Structure and function of the chloroplast signal recognition particle.

The targeting of proteins, including the insertion and translocation of proteins in or across membranes, is a fundamental process within a cell, and a variety of specialized mechanisms for protein transport have been developed during evolution. The signal recognition particle (SRP) is found in the cytoplasm of most, if not all, eukaryotes and prokaryotes where it plays a central role in the co-translational insertion of membrane proteins into the endoplasmic reticulum and plasma membrane, respectively. SRP is a ribonucleoprotein consisting of an RNA and at least one polypeptide of approximately 54 kDa (SRP54). Interestingly, chloroplasts contain a specialized type of signal recognition particle. Chloroplast SRP (cpSRP) contains a SRP54 homologue but differs strikingly from cytosolic SRP in various aspects of structure and function. In contrast to cytosolic SRP, it contains a novel protein subunit (cpSRP43) and lacks RNA. CpSRP is also distinctive in its ability to interact with its substrate, light-harvesting chlorophyll a/ b-binding protein, post-translationally. Furthermore, it is remarkable that the 54 kDa subunit of cpSRP is also involved in the co-translational transport of chloroplast-encoded thylakoid proteins, and is therefore able to switch between the co- and post-translational means of interaction with its respective substrate proteins.