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 and depends on the essential import receptor TOC159, mutation of which results in non-photosynthetic albino plants. We previously showed that ubiquitin-proteasome system (UPS)-dependent regulation of TOC159 levels contributes to the regulation of chloroplast biogenesis during early plant development. Here, we demonstrate that the SUMO (Small Ubiquitin-related Modifier) 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 (G-) domain and a SUMO3 covalent SUMOylation site in the membrane (M-) domain. A single K to R substitution (K1370R) in the M-domain disables SUMOylation. Expression of the TOC159K1370R mutant in the toc159 mutant (ppi2) complemented the albino phenotype. 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.
Competing Interest Statement
The authors have declared no competing interest.
