Structure and functions of Orange Carotenoid Protein homologs in cyanobacteria

Rapidly-induced photoprotection in cyanobacteria involves thermal dissipation of excess energy absorbed by the phycobilisome (PBS), the primary light-harvesting antenna. This process is called non-photochemical quenching (NPQ), and is mediated by a water-soluble photoactive protein, the Orange Carotenoid Protein (OCP). The OCP is structurally and functionally modular, consisting of a sensor domain, an effector domain, and a carotenoid. Blue-green light induces a structural transition of the OCP from the orange inactive form, OCP^o, to the red active form, OCP^R. Translocation of the carotenoid into the effector domain accompanies photoactivation. The OCP^R binds to the PBS core, where it triggers dissipation of excitation energy and quenches fluorescence. To recover the antenna capacity under low light conditions, the Fluorescence Recovery Protein (FRP) participates in detaching the OCP from the PBS and accelerates back-conversion of OCP^R to OCP^o. Increased sequencing of cyanobacterial genomes has allowed the identification of new paralogous families of the OCP and its domain homologs, the Helical Carotenoid Proteins (HCPs), which have been found distributed widely among taxonomically and ecophysiologically diverse cyanobacteria. Distinct functions from the canonical OCP have been revealed for some of these paralogs by recent structural and functional studies.