Chapter 22 Targeting of proteins into and across the chloroplastic envelope

Abstract

Abstract The transport of precursor proteins into chloroplasts is composed of at least two steps: a specific binding of precursor proteins to the surface of the chloroplastic envelope followed by translocation of precursor proteins across the envelope. Precursor binding to the envelope can only be detected when the translocation step is blocked and is greatly stimulated by low concentrations of ATP. When envelope membranes were isolated, bound precursors were found in a fraction previously shown to contain contact sites connecting the two membranes of the envelope. This observation supports the hypothesis that protein transport into chloroplasts occurs at contact sites. Translocation of precursor proteins across the envelope requires ATP in the stroma as its sole energy source. Synthetic peptide analogues, corresponding to different regions of the transit peptide of the small subunit of ribulose-1,5-bisphosphate carboxylase precursor, can inhibit either the binding or the translocation step of protein transport into chloroplasts. These peptides can also inhibit import of proteins destined for various compartments of the chloroplast, except a protein destined for the outer membrane. This indicates that precursor proteins of most chloroplastic compartments share the same import apparatus for their transport across the envelope. Proteins destined for the outer membrane of the envelope follow a unique import pathway. They are not synthesized as higher molecular weight precursors. Neither ATP nor a thermolysin-sensitive receptor is required for their insertion. The targeting mechanisms of proteins destined for the inner envelope membrane and for the intermembrane space of the envelope are still unknown. Some possible sorting pathways are discussed.