Protein Import Mechanism in Chloroplasts
DOI:
https://doi.org/10.24925/turjaf.v14i5.1377-1386.8402Keywords:
Chloroplast , Protein import , Membrane transport , TOC-TIC , EndosymbiosisAbstract
Chloroplasts evolved approximately 1.5 billion years ago from their endosymbiotic ancestors, cyanobacteria. This process resulted in large-scale gene transfer, and the majority of proteins required for proper chloroplast function are now encoded in the nucleus of the plant cell and subsequently imported into the chloroplast. This import is mediated by the TOC complex located in the outer membrane and the TIC complex located in the inner membrane of the chloroplast. Precursor proteins synthesized in the cytosol are directed to the chloroplast outer membrane via their transit peptides, accompanied by chaperones such as Hsp70, Hsp90, and 14-3-3; they are recognized by the Toc159 and Toc34 GTPase receptors of the TOC complex and translocated across the outer membrane through the Toc75 β-barrel channel, reaching the intermembrane space. The channel component of the TIC complex has not yet been definitively elucidated, and two competing models have been proposed: the Tic110 channel and the 1 MDa Tic20 complex. The basis of this functional ambiguity lies in the dual role of TIC as both a passive translocation channel and an ATP-dependent motor complex encompassing Hsp70/Hsp93 chaperones. Upon reaching the stroma, the transit peptide of the imported protein is cleaved by the stromal processing peptidase (SPP), and the protein folds into its mature conformation. The efficiency of protein import is modulated in a redox-dependent manner via the Tic62/Tic55/Tic32 redox regulon according to the NADP⁺/NADPH ratio, and proteolytically through the Ubiquitin-Proteasome System (UPS). In recent years, high-resolution structural data obtained by Cryo-Electron Microscopy (Cryo-EM) have revealed that Tic236 forms a physical bridge between Toc75 and Tic110, uncovering the atomic architecture of the TOC-TIC supercomplex. In this review, the structure, function, and regulation of these components are comprehensively discussed, along with open research questions and future perspectives in the field.
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