Figure 3: Model Showing Different Stages and Energetics of Chloroplast Protein Import

Based on energetic requirements determined in vitro, protein import into chloroplasts can be divided into three distinct stages. These are assumed to correspond to sequential steps on the import pathway that occurs in vivo. At stage 1 (‘energy-independent binding’), the preprotein interacts reversibly with receptor components of the TOC complex. Progression to stage 2 (formation of an ‘early import intermediate’) requires ATP at low concentrations in the intermembrane space, and GTP. At this step, the preprotein is inserted across the outer envelope membrane (OM) and is in contact with components of the TIC apparatus in the inner membrane (IM). Stage 3 (‘complete translocation’) requires high concentrations of ATP in the stroma. The preprotein is translocated simultaneously across both envelope membranes at a contact site (where the membranes are held in close proximity), the transit peptide is removed by the stromal processing peptidase (SPP; represented by scissors), and the mature protein takes on its final conformation. Taken from New Phytol., 2008, 179: 257-285.