Molecule of the Month: TOC-TIC Translocon

Plants and algae rely on chloroplasts to convert energy from sunlight into chemical energy that can be used to power cellular activity. Like mitochondria, chloroplasts are thought to have originated from the engulfment of a bacterial cell by an early eukaryotic cell. Although modern chloroplasts and mitochondria still retain their own circular genomes, a majority of the proteins needed for organelle function are encoded in the nuclear genome. Of the approximately 3000 proteins in the chloroplast, only about 100 can be synthesized in the organelle from its own genome. Most of the remaining chloroplast proteome is translated in the cytosol and imported into the chloroplast via the TOC-TIC supercomplex.

Chloroplasts are organized into multiple compartments that are separated by membranes. The outer membrane envelops the entire chloroplast. Within the chloroplast is an inner membrane which surrounds the stroma, a large compartment that contains photosynthetic structures, chloroplast ribosomes, and DNA. Between the inner and outer chloroplast membranes is a narrow intermembrane space. Passage of proteins through the outer membrane is mediated by the TOC (Translocon at the Outer envelope of Chloroplasts) complex, while passage through the inner membrane is controlled by the TIC (Translocon at the Inner envelope of Chloroplasts) complex. Recent structural studies in the green alga Chlamydomonas reinhardtii have shown that TOC and TIC complexes form a large single TOC-TIC supercomplex, as shown to the right (7VCF), which is thought to transport proteins from the cytosol to the stroma as well as into the intermembrane space.

Biochemical studies have shown that importing proteins from the cytosol into the chloroplast stroma is an energy-intensive process that can require the hydrolysis of hundreds of ATPs per protein. Much of this energy is likely used by motor complexes that associate with the TOC-TIC translocon and pull proteins into the stroma. One such motor complex in Chlamydomonas reinhardtii has recently been structurally characterized (left). The Ycf2-FtsHi complex (shown in pink/purple, 8XQX), is a large multi-subunit complex that spans the inner chloroplast membrane. Six of its component proteins form a hexameric ring that is thought to pull proteins through its central channel using the energy released from ATP hydrolysis.