Post-translational membrane protein targeting by the chloroplast signal recognition particle

Abstract

Post-translational transport of membrane proteins poses enormous challenges to the cells. The transport factors must accurately select and deliver the cargos to the appropriate target membranes. In addition, they have to provide chaperone for their hydrophobic cargos. To understand capacity and limitation of a post-translational transport factor, we studied one of the most efficient membrane protein transport pathways, the delivery of light-harvesting chlorophyll-binding (LHC) proteins to the thylakoid membrane. This targeting reaction is mediated by the chloroplast Signal Recognition Particle (cpSRP) and its receptor. Although the core SRP GTPases are close homologues of those in cytosolic SRP pathways, the unique features of cpSRP that might reflect its adaptation to the challenges in post-translational targeting include (i) the lack of the otherwise universally conserved SRP RNA, and (ii) the exclusive presence of a novel protein, cpSRP43. In the first part of this thesis, we define the thermodynamic and kinetic framework for the GTPase cycles of cpSRP and its receptor and uncover the molecular bases that enable their intrinsically fast interactions, such that they can bypass an SRP RNA, an essential accelerator for the cytosolic SRP–receptor interaction. The second part of the thesis is devoted to characterization of the chaperone function of cpSRP43. We show that cpSRP43 specifically and effectively prevents and reverses the aggregation of its cargo, LHC proteins. We further investigate the molecular mechanism of this novel disaggregase activity, using a combination of biochemical and structural approaches. In summary, this dissertation aims to understand how cpSRP and its receptor adapt to their unique requirements in efficiently transporting a family of highly abundant membrane proteins.