Evidence for two-step processing of nuclear-encoded chloroplast proteins during membrane assembly

Extremely large and slowly processed precursors to the Euglena light-harvesting chlorophyll a/b binding proteins of photosystem II

Antibody to the Euglena light-harvesting chlorophyll a/b binding protein of photosystem II (LHCPII) immunoprecipitated 207-, 161-, 122-, and 110-kDa proteins from total Euglena proteins pulse-labeled for 10 min with [(35)S]sulfate. The 25.6- and 27.2-kDa LHCPII were barely detectable in the immunoprecipitate. During a 40-min chase with unlabeled sulfate, the amount of radioactivity in the high molecular mass proteins decreased, and the amount of radioactivity in the 25.6- and 27.2-kDa LHCPII increased with kinetics consistent with a precursor-product relationship. The half-life of the high molecular mass proteins was approximately 20 min. The major proteins immunoprecipitated from a nuclease-treated rabbit reticulocyte cell-free translation system programmed with Euglena whole cell or poly(A)(+) RNA had molecular masses corresponding to the molecular masses of the proteins immunoprecipitated from the pulse-labeled in vivo translation products. RNAs of 6.6 and 8.3 kilobases were the only Euglena whole cell and poly(A)(+) RNAs that hybridized to a 0.7-kilobase EcoRI-BamHI fragment of plasmid pAB165, which contains a portion of the coding sequence for Arabidopsis LHCPII. RNAs of this size are more than sufficient to code for proteins of 207 kDa. Taken together, these findings demonstrate that the LHCPIIs of Euglena are initially synthesized as slowly processed precursors with molecular masses of 207, 161, 122, and 110 kDa.

Carboxyl-terminal processing protease for the D1 precursor protein: cloning and sequencing of the spinach cDNA

A previous study has demonstrated that the carboxyl-terminal (C-terminal) processing protease in spinach for the D1 precursor protein (pDl) of the photosystem II reaction center is a monomeric protein of about 45 kDa. Based on the amino acid sequence data of the purified protease, a cDNA clone encoding the enzyme has been identified and sequenced, from a spinach green leaf cDNA library. In order to determine the 5' end of the transcript, the rapid amplification of cDNA end (5'-RACE) technique was applied. By these analyses, the full-length transcript was established to consist of 1906 nucleotides and a poly(A) tail, containing an open reading frame (ORF) corresponding to a protein with 539 amino acid residues. By comparing the amino acid sequence of the purified protease with that deduced from nucleotide sequence of the cDNA clones, the enzyme was shown to be furnished with an extra amino-terminal extension characteristic of both a transit peptide and a signal sequence. This suggests that the protease is synthesized in the cytosol and translocated into the lumenal space of thylakoids. The mature part of the enzyme consists of 389 amino acid residues and exhibits a significant sequence homology with two groups of proteins as demonstrated by a computer homology search, i.e. (1) the deduced sequence of a protein proposed to be the C-terminal processing protease for pD1 in Synechocystis sp. PCC 6803, based on genetic experiments and (2) proteases for C-terminal cleavage identified in Escherichia coli and Bartonella bacilliformis.

Mutations in a signal sequence for the thylakoid membrane identify multiple protein transport pathways and nuclear suppressors

The apparatus that permits protein translocation across the internal thylakoid membranes of chloroplasts is completely unknown, even though these membranes have been the subject of extensive biochemical analysis. We have used a genetic approach to characterize the translocation of Chlamydomonas cytochrome f, a chloroplast-encoded protein that spans the thylakoid once. Mutations in the hydrophobic core of the cytochrome f signal sequence inhibit the accumulation of cytochrome f, lead to an accumulation of precursor, and impair the ability of Chlamydomonas cells to grow photosynthetically. One hydrophobic core mutant also reduces the accumulation of other thylakoid membrane proteins, but not those that translocate completely across the membrane. These results suggest that the signal sequence of cytochrome f is required and is involved in one of multiple insertion pathways. Suppressors of two signal peptide mutations describe at least two nuclear genes whose products likely describe the translocation apparatus, and selected second-site chloroplast suppressors further define regions of the cytochrome f signal peptide.

