Multiple pathways used for the targeting of thylakoid proteins in chloroplasts

The assembly of the chloroplast thylakoid membrane requires the import of n umerous proteins from the cytosol and their targeting into or across the th ylakoid membrane. It is now clear that multiple pathways are involved in th e thylakoid-targeting stages, depending on the type of protein substrate. T wo very different pathways are used by thylakoid lumen proteins: one is the Sec pathway which has been well-characterised in bacteria, and which invol ves the threading of the substrate through a narrow channel. In contrast, t he more recently characterised twin-arginine translocation (Tat) system is able to translocate fully folded proteins across this membrane. Recent adva nces on bacterial Tat systems shed further light on the structure and funct ion of this system. Membrane proteins, on the other hand, use two further p athways. One is the signal recognition particle-dependent pathway, involvin g a complex interplay between many different factors, whereas other protein s insert without the assistance of any known apparatus. This article review s advances in the study of these pathways and considers the rationale behin d the surprising complexity.