Crosstalk between the chloroplast protein import and SUMO systems revealed through genetic and molecular investigation in Arabidopsis.

Samuel James Watson,Na Li,Feijie Wu,Yiting Ye,Qihua Ling,R Paul Jarvis

doi:10.7554/elife.60960

Abstract

The chloroplast proteome contains thousands of different proteins that are encoded by the nuclear genome. These proteins are imported into the chloroplast via the action of the TOC translocase and associated downstream systems. Our recent work has revealed that the stability of the TOC complex is dynamically regulated by the ubiquitin-dependent chloroplast-associated protein degradation pathway. Here, we demonstrate that the TOC complex is also regulated by the small ubiquitin-like modifier (SUMO) system. Arabidopsis mutants representing almost the entire SUMO conjugation pathway can partially suppress the phenotype of ppi1, a pale-yellow mutant lacking the Toc33 protein. This suppression is linked to increased abundance of TOC proteins and improvements in chloroplast development. Moreover, data from molecular and biochemical experiments support a model in which the SUMO system directly regulates TOC protein stability. Thus, we have identified a regulatory link between the SUMO system and the chloroplast protein import machinery.

Highlights

The chloroplast is a membrane-bound organelle that houses photosynthesis in all green plants (Jarvis and Lopez-Juez, 2013)
To investigate whether the TOC complex is targeted by the SUMO system, we obtained several Arabidopsis SUMO system mutants, crossed them with ppi[1], and carefully examined the phenotypes of the resulting double mutants
The genetic experiments demonstrated that SUMO system mutations can suppress the phenotype of the Toc[33] mutant, ppi[1], while the molecular and biochemical experiments indicated that TOC proteins associate with key SUMO system proteins and are likely

Summary

Introduction

The chloroplast is a membrane-bound organelle that houses photosynthesis in all green plants (Jarvis and Lopez-Juez, 2013). Chloroplasts retain small genomes, almost all of the proteins required for chloroplast development and function are encoded by the central, nuclear genome (Jarvis, 2008). These proteins must be imported into the organelle after synthesis in the cytosol, and this import is mediated by the coordinate action of the TOC and TIC complexes (the translocons at the outer and inner envelope membranes of chloroplasts) (Jarvis, 2008). Toc[33] and Toc[159] project into the cytosol and bind incoming preproteins

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Results

Conclusion