Structural basis of the chloroplast protein-import channel
Protein translocation across membranes is a vital cellular process; however, the exact mechanism by which this occurs is not well understood. In the eukaryotic organelles, mitochondria and chloroplasts, the vast majority of proteins are nuclear-encoded and synthesised as preproteins on cytosolic ribosomes. The preproteins are then recognised by sophisticated molecular machineries to facilitate their import into the organelle. In chloroplasts (the organelles that conduct photosynthesis – a process upon which all life on Earth depends upon for food, energy, and oxygen), protein import is accomplished by translocases of the chloroplasts’ outer and inner envelope membranes, termed TOC and TIC, respectively. These translocases are multimeric protein complexes, and some of the main components are also well-conserved in apicomplexan parasites, for example, Plasmodium falciparum which causes malaria. The key players involved in preprotein recognition and import of the chloroplast TOC-TIC system have recently been described; however, the structural organisation of the TOC-TIC complexes remains elusive. To understand the mechanisms of protein transport by the TOC-TIC system at a molecular level, our lab uses an integrative structural biology approach that combines native mass spectrometry, cryo-EM, X-ray crystallography and mass photometry. A complete understanding of how these complexes assemble and function will then provide us with new means to enhance/manipulate the productivity of crop plants to tackle the growing population needs and climate change.