16.18: Protein Transport to the Inner Chloroplast Membrane
Proteins targeted to the inner chloroplast membrane, or plastid proteins, are transported by two general pathways: the stop-transfer and the re-insertion or post-import pathways. Most plastid proteins carry N-terminal transit sequences and internal import sequences targeting it to the specific chloroplast subcompartment. Proteins targeted by the stop-transfer pathway have internal hydrophobic sequences that inhibit their translocation into the stroma. As a result, these precursors are arrested across the TIC complex and are laterally released into the inner membrane. The hydrophobic segment helps anchor these proteins to the inner membrane.
Plastid proteins targeted to the inner membrane by the re-insertion pathway are first translocated to the stroma through the general TOC/TIC import pathway. After the stromal processing peptidases cleave the transit signal, the re-insertion signal is exposed. The re-insertion signal then guides the precursor to Hsp93-TIC40-TIC 110 complex. The precursor is folded and inserted by an unidentified protein, helped by TIC40 and TIC110.
A third pathway translocates plastid proteins that lack the N-terminal transit sequences. HP30-HP30-2 heteromer interacts with HP20 protein to translocate and insert these precursors. Proteins sorted by HP30-HP30-2 pathways include the chloroplast envelope quinone oxidoreductase homologs (ceQORH) and other proteins.