Chloroplast Evolution – the Requirement and Adaptation of Protein Transport

Chloroplast evolution began when the ancestral cyanobacterium was engulfed by a eukaryotic cell and became an endosymbiont. Successful conversion to the chloroplast depended on relocation of the genes from the endosymbiont to the host nucleus. This process required establishment of the protein import apparatus that allows uptake of cytosolically-synthesized proteins across the double-membrane envelope of the pre-organelle. In bacteria, membrane protein insertion usually occurs co-translationally. By contrast, nuclear-encode proteins found in the thylakoid membrane within the chloroplast are targeted post-translationally, thus need a novel strategy that prevents pre-mature folding or aggregation of hydrophobic proteins. Our research is centered on protein transport, which is required for and adapted during chloroplast evolution. More specifically, we address various questions about the protein import channel Toc75 (for translocon at the outer-envelope-membrane of chloroplasts! 75) and its paralog OEP80 (for outer envelope protein 80). These proteins are present in the outer membrane and share the common ancestor with BamA, which are essential proteins found in the outer membranes of Gram negative bacteria but do not catalyze protein import. We also study targeting of Plsp1 (for plastid type I signal peptidase 1), a single-pass protein of bacterial origin, to the envelope and thylakoid membranes within the chloroplast. I will discuss how we got there and what we are excited about. Supported by NSF-MCB and DOE-BES. Selected reference: Midorikawa T, Endow, JK, Dufour J, Zhu J, Inoue K (2014) Plastidic type I signal peptidase 1 is a redox-depenent thylakoid processing peptidase. The Plant Journal, 80, 592-603. Midorikawa T, Inoue K (2013) Multiple fates of nonmature lumenal proteins in thylakoids. The Plant Journal 76, 73-86. Nafati M, Inoue K (2013) Indispensable roles of plastids in Arabidopsis thaliana embryogenesis – update. Advances in Genomic Science 1, 234-252.