Specificity of leaf mitochondrial and chloroplast processing systems for nuclear-encoded precursor proteins

The specificity of the mitochondrial and chloroplast processing enzymes for the nuclear-encoded precursor proteins was investigated. Mitochondrial precursor proteins of the Nicotiana plumbaginifolia and the Neurospora crassa beta subunits of F1-ATPase and the Neurospora Rieske FeS precursor protein were processed to the correct mature size by matrix extracts isolated from spinach leaves, yeast, rat liver and beef heart. The mitochondrial extracts failed to process chloroplast precursor proteins of the stromal small subunit of ribulose 1,5-bisphosphate carboxylase and the thylakoid 33 kDa protein of the oxygen-evolving complex. Both mitochondrial F1 beta precursors were specifically processed by a soluble stromal extract from chloroplasts. However, no processing of the Rieske FeS precursor protein was observed under the same conditions with the chloroplast extract. The cleavage of the mitochondrial F1 beta precursors by the chloroplast extract was shown to be sensitive to the metal chelators EDTA and ortho-phenanthroline. The cleavage site of the mitochondrial F1 beta precursor by the chloroplast soluble extract appears to be located at the N-terminus.

A plastome mutation affects processing of both chloroplast and nuclear DNA-encoded plastid proteins

Immunoblotting of a chloroplast mutant (pm7) of Oenothera showed that three proteins, cytochrome f and the 23 kDa and 16 kDa subunits of the oxygen-evolving subcomplex of photosystem II, were larger than the corresponding mature proteins of the wild type and, thus, appear to be improperly processed in pm7. The mutant is also chlorotic and has little or no internal membrane development in the plastids. The improperly processed proteins, and other proteins that are completely missing, represent products of both the plastid and nuclear genomes. To test for linkage of these defects, a green revertant of pm7 was isolated from cultures in which the mutant plastids were maintained in a nuclear background homozygous for the plastome mutator (pm) gene. In this revertant, all proteins analyzed co-reverted to the wild-type condition, indicating that a single mutation in a plastome gene is responsible for the complex phenotype of pm7. These results suggest that the defect in pm7 lies in a gene that affects a processing protease encoded in the chloroplast genome.

Membrane composition and physiological activity of plastids from an oenothera plastome mutator-induced chloroplast mutant

Plastids were isolated from a plastome mutator-induced mutant (pm7) of Oenothera hookeri and were analyzed for various physiological and biochemical attributes. No photosynthetic electron transport activity was detected in the mutant plastids. This is consistent with previous ultrastructural analysis showing the absence of thylakoid membranes in the pm7 plastids and with the observation of aberrant processing and accumulation of chloroplast proteins in the mutant. In comparison to wild type, the mutant tissue lacks chlorophyll, and has significant differences in levels of four fatty acids. The analyses did not reveal any differences in carotenoid levels nor in the synthesis of several chloroplast lipids. The consequences of the altered composition of the chloroplast membrane are discussed in terms of their relation to the aberrant protein processing of the pm7 plastids. The pigment, fatty acid, and lipid measurements were also performed on two distinct nuclear genotypes (A/A and A/C) which differ in their compatibility with the plastid genome (type I) contained in these lines. In these cases, only chlorophyll concentrations differed significantly.

Analysis of the genes of the OEE1 and OEE3 proteins of the photosystem II complex from Chlamydomonas reinhardtii

The sequences of the nuclear genes of the 33 kDa (OEE1) and the 16 kDa (OEE3) polypeptides of the oxygen evolving complex of Chlamydomonas reinhardtii have been established. Comparison between the OEE1 protein sequences of C. reinhardtii and higher plants and cyanobacteria reveals 67 and 47% homology. In contrast, C. reinhardtii and higher plants have only 28% overall homology for OEE3 which is mostly limited to the central portion of the protein. The transit peptides of the C. reinhardtii proteins consist of 52 (OEE1) and, most likely, 51 (OEE1) amino acids. They have a basic amino terminal region and, at least in the case of OEE1, a hydrophobic segment at their carboxy terminal end typical of thylakoid lumen proteins. Comparison of the genomic and cDNA clones indicates that the OEE1 and OEE3 genes contain five and four introns, respectively, some of which are located within the coding sequences of the transit peptides.

