Ubiquitin-dependent chloroplast-associated protein degradation in plants

Chloroplast-associated protein degradation Protein degradation is vital for cellular functions, and it operates selectively with distinct mechanisms in different organelles. Some organellar proteins are targeted by the ubiquitin-proteasome system (UPS)—a major proteolytic network in the eukaryotic cytosol. In such cases, the organelle membrane presents a substantial barrier to protein degradation. Working in the model plant Arabidopsis , Ling et al. identified mechanisms underlying the UPS-mediated degradation of proteins in the outer membrane of chloroplasts (the organelles responsible for photosynthesis). They identified an Omp85-type β-barrel outer membrane channel and a cytosolic AAA + chaperone that fulfill conductance and motor functions in the retrotranslocation of target proteins from chloroplasts. This process thus enabled outer membrane protein processing by the cytosolic proteasome. Such chloroplast-associated protein degradation was initiated by ubiquitination of the targets by the chloroplast-localized E3 ubiquitin ligase SP1. Science , this issue p. eaav4467