SUMOylation contributes to proteostasis of the chloroplast protein import receptor TOC159 during early development.

Sonia Accossato,Felix Kessler,Venkatasalam Shanmugabalaji

doi:10.7554/elife.60968

Abstract

Chloroplast biogenesis describes the transition of non-photosynthetic proplastids to photosynthetically active chloroplasts in the cells of germinating seeds. Chloroplast biogenesis requires the import of thousands of nuclear-encoded preproteins by essential import receptor TOC159. We demonstrate that the small ubiquitin-related modifier (SUMO) pathway crosstalks with the ubiquitin-proteasome pathway to affect TOC159 stability during early plant development. We identified a SUMO3-interacting motif (SIM) in the TOC159 GTPase domain and a SUMO3 covalent SUMOylation site in the membrane domain. A single K to R substitution (K1370R) in the M-domain disables SUMOylation. Compared to wild-type TOC159, TOC159K1370R was destabilized under UPS-inducing stress conditions. However, TOC159K1370R recovered to same protein level as wild-type TOC159 in the presence of a proteasome inhibitor. Thus, SUMOylation partially stabilizes TOC159 against UPS-dependent degradation under stress conditions. Our data contribute to the evolving model of tightly controlled proteostasis of the TOC159 import receptor during proplastid to chloroplast transition.

Highlights

Chloroplasts are unique organelles that carry out photosynthesis
It has been demonstrated previously that TOC159 physically interacts with the Small Ubiquitin-like Modifier (SUMO) E2 enzyme in a yeast two-hybrid screen and that the SUMO3 isoform covalently
(“VKVLP”) in the G-domain of TOC159. We confirmed this prediction using a yeast two hybrid assay and co-immunoprecipitation experiment It revealed that the G-domain interacted with the SUMO3 isoform and not with SUMO1 and -2. (Figure 1B)

Summary

Introduction

Chloroplasts are unique organelles that carry out photosynthesis Chloroplasts contain their own genome, the majority of chloroplast proteins are encoded by the nuclear genome and synthesized as preproteins in cytosol, and these preproteins are imported into the chloroplast through TOC-TIC complexes (Translocon at the Outer or Inner membrane of the Chloroplast) [1]. The core of the TOC complex contains two related GTP-dependent preprotein receptor GTPases, Toc159 and Toc, which interact with a β-barrel membrane protein, Toc, that forms a protein-conducting channel, and is regulated by specific interactions with nuclear encoded preproteins [2-4]. Toc159 is a major point of entry for highly abundant photosynthesis-associated preproteins arriving at the translocon complex. It is regarded as the major chloroplast protein import receptor. Chloroplast biogenesis, the transition of a non-photosynthetic proplastid to a photosynthetically active chloroplast, depends on the essential import receptor Toc159 and its mutation results in nonphotosynthetic albino plants [5]

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Conclusion