Phagocytosis in Dictyostelium discoideum is inhibited by antibodies directed primarily against common carbohydrate epitopes of a major cell-surface plasma membrane glycoprotein

Using a water-soluble, reversible biotinylating reagent, we retrieved three surface-exposed proteins from a complex mixture of crude membrane proteins. The compound, sulfosuccinimidyl 2-(biotinamido)ethyl-1-3'-dithiopropionate (sulfo-NHS-SS-biotin), which has a cleavable disulfide bond, was used to label Dictyostelium discoideum amebae. Cells were lysed and a crude membrane preparation was isolated and solubilized with Triton X-100. Biotinylated molecules were bound to immobilized streptavidin and then eluted from the affinity matrix with dithiothreitol. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that out of the original complex mixture of detergent-solubilized membrane proteins, three major species at 130, 100, and 77 kDa were specifically bound and eluted with thiol reagents. These three proteins were glycoproteins (gp) since they bound concanavalin A. As demonstrated by one-dimensional peptide mapping, the retrieved gp130 and gp100 also were present in specialized plasma membrane subdomains called contact regions which are regions of cell-cell cohesion isolated from aggregated, developed amebae. This finding provides preliminary evidence that the two proteins may be involved in cell-cell interactions during both the vegetative and aggregation stages of the D. discoideum life cycle. The retrieved gp130 species has a relative mobility on SDS-gels similar to that of gp126, a surface-exposed glycoprotein. gp126 has been suggested to play roles both as a phagocytosis receptor and as a cohesion molecule (C.M. Chadwick, J.E. Ellison, and D.R. Garrod, (1984) Nature (London) 307, 646). To test if the retrieved gp130 was the same as gp126, a polyclonal antiserum was raised against gel-purified, endoglycosidase F-treated gp130. The immune serum recognized epitopes, apparently carbohydrates, present on many D. discoideum membrane proteins. Univalent IgG fragments from this antiserum inhibited phagocytosis, suggesting that anti-carbohydrate activity was responsible for the functional inhibition of phagocytosis.

Transcending the impenetrable: how proteins come to terms with membranes

In the living cell, proteins are efficiently sorted to a whole range of subcellular compartments. In many cases, sorting specificity is mediated by short 'sorting signals' attached either permanently or transiently to the protein. At long last, a fairly coherent picture of the design and function of many such sorting signals is beginning to emerge.

Cloning, nucleotide sequence and mutational analysis of the gene encoding the Photosystem II manganese-stabilizing polypeptide of Synechocystis 6803

Affinity purified, polyclonal antibodies raised against the Photosystem II 33 kDa manganese-stabilizing polypeptide of the spinach oxygen-evolving complex were used to isolate the gene encoding the homologous protein from Synechocystis 6803. Comparison of the amino acid sequence deduced from the Synechocystis psb1 nucleotide sequence with recently published sequences of spinach and pea confirms the homology indicated by antigenic cross-reactivity and shows that the cyanobacterial and higher plant sequences are 43% identical and 63% conserved. Regions of identity, varying in length from 1 to 10 consecutive residues, are distributed throughout the protein. The 28 residues at the amino terminus of the psb1 gene product, characteristic of prokaryotic signal peptides, show homology with the carboxyl-terminal third of the transit sequences of pea and spinach and are most likely needed for the transport of the manganese-stabilizing protein across the thylakoid membrane to its destination of the lumen. Synechocystis mutants which contain a kanamycin resistance gene cassette inserted into the coding region for the 32 kDa polypeptide were constructed. These mutants contain no detectable 32 kDa polypeptide, do not evolve oxygen, and are incapable of photoautotrophic growth.

Recent developments in chloroplast protein transport

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

Light-dependent accumulation and localization of photosystem II proteins in maize

We have raised antibodies against several major components of photosystem II. These antisera, which are directed against the apoproteins of two chlorophyll-binding proteins (CPa-1 and CPa-2), the apoprotein of light-harvesting complex II and the 33-kDa extrinsic protein of the oxygen-evolving complex, were used to examine the light regulation of photosystem II assembly in maize. The principal findings of this study are as follows. The 33-kDa protein is present in dark-grown maize and the content increases 5-10-fold upon illumination. The level of the protein is mediated at least in part by phytochrome and is independent of the accumulation of chlorophyll. In contrast, none of the three chlorophyll-binding proteins examined was detectable in leaves of maize grown in darkness or under other light regimes where chlorophyll does not accumulate. Even in the absence of photosystem II assembly, the 33-kDa protein is properly transported across the thylakoid into the lumen. However, the protein does not attach in the normal way to the inner surface of the membrane under these conditions.