Mechanisms of Protein Import and Routing in Chloroplasts

Abstract

The vast majority of the approximately 3000 different proteins required to build a fully functional chloroplast are encoded by the nuclear genome and translated on cytosolic ribosomes. As chloroplasts are each surrounded by a double-membrane system, or envelope, sophisticated mechanisms are necessary to mediate the import of these nucleus-encoded proteins into chloroplasts. Once inside the organelle, many chloroplast proteins engage one of four additional protein sorting mechanisms that direct targeting to the internal thylakoid membrane system.

LTD is a protein required for sorting light-harvesting chlorophyll-binding proteins to the chloroplast SRP pathway

“…Nuclear-encoded chloroplast proteins are inserted into or translocated across the thylakoid membrane by four protein translocation pathways after import into the chloroplast 3,4 . We have attempted to uncover the molecular events that are responsible for routing imported proteins into these distinct pathways.…”

“…Chloroplasts contain about 3,000 proteins, but only 50-200 of them are encoded for in the plastid genome. Nuclear-encoded proteins are synthesized in the cytosol and post-translationally translocated across the chloroplast envelopes [1][2][3][4][5][6] . The protein import complexes located on the outer and inner envelope membranes are termed as TOC (translocon at the outer envelop membrane of chloroplast) and TIC (translocon at the inner envelop membrane of chloroplast) complexes, respectively 1,2,5,6 .…”

“…After translocation across the envelope, imported proteins are inserted into or transported across the thylakoid membrane through one of four mechanistically distinct pathways, which are classified on the basis of their energy and stromal factor requirements 3,4 : the Secdependent pathway, the twin-arginine translocation (Tat) pathway, the chloroplast signal recognition particle (cpSRP) pathway and the spontaneous insertion pathway. The Sec pathway requires a translocation ATPase, SecA, and the translocation channel consisting of SecY and SecE, and is involved in transport of several lumenal proteins including plastocyanin and 33-kDa protein of the oxygen-evolving complex (OE33) (ref.…”

“…4). The 'spontaneous' pathway does not seem to require soluble factors or energy and is the mode of insertion of CFoII, PSII-W, PSII-X and ELIP into the membrane 3,4 . Although these pathways can be distinguished by their protein factors and energy requirements, how imported chloroplast proteins are sorted to these distinct pathways is still unknown.…”

Plant organellar protein targeting: a traffic plan still under construction

“…Cellular protein trafficking is a complex, responsive and rapidly evolving process. For mitochondrial [1] and plastid [2] components, it most often involves specific N-terminal extensions to the nuclear-encoded protein, known as the targeting presequence (mitochondrial) or transit peptide (plastid), interacting with membrane-bound proteins sitting near the surface of the organelle to influence protein import. However, in recent years, the number of proteins found targeted to both mitochondria and plastids has increased significantly, obscuring our understanding of which peptide features dictate organelle targeting specificity and heightening interest in the plant-specific evolutionary processes that have expanded and refined protein targeting capacity.…”