text: The three-dimensional protein structure of OCP (in the OCP form) was solved in 2003, before its photoprotective role had been defined.  The 35 kDa protein contains two structural domains: an all-α-helical N-terminal domain (NTD) consisting of two interleaved 4-helix bundles, and a mixed α/β C-terminal domain (CTD).  The two domains are connected by an extended linker.  In OCP, the carotenoid spans both domains, which are tightly associated in this form of protein. In 2013 Kerfeld and co-workers showed that the NTD is the effector (quencher) domain of the protein while the CTD plays a regulatory role.