A converged ubiquitin-proteasome pathway for the degradation of TOC and TOM tail-anchored receptors

In plants, thousands of nucleus-encoded proteins translated in the cytosol are sorted to chloroplasts and mitochondria by binding to specific receptors of the TOC (translocon at the outer membranes of chloroplasts) and TOM (translocon at the outer membranes of mitochondria) complexes for import into those organelles. The degradation pathways for these receptors are unclear. Here, we discovered a converged ubiquitin-proteasome pathway for the degradation ofArabidopsis thalianaTOC and TOM tail-anchored receptors. The receptors are ubiquitinated by E3 ligase(s) and pulled from the outer membranes by the AAA+ATPase CDC48, after which a previously characterized cytosolic protein, TTOP, binds to the exposed transmembrane domains (TMDs) at the C termini of the receptors and CDC48, and delivers the complexes to the 26S proteasome.