Targeting of proteins into and within the chloroplast

Abstract

Chloroplasts are semiautonomous organelles that cannot be made de novo. They possess their own DNA, which encodes only a small part of chloroplastic proteins. The nucleotide sequences for ten different chloroplast genomes: liverwort, maize, tobacco, rice, black pine, Euglena, Epifagus, ChloreIla vulgaris, Cyanidum caldarium, Odontella sinensis, Toxoplasma gondii, have been determined (see NCBI, Entrez, Genomes, Database). There are about 130 structural genes in the chloroplast genome, coding for about 100 different proteins (Sugita and Sugiura 1996). Most proteins in chloroplast are encoded by the nuclear genome and synthesised in the cytosol. With the exception of most outer envelope membrane proteins, nuclearencoded chloroplastic proteins are synthesised with N-terminal extension that contains the chloroplast targeting information of these proteins. This Nterminal extension is referred to as a transit peptide (also termed transit sequence) to distinguish it from other sorting signals such as the signal sequence involved in the secretion of protein in prokaryotes. Transit sequence is necessary and sufficient to target a protein to the chloroplast and is removed after translocation by a stromal processing protease. Import is initiated by binding of the precursor protein to proteinaceous receptor localised on the surface of the chloroplast fol lowed by translocation through the outer and inner envelope membranes. Protein transpor t into chloroplast usually requires ATP and GTP for binding and translocation. Sometimes cytosolic factors are required for keeping the precursor soluble and to provide an import competent conformation. Beside the transit sequence there is information present in the protein for proper routing inside the chloroplast. Proteins that have to cross the thylakoid membrane contain a second Nterminal extension behind the transit peptide that is cleaved by a thylakoid processing peptidase. Protein transport and routing inside chloroplast are outlined on Fig. 1. Chloroplasts are structurally complex organelles with three different membranes: the