Import of proteins into chloroplasts

Abstract

ABSTRACT Chloroplasts are the sites of photosynthesis in eukaryotic plants and are able to harvest solar energy for the synthesis of carbon skeletons. They, like mitochondria, are enclosed by two membranes; the outer and the inner envelope. The latter is the actual permeability barrier between the cytosol and the chloroplast stroma and the site of different metabolite translocators that coordinate the metabolism between compartments (Heldt and Flügge, 1987). Distinct from the envelope membranes are the energy-transducing thylakoid membranes that are located within the chloroplasts, and which enclose the thylakoid lumen. Stroma and thylakoid proteins each account for about 50% of the total chloroplast protein, whereas the two envelope membranes represent less than 1%. In spite of the chloroplasts’ semi-autonomy the majority of the chloroplasts proteins and also those of other cell compartments are coded for in the nucleus. These nuclear-coded proteins are synthesized in the cytosol, normally with N-terminal extensions called transit peptides, and are subsequently transported into the chloroplasts where they are processed to their mature sizes by specific proteases. Mechanisms must therefore exist to ensure that these proteins are (1) correctly targeted to chloroplasts and (2) subsequently localized into the correct chloroplast compartment: namely, the outer or inner envelope membrane, the intermembrane space, the chloroplast stroma, the thylakoid membrane or the thylakoid lumen. Major aspects of protein transport have been reviewed quite recently (Schmidt and Mishkind, 1986; Lubben et al. 1988; Keegstra et al. 1989). This Commentary will focus on the basic phenomena of this topic and will also present recent findings that extend the view of the reviews cited above.