Arabidopsis OR protein regulates plastid preprotein import through interacting with Tic proteins

Authors: YUAN, HUI - Cornell University; PAWLOWSKI, EMILY - Michigan State University; Yang, Yong; SUN, TIANHU - Cornell University; Thannhauser, Theodore - Ted; MAZOUREK, MICHAEL - Cornell University; SCHNELL, DANNY - Michigan State University; Li, Li

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2020
Publication Date: 11/9/2020
Citation: Yuan, H., Pawlowski, E., Yang, Y., Sun, T., Thannhauser, T.W., Mazourek, M., Schnell, D., Li, L. 2020. Arabidopsis OR protein regulates plastid preprotein import through interacting with Tic proteins. Journal of Experimental Botany. 2(4):1059-1072. https://doi.org/10.1093/jxb/eraa528.
DOI: https://doi.org/10.1093/jxb/eraa528

Interpretive Summary: Plastids are plant organelles that perform various functions. The import of nucleus-encoded proteins into plastids is critical to meet the requirement for all aspects of plastid formation and function. While the key components of chloroplast import pathway are well defined, additional proteins are expected to interact with the core components to facilitate plastid preprotein import. In this study, we discovered that the ORANGE (OR) protein, a key regulator of carotenoid biosynthesis and plastid development, physically interacts with several core proteins in the classic chloroplast import pathway. Alteration of OR expression affects preprotein import efficiency, which occurs at the inner envelope membrane of chloroplasts. OR was found to mediate plastid preprotein import by interfering with the interaction between Tic40 and Tic110, and reducing the binding of preproteins to Tic110 to aid their release for translocation and processing. Our findings demonstrate that OR is a key protein to facilitate the later stage of plastid preprotein translocation in regulating chloroplast preprotein import.

Technical Abstract: Chloroplast targeted proteins are actively imported into chloroplasts via the machinery spanning the double layered membranes of chloroplasts. While the key translocons at the outer (TOC) and inner (TIC) membranes of chloroplasts are defined, proteins that interact with the core machinery to facilitate preprotein import are continuously being discovered. A DnaJ-like chaperone ORANGE (OR) protein is known to regulate carotenoid biosynthesis as well as plastid biogenesis and development. In this study, we found that OR physically interacts with several Tic proteins including Tic20, Tic40, and Tic110 in the classic TIC core complex of the chloroplast import machinery. Knocking out or and its homologous gene or-like greatly affects the import efficiency of some photosynthetic and non-photosynthetic preproteins. Consistent with the direct interactions of OR with Tic proteins, the binding efficiency assay revealed that the effect of OR likely occurs at translocation at the inner envelope membrane (i.e. at the TIC complex). OR is able to reduce Tic40 protein turnover rate through its chaperone activity. Moreover, OR was found to interfere with the interaction between Tic40 and Tic110, and reduces the binding of preproteins to Tic110 in aiding their release for translocation and processing. Our findings suggest that OR plays a new and regulatory role in stabilizing key translocons and in facilitating the late stage of plastid preprotein translocation to regulate plastid preprotein import.