acid. Assayed against a mixture of triacylglycerol substrates, the val209trp + phe112trp mature lipase mutant showed an 80-fold increase in the hydrolysis of tributyrin relative to the hydrolysis of tricaprylin while no triolein hydrolysis was detected. By comparison, the val94Trp mutant, predicted to pose steric or geometric constraints for docking fatty acids longer than caprylic acid in the acyl binding groove, resulted in a modest 1.4-fold increase in tricaprylin hydrolysis relative to the hydrolysis of tributyrin. Molecular models of the double mutant phe95asp + phe214arg indicated the creation of a salt bridge between asp95 and arg214 across the distal end of the acyl binding groove. When challenged with a mixture of triacylglycerols, the phe95asp + phe214arg substitutions resulted in an enzyme with 3-fold enhanced relative activity for tricaprylin compared to triolein, suggesting that structural determinants for medium-chain length specificity may reside in the distal end of the acyl binding groove. Attempts to introduce a salt bridge within 8 A of the active site by the double mutation leu146lys + ser115asp destroyed catalytic activity entirely. Similarly, the substitution of polar Gln at the rim of the acyl binding groove for phe112 largely eliminated catalytic activity of the lipase.

In vitro interaction between a chloroplast transit peptide and chloroplast outer envelope lipids is sequence-specific and lipid class-dependent

Interaction of artificial lipid bilayers (liposomes) with the purified transit peptide (SS-tp) of the precursor form of the small subunit for ribulose-2,5-bisphosphate carboxylase/oxygenase (prSSU) has been studied using a vesicle-disruption assay (calcein dye release) and electron microscopy. Employing purified forms of Escherichia coli-expressed prSSU, mature small subunit, glutathione S-transferase-transit peptide fusion protein, and SS-tp in dye release studies demonstrated that lipid interaction is mediated primarily through the transit peptide. Using chemically synthesized peptides (20-mers), the lipid-interacting domain of the transit peptide was partially mapped to the C-terminal 20 amino acids of the transit peptide. Peptides corresponding to other regions of the transit peptide and control peptides promoted significantly less calcein release. Interaction between the transit peptide and the bilayer was very rapid and could not be resolved by stopped-flow fluorometry with a mixing time of <50 ms. Interaction between the peptides and bilayer was also lipid class-dependent. Disruption occurred only when the bilayer contained the galactolipid monogalactosyldiacylglycerol (MGDG). The extent of bilayer disruption directly correlated with the relative concentration of MGDG in the liposome, with maximum calcein release occurring in 20 mol % MGDG liposomes. Lipid bilayers with greater than 20 mol % MGDG could not be achieved as determined by calcein entrapment. Electron microscopy of the liposomes before and after addition of the transit peptide suggested that the transit peptide induced a dramatic reorganization of lipids. These results are discussed in light of a possible mechanism for the early steps in protein transport that may involve polymorphic changes in the envelope membrane organization to include localized non-bilayer HII structures.

Phosphorylation of the transit sequence of chloroplast precursor proteins

A protein kinase was located in the cytosol of pea mesophyll cells. The protein kinase phosphorylates, in an ATP-dependent manner, chloroplast-destined precursor proteins but not precursor proteins, which are located to plant mitochondria or plant peroxisomes. The phosphorylation occurs on either serine or threonine residues, depending on the precursor protein used. We demonstrate the specific phosphorylation of the precursor forms of the chloroplast stroma proteins ferredoxin (preFd), small subunit of ribulose-bisphosphate-carboxylase (preSSU), the thylakoid localized light-harvesting chlorophyll a/b-binding protein (preLHCP), and the thylakoid lumen-localized proteins of the oxygen-evolving complex of 23 kDa (preOE23) and 33 kDa (preOE33). In the case of thylakoid lumen proteins which possess bipartite transit sequences, the phosphorylation occurs within the stroma-targeting domain. By using single amino acid substitution within the presequences of preSSU, preOE23, and preOE33, we were able to tentatively identify a consensus motif for the precursor protein protein kinase. This motif is (P/G)X(n)(R/K)X(n)(S/T)X(n) (S*/T*), were n = 0-3 amino acids spacer and S*/T* represents the phosphate acceptor. The precursor protein protein kinase is present only in plant extracts, e.g. wheat germ and pea, but not in a reticulocyte lysate. Protein import experiments into chloroplasts revealed that phosphorylated preSSU binds to the organelles, but dephosphorylation seems required to complete the translocation process and to obtain complete import. These results suggest that a precursor protein protein phosphatase is involved in chloroplast import and represents a so far unidentified component of the import machinery. In contrast to sucrose synthase, a cytosolic marker protein, the precursor protein protein kinase seems to adhere partially to the chloroplast surface. A phosphorylation-dephosphorylation cycle of chloroplast-destined precursor proteins might represent one step, which could lead to a specific sorting and productive translocation in plant cells.

