Transport of proteins into chloroplasts

Cleavage of the precursor of pea chloroplast cytochrome f by leader peptidase from Escherichia coli

Leader peptidase from Escherichia coli was able to process the precursor of pea cytochrome f synthesised in vitro. N-Terminal sequencing established that cleavage by leader peptidase generated the same mature sequence as in pea chloroplasts. Processing by leader peptidase was much more efficient co-translationally rather than post-translationally, and the extent of post-translational processing declined with time suggesting that the cytochrome f precursor folded to an uncleavable conformation. Detergent extracts of pea thylakoid membranes were unable to process the cytochrome f precursor co- or post-translationally.

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.

Protein transport into and within chloroplasts

The chloroplast is a complex organelle which carries out a wide range of metabolic processes such as light capture and the biosynthesis of carbohydrates, fatty acid and amino acids. This organelle consists of three separate membrane systems which enclose three distinct soluble phases. Most of the chloroplast proteins are imported from the cytosol and directed into the six different compartments. This import and intraorganellar sorting process makes the chloroplast an interesting and promising system for the analysis of how proteins interact with and are translocated across biological membranes.