Chloroplast Biogenesis Is Regulated by Direct Action of the Ubiquitin-Proteasome System

TLDR

Chloroplast development relies on import of nucleus‑encoded proteins through TOC complexes, and modulation of this import is thought to control plastid proteome, developmental fate, and functions. Forward‑genetic screens identified SP1, a RING‑type ubiquitin‑E3 ligase localized to the chloroplast outer membrane. SP1 associates with TOC complexes, ubiquitinates and promotes degradation of TOC components, and sp1 mutants show impaired developmental transitions that involve plastid proteome changes, demonstrating that the ubiquitin‑proteasome system regulates plastid development.

Abstract

Development of chloroplasts and other plastids depends on the import of thousands of nucleus-encoded proteins from the cytosol. Import is initiated by TOC (translocon at the outer envelope of chloroplasts) complexes in the plastid outer membrane that incorporate multiple, client-specific receptors. Modulation of import is thought to control the plastid's proteome, developmental fate, and functions. Using forward genetics, we identified Arabidopsis SP1, which encodes a RING-type ubiquitin E3 ligase of the chloroplast outer membrane. The SP1 protein associated with TOC complexes and mediated ubiquitination of TOC components, promoting their degradation. Mutant sp1 plants performed developmental transitions that involve plastid proteome changes inefficiently, indicating a requirement for reorganization of the TOC machinery. Thus, the ubiquitin-proteasome system acts on plastids to control their development.