Molecular chaperones are present in the thylakoid lumen of pea chloroplasts

A soluble protein fraction was obtained from pea chloroplast thylakoids, which represents highly enriched lumenal components. Using antisera against chaperonin 60 (cpn60), chaperonin 10 (cpn10) and the heat shock cognate protein of 70 kDa (hsc70) we are able to demonstrate, that the thylakoid lumen contains a separate set of molecular chaperones, which is distinct from the stromal one. In contrast to the alpha and beta subunits of cpn60 present in the stroma the lumen contains only one cpn60 isoform of distinct isoelectric point. Furthermore the lumenal cpn10 is of 'normal' size and not like its stromal counterpart of a double-domain tandem architecture. The immunoreactive hsc70 isoforms in the lumen seem also to be different from the stromal ones. Thus, chloroplasts seem to contain the largest number of molecular chaperone isoforms present in one organelle.

Rubisco, rubisco activase and ribulose-5-phosphate kinase gene expression and polypeptide accumulation in a tobacco mutant defective in chloroplast protein synthesis

Expression of the genes for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; rbcS and rbcL), Rubisco activase (rca) and ribulose-5-phosphate (Ru5-P) kinase (prk) and accumulation of the polypeptides was examined in chlorophyllous and chlorotic sectors of the DP1 mutant of Nicotiana tabacum. Plastids from chlorotic sectors of this variegated plastome mutant contained 30S and 50S ribosomal subunits, but had abnormally low levels of plastid polysomes. Consequently, mutant plastids were translationally repressed, unable to synthesize plastid-encoded polypeptides including the large subunit of Rubisco despite the presence of the corresponding mRNAs. Transcripts of rbcS accumulated to near wild type levels in chlorotic sectors, but there was little accumulation of the Rubisco small subunit (SS) polypeptide or holoenzyme. Messenger-RNA isolated from chlorotic sectors effectively directed the synthesis of Rubisco SS in vitro suggesting that posttranslational factors were responsible for the decrease in Rubisco SS abundance. Transcripts of rca and prk also accumulated to near wild type levels in chlorotic sectors and a diurnal rhythm in the abundance of rca mRNA was detected in green and chlorotic sectors. Despite the low abundance of Rubisco holoenzyme in chlorotic sectors, Rubisco activase and Ru5-P kinase polypeptides accumulated to significant levels. Activities of Rubisco and Ru5-P kinase paralleled protein levels, indicating that active forms of these enzymes were present in chlorotic sectors. The data indicate that the developmental events governing the accumulation of Rubisco activase and Ru5-P kinase polypeptides and the diurnal regulation of rca expression were not dependent on the attainment of photosynthetically competent plastids or the accumulation of Rubisco.

Import of a new chloroplast inner envelope protein is greatly stimulated by potassium phosphate

A cDNA clone encoding a major chloroplast inner envelope membrane protein of 96 kDa (IEP96) was isolated and characterized. The protein is synthesized as a larger-molecular-weight precursor (pIEP96) which contains a cleavable N-terminal transit sequence of 50 amino acids. The transit peptide exhibits typical stromal targeting information. It is cleaved in vitro by the stromal processing peptidase, though the mature protein is clearly localized in the inner envelope membrane. Translocation of pIEP96 into chloroplasts is greatly stimulated in the presence of 80 mM potassium phosphate which results in an import efficiency of about 90%. This effect is specific for potassium and phosphate, but cannot be ascribed to a membrane potential across the inner envelope membrane. Protein sequence analysis reveals five stretches of repeats of 26 amino acids in length. The N-terminal 300 amino acids are 45% identical (76% similarity) to the 35 kDa alpha-subunit of acetyl-CoA carboxyl-transferase from Escherichia coli. The C-terminal 500 amino acids share significant similarity (69%) with USOI, a component of the cytoskeleton in yeast.

