Ubiquitin-based pathway acts inside chloroplasts to regulate photosynthesis

Abstract

Photosynthesis is the energetic basis for most life on Earth, and in plants it operates inside double-membrane-bound organelles called chloroplasts. The photosynthetic apparatus comprises numerous proteins encoded by the nuclear and organellar genomes. Maintenance of this apparatus requires the action of internal chloroplast proteases, but a role for the nucleocytosolic ubiquitin-proteasome system (UPS) was not expected owing to the barrier presented by the double-membrane envelope. Here, we show that photosynthesis proteins (including those encoded internally by chloroplast genes) are ubiquitinated, and processed via the CHLORAD pathway: they are degraded by the 26S proteasome following CDC48-dependent retrotranslocation to the cytosol. This demonstrates that the reach of the UPS extends to the interior of endosymbiotically-derived chloroplasts, where it acts to regulate one of the most fundamental processes of life.

Functions of autophagy in chloroplast protein degradation and homeostasis

“…Nonetheless, there is no clear evidence that these two E3 ligases are directly related to chlorophagy. Very recently, it has been shown by two independent laboratories that much broader chloroplast proteins, even including those resident in internal compartments of the chloroplast, can be targeted for ubiquitination ( Li et al., 2022 ; Sun et al., 2022 ; Trosch, 2022 ). It is therefore intriguing to investigate whether ubiquitin-modified chloroplast proteins can also be a signal to induce chlorophagy.…”

The Role of E3 Ubiquitin Ligases in Chloroplast Function

“…Among these ubiquitinated proteins were the Rubisco large subunit (RbcL) and ATP synthase subunit beta (AtpB), both of which are chloroplast encoded and degraded through the CHLORAD pathway [ 39 ]. Most recently, an unpublished study by Sun et al demonstrated additional targets of the CHLORAD pathway involved in photosynthesis (i.e., electron transport, energy transduction, and carbon fixation), gene expression, and fatty acid/lipid metabolism [ 126 ]. One important target substrate of interest is fatty acid export 1 (FAX1), an inner chloroplast envelope protein and key player in fatty acid (FA) export.…”

“…One important target substrate of interest is fatty acid export 1 (FAX1), an inner chloroplast envelope protein and key player in fatty acid (FA) export. In the presence of defective CHLORAD-mediated degradation of FAX1, disturbances in cellular metabolic homeostasis (i.e., a notable decrease in cellular lipid species and chloroplast-produced FAs) were observed [ 126 ]. Altogether, these findings raise questions about how intra-chloroplast proteins are selected by the CHLORAD pathway and if intra-chloroplast proteins are marked by ubiquitin inside the organelle [ 39 , 127 ].…”

The Role of E3 Ubiquitin Ligases in Chloroplast Function

“…Beyond chloroplast protein import machinery, extensive ubiquitination was recently found in chloroplast fractions, and in greater abundance under UPS or retrotranslocation inhibition. Among these ubiquitinated proteins were the Rubisco large subunit (RbcL) and ATP synthase subunit beta (AtpB), both of which are chloroplast encoded and degraded through the CHLORAD pathway [38] a notable decrease in cellular lipid species and chloroplast-produced FAs) were observed [125]. Altogether, these findings raise questions about how intra-chloroplast proteins are selected by the CHLORAD pathway and if intrachloroplast proteins are marked by ubiquitin inside the organelle [38,126].…”

“…and carbon fixation), gene expression, and fatty acid/lipid metabolism[125]. One important target substrate of interest is fatty acid export 1 (FAX1), an inner chloroplast envelope protein and key player in fatty acid (FA) export.…”