Studies on the Mechanisms Underlying the Targeting and Membrane Insertion of Chloroplast Inner Membrane Proteins

The chloroplast is a complex subcellular organelle composed of three membrane systems and three soluble compartments. Reflecting its endosymbiotic origin, it is enclosed by two envelope membranes: the outer and inner membranes. While most outer membrane proteins are directly inserted from the cytosol, nearly all inner membrane proteins are first imported into the chloroplast through the translocons at the outer and inner envelope membranes of chloroplasts (TOC and TIC complexes). During translocation through the TIC complex, some inner membrane proteins are laterally inserted into the inner membrane via the stop-transfer pathway. In contrast, others are fully imported into the stroma and subsequently inserted into the inner membrane through the post-import pathway. In this review, We summarize the overall process of protein import into chloroplasts and highlight recent advances in understanding the mechanisms that govern the targeting and membrane insertion of chloroplast inner membrane proteins.