Figure 9: Chloroplast-Associated Protein Degradation

Chloroplast-associated protein degradation (CHLORAD) is a proteolytic system that selectively removes chloroplast-resident proteins, including TOC components of the chloroplast protein import apparatus. The E3 ligase SP1 mediates the ubiquitination (Ub) of target proteins; it has a RING finger (RNF) domain for ubiquitin-conjugating enzyme (E2) recruitment and an intermembrane space (IMS) domain that binds to its targets. The beta-barrel SP2 and the cytosolic ATPase CDC48 together mediate target protein "retrotranslocation" to the cytosol, respectively providing a channel and driving force for the process. On release to the cytosol, target proteins are degraded by the 26S proteasome (26SP). Additional, as yet unknown factors are shown in gray. Taken from Science 363: eaav4467.