Orf2971: a link between chloroplast import and quality control in Chlamydomonas

Xing et al. investigate the link between chloroplast import and quality control in Chlamydomonas reinhardtii. Plant Cell https://doi.org/10.1093/plcell/koac180

By Jiale Xing, Junting Pan and Wenqiang Yang

Background: The chloroplast is an important bioreactor as well as a photosynthetic site. Approximately 3,000 plastid proteins encoded in the nucleus are translocated into the chloroplast envelope via the TOC and TIC machineries. Most nucleus-encoded preproteins that enter the plastid are unfolded as they traverse the TOC-TIC import complexes. To prevent these unfolded or misfolded proteins from causing chloroplast damage, a quality control mechanism comprising molecular chaperones and proteases ensures that all polypeptides entering chloroplasts are either correctly folded or degraded. However, there is still no consensus on the TIC complex’s components, motor proteins, or mechanism for refolding proteins entering the chloroplast.

Question: What is the precise function of each of the proteins in the TIC complex? What is the composition of the chloroplast protein import machinery motor? How are the newly imported chloroplast proteins refolded and assembled into functional complexes?

Findings: We found that Orf2971, encoded by the largest gene in the Chlamydomonas reinhardtii chloroplast genome and proposed to be an ortholog of Ycf2, is directly associated with the protein import machinery and plays a crucial role in ensuring the quality of proteins targeted to the chloroplast. Orf2971 deficiency induces protein precursor accumulation, partial proteolysis and protein aggregation, increased expression of chaperones and proteases, and autophagy. We hypothesize that Orf2971 is intimately linked to the protein import machinery and plays a critical role in chloroplast protein quality control.

Next steps: The next challenge is to identify the sorting components associated with this complex on the stromal side. Furthermore, additional experimental evidence is required to investigate the relationship between different import machineries, including the analysis of the accumulation of precursor proteins in the various import mutants.

Jiale Xing, Junting Pan, Heng Yi, Kang Lv, Qiuliang Gan, Meimei Wang, Haitao Ge, Xiahe Huang, Fang Huang, Yingchun Wang, Jean-David Rochaix, Wenqiang Yang. (2022). The plastid-encoded protein Orf2971 is required for protein translocation and chloroplast quality control. Plant Cell. https://doi.org/10.1093/plcell/koac180.