Light‐controlled carotenoid transfer between water‐soluble proteins related to cyanobacterial photoprotection

Abstract

Carotenoids are lipophilic pigments with multiple biological functions from coloration to vision and photoprotection. Still, the number of water‐soluble carotenoid‐binding proteins described to date is limited, and carotenoid transport and carotenoprotein maturation processes are largely underexplored. Recent studies revealed that CTDHs, which are natural homologs of the C‐terminal domain (CTD) of the orange carotenoid protein (OCP), a photoswitch involved in cyanobacterial photoprotection, are able to bind carotenoids, with absorption shifted far into the red region of the spectrum. Despite the recent discovery of their participation in carotenoid transfer processes, the functional roles of the diverse family of CTDHs are not well understood. Here, we characterized CTDH carotenoproteins from Anabaena variabilis (AnaCTDH) and Thermosynechococcus elongatus and examined their ability to participate in carotenoid transfer processes with a set of OCP‐derived proteins. This revealed that carotenoid transfer occurs in several directions guided by different affinities for carotenoid and specific protein–protein interactions. We show that CTDHs have higher carotenoid affinity compared to the CTD of OCP from Synechocystis, which results in carotenoid translocation from the CTD into CTDH via a metastable heterodimer intermediate. Activation of OCP by light, or mutagenesis compromising the OCP structure, provides AnaCTDH with an opportunity to extract carotenoid from the full‐length OCP, either from Synechocystis or Anabaena. These previously unknown reactions between water‐soluble carotenoproteins demonstrate multidirectionality of carotenoid transfer, allowing for efficient and reversible control over the carotenoid‐mediated protein oligomerization by light, which gives insights into the physiological regulation of OCP activity by CTDH and suggests multiple applications.

Soluble cyanobacterial carotenoprotein as a robust antioxidant nanocarrier and delivery module

“…We scrutinized the process of carotenoid uptake from membranes by CTDH and showed that ECN distribution is in a dynamic equilibrium which is shifted from the protein to membrane (35 vs. 65 %, respectively), permitting efficient delivery of carotenoids into membranes ( Figure 5). Moreover, light could be potentially used in order to activate the process of carotenoid delivery into membranes by CTDH from the photoconvertible OCP ( Figure S4) (24).…”

“…The presence of two keto-groups in CAN results in the most efficient carotenoid binding in the CTDH dimer, while ECN binding is apparently weaker. Since both types of CTDH can transfer carotenoids into other proteins we postulate that spontaneous monomerization of protein dimer occurs regardless of the carotenoid type (24). However, only CTDH monomers in which ketogroup of ECN loses connection with the protein give carotenoid an opportunity to escape another protein subunit and get to the membrane once again.…”

“…In contrast to OCP-CTD dimer (also called COCP), almost exclusively binding CAN (29,44), Apo-CTDH may bind both ECN and CAN, which confers different spectral properties and colours to the corresponding holoprotein forms (24). To monitor the dynamics of the holoform assembly or kinetics of the carotenoid transfer, we first studied spectroscopic signatures of ECN and CAN in different environments in more detail ( Figure S1).…”

“…Besides that, OCP is also an efficient ROS quencher (23). Upon expression in carotenoid-producing Escherichia coli strains, OCP-like proteins can bind echinenone (ECN), canthaxanthin (CAN), zeaxanthin and β-carotene (24)(25)(26). Assembly of these water-soluble carotenoproteins requires carotenoid absorption from membranes, however, the mechanism of this process is barely known.…”

“…Very recently, it was demonstrated in vitro that a natural homolog of the C-terminal domain of OCP (hereinafter CTDH) from Anabaena can take keto-carotenoids from membranes of overproducing E.coli strains (27,28). This leads to the maturation of the initially colorless CTDH apoprotein into a violet, soluble nm-sized holoprotein via the carotenoid-mediated protein homodimerization (24,28).…”