Orange carotenoid N-terminal domain

In molecular biology the orange carotenoid N-terminal domain is a protein domain found predominantly at the N-terminus of prokaryotic orange carotenoid proteins and in related carotenoid-binding proteins. It adopts an alpha-helical structure consisting of two four-helix bundles.

Orange carotenoid-binding proteins (OCP) were first identified in cyanobacterial species, where they occur associated with phycobilisome in the cellular thylakoid membrane. These proteins function in photoprotection, and are essential for inhibiting white and blue-green light non-photochemical quenching (NPQ). Carotenoids improve the photoprotectant activity by broadening OCP's absorption spectrum and facilitating the dissipation of absorbed energy. OCP acts as a homodimer, and binds one molecule of carotenoid (3'-hydroxyechinenone) and one chloride ion per subunit, where the carotenoid binding site is lined with a striking number of methionine residues. The carotenoid 3'-hydroxyechinenone is not found in higher plants. OCP has two domains: an N-terminal helical domain and a C-terminal domain that resembles a NTF2 (nuclear transport factor 2) domain. OCP can be proteolytically cleaved into a red form (RCP), which lacks 15 residues from the N terminus and approximately 150 residues from the C terminus.