Abstract: During their biogenesis, chloroplasts import most proteins via the TOC-TIC pathway, but some of them must be transported further across the photosynthetic thylakoid membrane into its lumen. This requires evolutionarily conserved SEC (Secretory) and TAT (Twin Arginine Translocation) pathways. These are energized by ATP and the trans-thylakoid proton gradient, respectively. Most luminal proteins are synthesized in the cytoplasm with bi-partite, cleavable targeting sequences (first for the chloroplast envelope, second for the thylakoid membrane). Two-stage cleavage of these peptides is a critical step of chloroplast biogenesis. Here, we present two mutants (var2 and abc1k1) in which photosynthesis can be reversibly switched off by red light. Red light arrests chloroplast biogenesis and accumulation of higher molecular mass protein bands can be observed. Deep proteomics reveal that the higher molecular mass bands belong to a specific module of proteins extrinsically associated with luminal side of Photosystems I and II. They are rarely observed partially processed intermediates that still possess the targeting sequences for the thylakoid SEC and TAT machineries but not for the TOC-TIC pathway. The results show that the processing of a specific module of Photosystem-associated proteins and concomitantly progression of chloroplast biogenesis depend on active photosynthesis early in plant development.

Biography: Professor Felix Kessler completed his PhD at the ETH Zürich, Switzerland, researching isoforms of the plasma membrane Ca-ATPase with Prof. Dr. Ernest Carafoli. At the early stages of his career as a postdoc, he was a co-discoverer of the chloroplast protein import machinery. The work was carried out in the laboratory of Prof. Günter Blobel (Nobel Prize 1999) at the Rockefeller University (New York, USA). Since 2002 he is a full professor at the Université de Neuchâtel, Switzerland. His current research focuses on chloroplast biogenesis and protein import during early stages of plant development. In addition, he has developed a new area of research aimed at plastoglobules (PG), lipid droplets inside the chloroplast. For a long time, PG were considered a simple lipid storage site. But the Kessler lab’s work demonstrated that they contribute in multiple and surprising ways to metabolism of lipid and membrane antioxidants, including carotenoids, tocopherol (Vitamin E), phylloquinone (Vitamin K) and plastoquinone. The activities depend on a small set of PG-associated proteins consisting of structural proteins, enzymes and atypical ABC1K-like kinases. In their most recent work, the Kessler lab demonstrated that plastoglobule-localized ABC1Ks control the supply of plastoquinone to the photosynthetic electron transport chain in the thylakoid membrane. This new regulatory mechanism of photosynthesis was termed "Plastoquinone Homeostasis". Unexpectedly, PG in the chromoplasts of tomato fruit contain the complete carotenoid biosynthetic pathway and act as a site of carotenoid biosynthesis. This is also a current focus of research in the Kessler lab.