PSC Frontier Seminar series
Title: The Regulation of Chloroplast Protein Import by the Ubiquitin-Proteasome System
Speaker: Prof. Qihua Ling, CAS Center for Excellence in Molecular Plant Sciences, Institute of Plant Physiology and Ecology
Time: 3:30 pm
Day/Date: Oct. 15th, 2020 (Thursday)
Host: Prof. Chanhong Kim
Venue: PSC Auditorium
Chloroplasts belong to a family of plant organelles called plastids, which includes several nonphotosynthetic variants, such as etioplasts in dark-grown seedlings and gerontoplasts in senescent leaves. Development of 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 suppressor of ppi1 locus 1 (sp1), and showed that it could specifically suppress a TOC component mutant, ppi1, in the plant phenotype, the ultrastructure of chloroplasts, and the import defect. SP1 encodes a ubiquitin E3 ligase, a component of the ubiquitin-proteasome system (UPS) for protein degradation. SP1 is located at the outer envelope of chloroplasts, which gives evidencek that the UPS directly regulates chloroplast-resident proteins. The SP1 protein associates with TOC complexes and mediates ubiquitination of TOC components, promoting their degradation. Recently, we showed that SP1 operates as part of a broader pathway, which we named CHLORAD. In this, a retrotranslocation system comprising SP2 (channel) and CDC48 (motor) delivers ubiquitinated chloroplast proteins to the cytosolic proteasome for degradation. SP2 is an Omp85-type β-barrel of chloroplast, whereas CDC48 is cytosolic. CHLORAD is important for plastid type transitions, for example during de-etiolation (dark-to-light transition) and leaf senescence. In addition. It also plays a critical role in plant responses to abiotic stress, including salinity, drought and oxidative stresses.
The seminar will be delivered in English .