Chloroplasts are organelles found in plant and algal cells that use photosynthesis to transform light energy into chemical energy. Chloroplasts, being a distinct organelle surrounded by two membranes, have their own genome, and their expression is closely coordinated with that of the nuclear genome. A tiny percentage (50-200) of chloroplast proteins are encoded by the chloroplast genome, while the majority of other chloroplast proteins (2000-3000) are encoded by nuclear genes. Nuclear genes encode chloroplast preproteins, which are produced in the cytoplasm from 80 S ribosomes with a transit peptide at their amino terminus. The transit peptide serves as a ticket into the chloroplast. The protein transporter complex found on the inner and outer chloroplast membranes should transport the preprotein into the chloroplast. The protein transporter on the outer chloroplast membrane is known as TOC, whereas the transporter on the inner membrane is known as TIC. TOC and TIC are membrane protein complexes made up of several distinct protein components that mediate the transmembrane transport of many different chloroplast proteins. They are essential for the formation of chloroplasts, the formation of photosynthetic complexes, and the functioning of several metabolic processes. Over the last 30 years, many protein subunits that comprise TOC and TIC have been found, identified, and researched one after the other. Recent research has revealed the presence of a supramolecular complex (TOC-TIC supercomplex) in plant and green algal chloroplasts. However, how do the subunits in TOC and TIC assemble together to form pore channels for transporting proteins? How do the two further assemble to form supramolecular complexes across the inner and outer membranes? Where are the transport pathways of the precursor proteins located in the transporters? The answers to the set of key scientific questions are not yet clear and further studies are needed to elucidate them. Zhenfeng Liu’s group at the Chinese Academy of Sciences’ Institute of Biophysics, in collaboration with Professor Jean-David Rochaix of the University of Geneva, Switzerland, published a research paper titled “Architecture of chloroplast TOC-TIC translocon supercomplex” online in the journal Nature on January 26, 2023. The study identified and localized 13 different protein subunits that make up the TOC-TIC supramolecular complex of Chlamydomonas reinhardtii origin by resolving the cryoelectron microscopic structure of the complex. All proteins are encoded by nuclear genes, except for the Tic214 protein, which is encoded by chloroplast genes. Together, these proteins form the TOC complex located in the outer membrane, the intermembrane-space complex (ISC), and the TIC complex located in the inner membrane. Surprisingly, Tic214, the largest membrane protein, crosses TIC, ISC, and TOC, operating as a bridge linking distinct protein subunits found in the inner and outer membranes, as well as the membrane gap, and most likely performing a scaffold-like role. The team performed a detailed analysis of the pore characteristics in the TOC and TIC complexes and predicted the interaction between the transit peptide and the TIC complex by molecular dynamics simulations. In addition, it was observed in this study that the two pore channels located in TOC and TIC, respectively, are connected by surface grooves located in the interstitial region of the membrane. Based on the results of previous biochemical and functional studies, it is proposed that the precursor proteins can sort and enter different micro-regions inside the chloroplast through several different transport pathways in the TOC-TIC supercomplex.
