The cotranslational sorting and integration of chloroplast-encoded multi-span thylakoid membrane proteins

• Applicant: Professorin Dr. Danja Schünemann
• Subject Area: Plant Physiology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 553203845

The biogenesis and maintenance of photosynthetic protein complexes of the chloroplast thylakoid membrane require well-orchestrated sorting and assembly mechanisms of nucleus and plastid encoded subunits. Key proteins of these complexes are synthesized on chloroplast ribosomes and cotranslationally inserted into the thylakoid membrane. However, little is known about the molecular mechanisms underlying this multi-step process which requires the precise spatiotemporal coordination of nascent chain processing, protein quality control, sorting of the ribosome nascent chain complex (RNC) and integration of the nascent chain into the thylakoid membrane. In our previous work, we showed that ribosome-associated cpSRP54 is required for efficient cotranslational sorting of central components of the photosynthetic apparatus and revealed a role of the cpSec/Alb3 insertase machinery and the membrane maintenance factor Vipp1 in the biogenesis of D1, the reaction center protein of photosystem II. Furthermore, we showed that the C-terminal region of Alb3 binds chloroplast ribosomes and is therefore likely involved in the recruitment of the RNC to the insertion machinery. In a recent project, we identified the interactome of a ribosome subpopulation, that specifically translates D1. Here, we identified STIC2 as a novel interactor of ribosome D1-nascent chain complexes. Preliminary results indicate that STIC2 cooperates with cpSRP54 in the biogenesis of D1 and potentially further photosynthetic proteins. The aim of this proposal is the comprehensive dissection of STIC2’s function in thylakoid membrane protein biogenesis. Furthermore, we aim to elucidate the role of proteins of yet unknown function in D1 biogenesis that we identified as D1-RNC associated factors.

• DFG Programme: Research Grants