Cloning and developmental expression of pea ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit N-methyltransferase

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit (LS) N-methyltransferase (protein methylase III, Rubisco LSMT, EC 2.1.1.43) catalyzes methylation of the epsilon-amino group of Lys-14 in the LS of Rubisco. With limited internal amino acid sequence information obtained from HPLC-purified peptic polypeptides from Rubisco LSMT, a full-length cDNA clone was isolated utilizing polymerase chain reaction-based technology and conventional bacteriophage library screening. The 1802 bp cDNA of Rubisco LSMT encodes a 489 amino acid polypeptide with a predicted molecular mass of ca. 55 kDa. A derived N-terminal amino acid sequence with features common to chloroplast transit peptides was identified. The deduced sequence of Rubisco LSMT did not exhibit regions of significant homology with other protein methyltransferases. Southern blot analysis of pea genomic DNA indicated a low gene copy number of Rubisco LSMT in pea. Northern analysis revealed a single mRNA species of about 1.8 kb encoding for Rubisco LSMT which was predominately located in leaf tissue. Illumination of etiolated pea seedlings showed that the accumulation of Rubisco LSMT mRNA is light-dependent. Maximum accumulation of Rubisco LSMT transcripts occurred during the initial phase of light-induced leaf development which preceded the maximum accumulation of rbcS and rbcL mRNA. Transcript levels of Rubisco LSMT in mature light-grown tissue were similar to transcript levels in etiolated tissues indicating that the light-dependent accumulation of Rubisco LSMT mRNA is transient. This is the first reported DNA and amino acid sequence for a protein methylase III enzyme.

A receptor component of the chloroplast protein translocation machinery

The chloroplast outer envelope protein OEP86 functions as a receptor in precursor protein translocation into chloroplasts. Sequence analysis suggests that the precursor of OEP86 is directed to the chloroplast outer envelope by a cleavable, negatively charged, and unusually long amino-terminal peptide. This presequence is unlike other potential targeting signals and suggests the existence of another membrane insertion pathway. Insertion of precursor OEP86 required the hydrolysis of adenosine triphosphate and the existence of surface exposed chloroplast membrane components, and it was not competed by another precursor protein destined for the internal plastid compartments.

Covalent modification of a highly reactive and essential lysine residue of ribulose-1,5-bisphosphate carboxylase/oxygenase activase

Chemical modification of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase with water-soluble N-hydroxysuccinimide esters was used to identify a reactive lysyl residue that is essential for activity. Incubation of Rubisco activase with sulfosuccinimidyl-7-amino-4-methylcoumarin-3-acetate (AMCA-sulfo-NHS) or sulfosuccinimidyl-acetate (sulfo-NHS-acetate) caused progressive inactivation of ATPase activity and concomitant loss of the ability to activate Rubisco. AMCA-sulfo-NHS was the more potent inactivator of Rubisco activase, exhibiting a second-order rate constant for inactivation of 239 M-1 s-1 compared to 21 M-1 s-1 for sulfo-NHS-acetate. Inactivation of enzyme activity by AMCA-sulfo-NHS correlated with the incorporation of 1.9 mol of AMCA per mol of 42-kD Rubisco activase monomer. ADP, a competitive inhibitor of Rubisco activase, afforded considerable protection against inactivation of Rubisco activase and decreased the amount of AMCA incorporated into the Rubisco activase monomer. Sequence analysis of the major labeled peptide from AMCA-sulfo-NHS-modified enzyme showed that the primary site of modification was lysine-247 (K247) in the tetrapeptide methionine-glutamic acid-lysine-phenylalanine. Upon complete inactivation of ATPase activity, modification of K247 accounted for 1 mol of AMCA incorporated per mol of Rubisco activase monomer. Photoaffinity labeling of AMCA-sulfo-NHS- and sulfo-NHS-acetate-modified Rubisco activase with ATP analogs derivatized on either the adenine base or on the gamma-phosphate showed that K247 is not essential for the binding of adenine nucleotides per se. Instead, the data indicated that the essentiality of K247 is probably due to an involvement of this highly reactive, species-invariant residue in an obligatory interaction that occurs between the protein and the nucleotide phosphate during catalysis.

