Fig. 5.

CTDH carotenoid uptake and delivery cycle. apo-AnaCTDH monomer (1) or a dimer in its A-type state (2′) may extract a carotenoid molecule from the membrane with the assistance of the C-terminal tail (labeled as CTT in the figure). Then a holo-dimer is formed (3) in an F-type state (with association of another monomer; 2) Then the holo-CTDH can either be redox regulated in its F-type state between the reduced (3) and oxidized states (7) or interact with apo-HCP to allow carotenoid transfer through the A-type state with the assistance of the C-terminal tail (4). This yields the holo-HCP and the apo-CTDH dimer that can undergo A-to-F type transition and either be in its reduced state (5) and thus can take another carotenoid molecule or be modified to its oxidized states (6). Different carotenoid molecule colors represent different binding modes and thus different spectroscopic characteristics