Protein Routing Processes in the Thylakoid

Abstract

Thylakoids are complex sub-organellar membrane systems whose role in photosynthesis makes them critical to life. To function properly, thylakoids require the coordinated incorporation of both nuclear- and plastid-encoded proteins allowing rapid response to changing environmental conditions. Protein trafficking to thylakoids is complex; the processes occurring in thylakoids result in an exceptionally protein-dense membrane in which some proteins experience rapid turn-over. Protein transport in thylakoids is accomplished via an intriguing mix of conserved ancestral translocases with novel adaptations to a sub-organellar location. This chapter describes the four known transport pathways into the thylakoid membrane and the thylakoid lumen, namely the chloroplast general secretory system (cpSec), signal recognition particle (cpSRP), twin arginine transport (cpTat), and spontaneous insertion pathways as well as a potential secondary Sec system proposed to be in the inner chloroplast envelope. An overview is provided of known aspects of translocase components, energy requirements, and mechanisms with a focus on recent discoveries. Some of the most exciting new studies have been in determining the structure and binding features of the translocase components and substrate proteins. This chapter highlights the connection between structural and biochemical data and how these complementary avenues of study allow for a more detailed understanding and confirmation of mechanistic models and a means to imagine new areas of pursuit. The cpTat system is of particular interest because it transports folded protein domains using only the proton motive force for energy. Recent structural data has contributed to rapid progress in studying the individual cpTat components by translating their structural features into mechanistic functions.