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

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.