Fundamental mechanisms and application aspects of protein-mediated carotenoid transport

• Applicant: Professor Dr. Thomas Friedrich
• Subject Area: Biochemistry ; Biophysics ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429542475

Carotenoids abound in photosynthetic organisms and are essential nutrients for animals. The more than 700 species perform multifaceted functions ranging from coloration, energy harvesting and excitation energy dissipation in photosynthesis, light protection and antioxidant activity, and they are precursors for vitamins, visual pigments and hormones. Though carotenoids are hydrophobic and partition into lipid compartments or membranes, only very few specific carotenoid-binding proteins are known, which keep carotenoids in water-soluble form to shuttle them between membranes, between carotenoproteins or between cells in the organism. One of these examples is the 35-kDa Orange Carotenoid Protein (OCP), which plays a central role in cyanobacterial photoprotection. Upon illumination with blue light, OCP is photoconverted from the basic, orange form OCPo into the red signaling form OCPr, the latter quenching the fluorescence of phycobilisome (PBs) antennae, thus preventing excessive energy flow to the photosystems to enact non-photochemical quenching (NPQ).During our studies on a series of OCP variants, we discovered that carotenoids can be transferred between the isolated domains of OCP. Biochemical and spectroscopic data hinted at several intermediates of this process suggesting a multi-step nature of this unique carotenoid transfer mechanism. It was later shown that naturally occurring homologs of the OCP C-terminal domain (CTDH proteins) are able to regulate photoprotection in several cyanobacteria lacking full-length OCP by transferring the carotenoid from membranes to homologs of the OCP N-terminal domain (HCP proteins), which are constantly able to quench PBs fluorescence. This novel type of regulation of photosynthetic activity is only superficially studied yet. Given the huge number of CTDHs with vastly different sequences, as well as the existence of eukaryotic proteins, which are similar to CTDHs in spatial structure (nuclear transport factor 2 [NTF-2] family), the principles of protein-mediated carotenoid transport can be explored in detail. We have shown that various CTDH proteins specifically bind certain types of carotenoids, which strongly influences the spectral and structural properties, and the interaction of CTDH proteins with membranes. Since other NTF-2-like proteins specifically bind carotenoids as well (e.g. the lutein-binding StARD3 protein in primate retinae), the identification of common mechanisms of carotenoid transport systems is within reach. This project aims to explore these fundamental features of protein-mediated carotenoid transport and establish the overarching functional principles in molecular detail, taking benefit from a multidisciplinary, systemic approach (biochemistry, biophysics, structural and molecular biology, steady-state and time-resolved spectroscopy). As a project result, the potentials for using CTDH and HCP protein variants as modules for the targeted delivery of antioxidants will be explored.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Foundation for Basic Research
• Cooperation Partner: Nikolai N. Sluchanko, Ph.D.