RT Journal Article SR Electronic T1 Assembly of photoactive Orange Carotenoid Protein from its domains unravels a carotenoid shuttle mechanism JF bioRxiv FD Cold Spring Harbor Laboratory SP 096651 DO 10.1101/096651 A1 Marcus Moldenhauer A1 Nikolai N. Sluchanko A1 David Buhrke A1 Dmitry V. Zlenko A1 Neslihan N. Tavraz A1 Franz-Josef Schmitt A1 Peter Hildebrandt A1 Eugene G. Maksimov A1 Thomas Friedrich YR 2016 UL http://biorxiv.org/content/early/2016/12/24/096651.abstract 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.