PT - JOURNAL ARTICLE AU - Marcus Moldenhauer AU - Nikolai N. Sluchanko AU - David Buhrke AU - Dmitry V. Zlenko AU - Neslihan N. Tavraz AU - Franz-Josef Schmitt AU - Peter Hildebrandt AU - Eugene G. Maksimov AU - Thomas Friedrich TI - Assembly of photoactive Orange Carotenoid Protein from its domains unravels a carotenoid shuttle mechanism AID - 10.1101/096651 DP - 2016 Jan 01 TA - bioRxiv PG - 096651 4099 - http://biorxiv.org/content/early/2016/12/24/096651.short 4100 - http://biorxiv.org/content/early/2016/12/24/096651.full AB - The Orange Carotenoid Protein (OCP) is indispensable for cyanobacterial photoprotection by quenching phycobilisome fluorescence upon photoconversion from the orange OCPO to the red OCPR form. Cyanobacterial genomes frequently harbor, besides genes for Orange Carotenoid Proteins (OCPs), several genes encoding homologs of OCP’s N- or C-terminal domains (NTD, CTD). Unlike the well-studied NTD homologs, called Red Carotenoid Proteins (RCPs), the role of CTD homologs remains elusive. We show how OCP can be reassembled from its functional domains. Expression of Synechocystis OCP-CTD in carotenoid-producing Escherichia coli yielded violet-colored proteins, which, upon mixing with the RCP-apoprotein, produced an orange-like photoswitchable form that further photoconverted into a species spectroscopically indistinguishable from RCP, thus demonstrating a unique carotenoid shuttle mechanism. The CTD itself is a novel, dimeric carotenoid-binding protein, which effectively quenches singlet oxygen and interacts with the Fluorescence Recovery Protein, assigning physiological roles to CTD homologs and explaining the evolutionary process of OCP formation.One Sentence Summary The C-domain of cyanobacterial OCP dimerizes, binds a carotenoid, and delivers it to the N-domain forming photoactive OCP.