Protein Sorting within Chloroplasts

Identification and functional characterization of novel components, facilitating translocation across the envelope, and subsequent sorting at the thylakoid membranes, implies that the ancient thylakoid transport systems have acquired new components to adapt to their eukaryotic environment. The newly identified stromal components play key roles not only in recognizing preproteins to prevent mistargeting, but also in ensuring efficient sorting. Dynamic interactions of chloroplast signal recognition particle (cpSRP) 43 with light-harvesting chlorophyll-binding protein (LHCP), cpSRP54, and chloroplast filamentation temperature sensitive Y (cpFtsY) provide further insight on how cpSRP54 drives cpSRP43 binding to the L18 region of LHCP and the subsequent interaction of LHCP with the Alb3 translocase through structural rearrangements. The ankyrin proteins, sorting of cpTat substrates to thylakoid membranes (STT)1 and STT2, form a heterocomplex to selectively direct chloroplast twin-arginine translocation (cpTat) proteins to their thylakoid translocon through a liquid–liquid phase transition (LLPT) mechanism. Chloroplasts have multiple suborganellar membranes. Correct and efficient translocation of chloroplast proteins from their site of synthesis into or across membranes to their functional compartments are fundamental processes. In recent years, several new components and regulatory mechanisms involved in chloroplast protein import and sorting have been explored. Moreover, the formation of liquid–liquid phase transition (LLPT) has been recently reported as a novel mechanism for regulating chloroplast protein sorting. Here, we overview the recent advances of both nuclear- and chloroplast-encoded protein trafficking to their final destination within chloroplasts, and discuss the novel components and regulatory mechanisms of intrachloroplast sorting. Furthermore, we propose that LLPT may be a universal and conserved mechanism for driving organelle protein trafficking and organelle biogenesis. Chloroplasts have multiple suborganellar membranes. Correct and efficient translocation of chloroplast proteins from their site of synthesis into or across membranes to their functional compartments are fundamental processes. In recent years, several new components and regulatory mechanisms involved in chloroplast protein import and sorting have been explored. Moreover, the formation of liquid–liquid phase transition (LLPT) has been recently reported as a novel mechanism for regulating chloroplast protein sorting. Here, we overview the recent advances of both nuclear- and chloroplast-encoded protein trafficking to their final destination within chloroplasts, and discuss the novel components and regulatory mechanisms of intrachloroplast sorting. Furthermore, we propose that LLPT may be a universal and conserved mechanism for driving organelle protein trafficking and organelle biogenesis. protein containing ankyrin repeat domains. The ankyrin repeat is one of the most conserved and widely existing protein motifs in nature. It consists of 30–34 amino acid residues and functions to mediate protein–protein interactions. The intra- and inter-repeat hydrophobic and hydrogen bonding interactions can stabilize the global structure of ankyrin proteins. The unique features of ankyrin proteins in protein stability, folding, and binding specificity are based on the repetitive and elongated nature of ankyrin repeat domains. a chloroplast protein translocation system that imports unfolded proteins into lumen. Sec translocon is an evolutionary conserved system that exists in the bacterial plasma membrane, the eukaryotic endoplasmic reticulum, and in plant and algal chloroplasts. The cpSec system appears to be a minimal bacterial Sec system because only homologs of SecA (cpSecA), SecE (cpSecE), and SecY (cpSecY) have been identified whereas SecB, SecD, SecF, and SecG are all absent from the cpSec system. cpSecY and cpSecE form the narrow cpSec translocation channel (~12 Å, about the width of a polypeptide chain) and it can only handle the transport of unfolded proteins into the lumen. Like the bacterial Sec system, the cpSec translocon also utilizes cpSecA as a motor that powers preproteins translocation across the cpSecY/E channel. a chloroplast protein translocation system that translocates folded substrates into the lumen. The cpTat pathway is named for the twin arginines in the targeting signal sequence of cpTat substrates. The cpTat translocon comprises three membrane proteins, Tha4 (a homolog of bacterial TatA), Hcf106 (a homolog of bacterial TatB), and cpTatC (a homolog of bacterial TatC). The precursor proteins firstly bind the Hcf106-cpTatC receptor complex through a close contact between cpTatC and the RR motif of the substrate signal peptide. Then Tha4 is recruited by the Hcf106-cpTatC complex in the presence of the thylakoid PMF (proton motive force). When docking with the precursor-bound receptor complex, Tha4 oligomers clearly undergo a conformational reorganization to allow formfitting transport of precursor proteins. a chloroplast protein translocation system that targets the LHCPs to the thylakoid membranes. The cpSRP system is different from bacterial SRP systems because cpSRP lacks the associated RNA and consists of the conserved cpSRP54 and a unique chloroplast-specific cpSRP43 subunit. LHCP membrane integration requires the cpSRP43/cpSRP54 complex and its receptor cpFtsY, the integral translocase Alb3, and GTP, which is hydrolyzed by the GTPases cpSRP54 and cpFtsY. protein segments that are flexible and disordered, which are unlikely to form a defined 3D structure, but are nevertheless functional. IDRs are enriched in charged and in structure-breaking (Pro and Gly) and Ala residues but lack hydrophobic and aromatic residues and have a low content of Cys and Asn. IDRs are traditionally considered to be passive segments that link structured domains. However, it is now well established that IDRs are involved in diverse functions, including regulatory roles via diverse post-translational modifications, recruitment of binding partners, as well as mediating LLPT based on the conformational variability and adaptability. the process through which different biological molecules separate from each other by creating membraneless organelles; a crucial mechanism for spatiotemporal organization of various biochemical reactions. LLPT is highly dependent on the concentration and physicochemical properties of proteins and nucleic acids, and can be precisely regulated by temperature, pH, ionic strength, and so on. two translocation systems that translocate proteins across bacterial membranes. The β-barrel assembly machine (Bam) and the translocation and assembly module (Tam) are outer membrane protein transport systems in bacteria. The Bam complex consists of an outer membrane component BamA and several outer membrane associated proteins. The Tam complex also contains an outer membrane component TamA and an inner membrane protein TamB. BamA and TamA are homologues of chloroplast TOC75, and TamB is the homologue of chloroplast TIC236.