Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system

Abstract

Chloroplasts are photosynthetic plant organelles descended from a bacterial ancestor. The vast majority of chloroplast proteins are synthesized in the cytosol and then imported into the chloroplast post-translationally. Translocation complexes exist in the organelle's outer and inner envelope membranes (termed TOC and TIC, respectively) to facilitate protein import. These systems recognize chloroplast precursor proteins and mediate their import in an energy-dependent manner. However, many unanswered questions remain regarding mechanistic details of the import process and the participation and functions of individual components; for example, the cytosolic events that mediate protein delivery to chloroplasts, the composition of the TIC apparatus, and the nature of the protein import motor all require resolution. The flux of proteins through TOC and TIC varies greatly throughout development and in response to specific environmental cues. The import process is, therefore, tightly regulated, and it has emerged that the ubiquitin-proteasome system (UPS) plays a key role in this regard, acting at several different steps in the process. The UPS is involved in: the selective degradation of transcription factors that co-ordinate the expression of chloroplast precursor proteins; the removal of unimported chloroplast precursor proteins in the cytosol; the inhibition of chloroplast biogenesis pre-germination; and the reconfiguration of the TOC apparatus in response to developmental and environmental signals in a process termed chloroplast-associated protein degradation. In this review, we highlight recent advances in our understanding of protein import into chloroplasts and how this process is regulated by the UPS.

Coexpressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts

“…To this end, we used tandem mass spectrometric analysis of protein extracts from strain Y14 collected following Tic214 repression and compared the results to the A31 control strain ( Dataset S8 ). Since unimported chloroplast proteins are quickly degraded by the cytosolic ubiquitin-26S proteasome system ( 4 , 67 , 68 ), we incubated each culture with the proteasome inhibitor MG132 before sampling. We limited our analysis to those proteins for which at least 10 peptides could be identified in one of the six conditions used in the experiment (2, 4, and 6 d; −/+ Vit).…”

“…The translocons are multiprotein complexes, located in the outer and inner chloroplast envelope membranes. Receptor proteins intrinsic to the translocons recognize a signal peptide (for chloroplasts and mitochondria also called transit peptide) at the N terminus of chloroplast precursor proteins that is cleaved off and quickly degraded upon import ( 2 – 7 ). Based on the membrane in which the translocons reside, they are referred to as translocon of the outer and inner chloroplast membrane (TOC and TIC), respectively ( 8 ).…”

Co-expressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts

“…To this end, we used tandem mass spectrometric analysis of protein extracts from strain Y14 collected following Tic214 repression and compared the results to the A31 control strain (SI Appendix, Table S10). Since unimported chloroplast proteins are quickly degraded by the cytosolic ubiquitin-26S proteasome system (16,80,81), we incubated each culture with the proteasome inhibitor MG132 before sampling. We limited our analysis to those proteins for which at least ten peptides could be identified in one of the six conditions used in the experiment (2, 4, and 6 days; -/+Vit).…”

“…The translocons are multiprotein complexes, located in the outer and inner chloroplast envelope membranes. Receptor proteins intrinsic to the translocons recognize a signal-peptide (for chloroplasts and mitochondria also called transit-peptide) at the N-terminus of chloroplast precursor proteins that is cleaved off and quickly degraded upon import [original findings: (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12); recent reviews: (13)(14)(15)(16)(17)(18)(19)]. Based on the membrane in which the translocons reside, they are referred to as translocon of the outer and inner chloroplast membrane (TOC and TIC), respectively (20).…”

The Secret Life of Chloroplast Precursor Proteins in the Cytosol

“…Furthermore, precursor identification does not correlate with expression level, indicating that their detection is not a simple question of abundance (Bischof et al, 2011). Therefore, the turnover of cytosolic precursor proteins by the UPS seems tightly regulated, possibly by their specific interaction with either HSP70 or HSP90 family proteins and their recognition by cytosolic E3 ligases (Lee et al, 2009; reviewed in Thomson et al, 2020). The diverse interactions most likely generate specific cytosolic precursor profiles, and it is tempting to speculate that these may trigger precursor-specific signaling responses.…”