[2] Orange carotenoid-binding proteins (OCP) were first identified in cyanobacterial species, where they occur associated with phycobilisome in the cellular thylakoid membrane.

[3][4] In full-length OCP, the NPQ activity is regulated by photoactivation by strong blue-green light.

OCP seems to act as a homodimer, and binds one molecule of 3'-hydroxyechinenone (a ketocarotenoid) and one chloride ion per subunit.

The carotenoid binding site is lined with a striking number of methionine residues.

[5] This domain is implicated in binding the phycobilisome complex, which thereby facilitates thermal dissipation (quenching) of excess absorbed light energy.