Ubiquitin-based pathway acts inside chloroplasts to regulate photosynthesis

Photosynthesis is the energetic basis for most life on Earth, and in plants it operates inside doublemembrane- 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 photosynthesis, arguably the most fundamental process of life.