Synthesis and assembly of H+-ATPase complex by isolated "rough" thylakoids

Genome-wide analysis of thylakoid-bound ribosomes in maize reveals principles of cotranslational targeting to the thylakoid membrane

Chloroplast genomes encode ∼ 37 proteins that integrate into the thylakoid membrane. The mechanisms that target these proteins to the membrane are largely unexplored. We used ribosome profiling to provide a comprehensive, high-resolution map of ribosome positions on chloroplast mRNAs in separated membrane and soluble fractions in maize seedlings. The results show that translation invariably initiates off the thylakoid membrane and that ribosomes synthesizing a subset of membrane proteins subsequently become attached to the membrane in a nuclease-resistant fashion. The transition from soluble to membrane-attached ribosomes occurs shortly after the first transmembrane segment in the nascent peptide has emerged from the ribosome. Membrane proteins whose translation terminates before emergence of a transmembrane segment are translated in the stroma and targeted to the membrane posttranslationally. These results indicate that the first transmembrane segment generally comprises the signal that links ribosomes to thylakoid membranes for cotranslational integration. The sole exception is cytochrome f, whose cleavable N-terminal cpSecA-dependent signal sequence engages the thylakoid membrane cotranslationally. The distinct behavior of ribosomes synthesizing the inner envelope protein CemA indicates that sorting signals for the thylakoid and envelope membranes are distinguished cotranslationally. In addition, the fractionation behavior of ribosomes in polycistronic transcription units encoding both membrane and soluble proteins adds to the evidence that the removal of upstream ORFs by RNA processing is not typically required for the translation of internal genes in polycistronic chloroplast mRNAs.

Function of a chloroplast SRP in thylakoid protein export

Protein export systems derived from prokaryotes are used to transport proteins into or across the endoplasmic reticulum, the mitochondrial inner membrane, and the chloroplast thylakoid membrane. Signal recognition particle (SRP) and its receptor are essential components used exclusively for cotranslational export of endomembrane and secretory proteins to the endoplasmic reticulum in eukaryotes and export of polytopic membrane proteins to the cytoplasmic membrane in prokaryotes. An organellar SRP in chloroplasts (cpSRP) participates in cotranslational targeting of chloroplast synthesized integral thylakoid proteins. Remarkably, cpSRP is also used to posttranslationally localize a subset of nuclear encoded thylakoid proteins. Recent work has begun to reveal the basis for cpSRP's unique ability to function in co- and posttranslational protein localization, yet much is left to question. This review will attempt to highlight these advances and will also focus on the role of other soluble and membrane components that are part of this novel organellar SRP targeting pathway.

The stromal protein large subunit of ribulose-1,5-bisphosphate carboxylase is translated by membrane-bound ribosomes

Translation of the large subunit of ribulose-1,5-bisphosphate carboxylase (LSU) was investigated by labeling of isolated barley plastids with [35S]-methionine. In both chloroplasts and etioplasts, labeling of LSU was severely impaired if plastid membranes were removed from the reaction mixtures. Removal of membrane-bound polysomes with high salt or puromycin greatly decreased translation of LSU. Pulse-labeled chloroplast membranes were shown to release LSU if chased with unlabeled methionine in the presence of stroma. Immunoprecipitation detected higher amounts of labeled LSU translation intermediates associated with the membrane fraction than in the soluble fraction. We therefore conclude that, in plastids, membrane-bound polysomes are required not only for translation of membrane-intrinsic proteins but also for translation of a soluble protein.

Import and processing of the precursor of the delta subunit of tobacco chloroplast ATP synthase

A cDNA clone encoding the complete precursor of the delta subunit of chloroplast ATP synthase has been isolated from a tobacco (Nicotiana tabacum) leaf cDNA library in lambda gt11. The 880 bp insert encodes a polypeptide of 248 amino acid residues, of which 61 residues constitute an N-terminal presequence and 187 residues make up the mature delta subunit. Transcription and translation of the cDNA in vitro produced a protein of 29 kDa which was imported by isolated pea chloroplasts and processed to the mature 20 kDa subunit. The delta subunit precursor was processed to the mature size by a processing peptidase present in pea stromal extracts. Hybridisation of the cDNA to Southern blots of tobacco genomic DNA suggests the presence of two genes in the haploid genome.

Import and processing of the precursor form of the gamma subunit of the chloroplast ATP synthase from tobacco

A cDNA clone encoding the complete precursor of the gamma subunit of chloroplast ATP synthase has been isolated from a tobacco (Nicotiana tabacum) leaf cDNA library in lambda gt11. The 1.4 kb insert encodes a polypeptide of 377 amino acid residues, of which 55 residues constitute an N-terminal presequence and 322 residues make up the mature gamma subunit. Hybridisation of the cDNA to Southern blots of tobacco genomic DNA indicates the presence of two genes in the haploid genome. Transcription and translation of the cDNA in vitro produced a protein of 41 kDa which was imported by isolated pea chloroplasts and processed to the mature 36 kDa subunit. The gamma subunit precursor was processed to the mature size by a processing peptidase of 180 kDa present in pea stromal extracts.