Split-Ubiquitin Two-Hybrid Screen for Proteins Interacting with slToc159-1 and slToc159-2, Two Chloroplast Preprotein Import Receptors in Tomato (Solanum lycopersicum)

The post-translational import of nuclear-encoded chloroplast preproteins is critical for chloroplast biogenesis, and the Toc159 family of proteins is the receptor for this process. Our previous work identified and analyzed the Toc GTPase in tomato; however, the tomato-specific transport substrate for Toc159 is still unknown, which limits the study of the function of the TOC receptor in tomato. In this study, we expand the number of preprotein substrates of slToc159 receptor family members using slToc159-1 and slToc159-2 as bait via a split-ubiquitin yeast two-hybrid membrane system. Forty-one specific substrates were identified in tomato for the first time. Using slToc159-1GM and slToc159-2GM as bait, we compared the affinity of the two bait proteins, with and without the A domain, to the precursor protein, which suggested that the A domain endowed the proproteins with subclass specificity. The presence of the A domain enhanced the interaction intensity of slToc159-1 with the photosynthetic preprotein but decreased the interaction intensity of slToc159-2 with the photosynthetic preprotein. Similarly, the presence of the A domain also altered the affinity of slToc159 to non-photosynthetic preproteins. Bimolecular fluorescence complementation (BiFC) analysis showed that A domain had the ability to recognize the preprotein, and the interaction occurred in the chloroplast. Further, the localization of the A domain in Arabidopsis protoplasts showed that the A domain did not contain chloroplast membrane targeting signals. Our data demonstrate the importance of a highly non-conserved A domain, which endows the slToc159 receptor with specificity for different protein types. However, the domain containing the information on targeting the chloroplast needs further study.