Identification of the uridine-binding domain of sucrose-phosphate synthase. Expression of a region of the protein that photoaffinity labels with 5-azidouridine diphosphate-glucose

The uridine diphosphate-glucose (UDP-Glc) binding domain of sucrose-phosphate synthase (SPS) was identified by overexpressing part of the gene from spinach (Spinacia oleracea). Degenerate oligonucleotide primers corresponding to two tryptic peptides common to both the full-length 120-kD SPS subunit and an 82-kD form that photoaffinity labeled with 5-azidouridine diphosphate-glucose (5-N3UDP-Glc) were used in a polymerase chain reaction to isolate a partial cDNA clone. Comparison of the deduced amino acid sequence of spinach SPS with the sequences of potato sucrose synthase showed that the partial cDNA included one region that was highly conserved between the proteins. Expression of the partial cDNA clone of SPS in Escherichia coli produced a 26-kD fusion protein that photoaffinity labeled with 5-N3UDP-Glc. Photoaffinity labeling of the 26-kD fusion protein was specific, indicating that this portion of the SPS protein harbors the UDP-Glc-binding domain. Isolation of a modified peptide from the photolabeled protein provided tentative identification of amino acid residues that make up the uridine-binding domain of SPS.

Cloning and developmental expression of the sucrose-phosphate-synthase gene from spinach

A 561-base-pair (bp) polymerase-chain-reaction (PCR) product of sucrose-phosphate synthase (SPS) was amplified using degenerate oligonucleotide primers corresponding to tryptic peptides of SPS (EC 2.4.1.14) from spinach (Spinacia oleracea L). Crucial to the primer specificity and the synthesis of the 561-bp product was the use of primer pools in which the number of degenerate primer species was limited. A full-length cDNA was subsequently obtained by screening a cDNA bacteriophage library with the 561-bp product of SPS and 5' PCR-RACE (Rapid Amplification of cDNA Ends). The 3530-bp cDNA of SPS encoded for a 1056-amino-acid polypeptide of predicted molecular mass of 117 kDa. The deduced amino-acid sequence of spinach SPS showed regions of strong homology with SPS from maize (A.C. Worrell et al., 1991, Plant Cell 3, 1121-1130); amino-acid identity was 54% over the entire protein. Western and Northern analyses of root, petiole and spinach leaf tissue showed that SPS was expressed in an organ-specific manner, being predominantly localized in the leaf. The accumulation of SPS protein and mRNA during leaf development coincided with the early rapid phase of leaf expansion and the apparent transition of the leaf from sink to source status. Levels of SPS mRNA and protein were reduced during the acclimation of leaves to low-irradiance conditions. Transfer of low-irradiance-adapted leaves to higher-irradiance conditions resulted in a gradual increase in SPS protein and mRNA. Diurnal changes in irradiance did not alter SPS protein or transcript levels, indicating that short-term regulation of SPS primarily involves a modulation of enzyme activity.

Subunit interactions of Rubisco activase: polyethylene glycol promotes self-association, stimulates ATPase and activation activities, and enhances interactions with Rubisco

The effect of polyethylene glycol (PEG) on the enzymatic and physical properties of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase was examined. In the presence of PEG, Rubisco activase exhibited higher ATPase and Rubisco activating activities, concomitant with increased apparent affinity for ATP and Rubisco. Specific ATPase activity, which was dependent on Rubisco activase concentration, was also higher in the presence of Ficoll, polyvinylpyrrolidone, and bovine serum albumin. The ability of Rubisco activase to facilitate dissociation of the tight-binding inhibitor 2-carboxyarabinitol 1-phosphate from carbamylated Rubisco was also enhanced in the presence of PEG. Mixing experiments with Rubisco activase from two different sources showed that tobacco Rubisco activase, which exhibited little activation of spinach Rubisco by itself, was inhibitory when included with spinach Rubisco activase. Polyethylene glycol improved the ability of tobacco and a mixture of tobacco plus spinach Rubisco activase to activate spinach Rubisco. Estimates based on rate zonal sedimentation and gel-filtration chromatography indicated that the apparent molecular mass of Rubisco activase was two- to fourfold higher in the presence of PEG. The increase in apparent molecular mass was consistent with the propensity of solvent-excluding reagents like PEG to promote self-association of proteins. Likewise, the change in enzymatic properties of Rubisco activase in the presence of PEG and the dependence of specific activity on protein concentration resembled changes that often accompany self-association. For Rubisco activase, high concentrations of protein in the chloroplast stroma would provide an environment conducive to self-association and cause expression of properties that would enhance its ability to function efficiently in vivo.