Structural rearrangements in the C-terminal domain homolog of orange carotenoid protein are crucial for carotenoid transfer

Dvir Harris ¹ Adjele Wilson ² Fernando Muzzopappa ² Nikolai N. Sluchanko ³ Thomas Friedrich ⁴ Eugene G. Maksimov ³ Diana Kirilovsky ² Noam Adir ¹

¹Schulich Faculty of Chemistry and Grand Technion Energy Program (GTEP), Technion – Israel Institute of Technology, Haifa, Israel
²Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, Université Paris-Saclay, Gif sur Yvette, France
³Department of Biophysics, Faculty of Biology, Lomonosov State University, Moscow, Russia
⁴Institute of Chemistry, Technical University of Berlin, Berlin, Germany

A recently reported family of soluble cyanobacterial carotenoproteins, homologs of the C-terminal domain (CTDH) of the photoprotective Orange Carotenoid Protein, is suggested to mediate carotenoid transfer from the thylakoid membrane to the Helical Carotenoid Proteins, which are paralogs of the N-terminal domain of the OCP. Here we present the three-dimensional structure of a carotenoid-free CTDH variant from Anabaena (Nostoc) PCC 7120. This CTDH contains a cysteine residue at position 103. Two dimer-forming interfaces were identified, one stabilized by a disulfide bond between monomers and the second between each monomer’s β-sheets, both compatible with small-angle X-ray scattering data and likely representing intermediates of carotenoid transfer processes. The crystal structure revealed a major positional change of the C-terminal tail. Further mutational analysis revealed the importance of the C-terminal tail in both carotenoid uptake and delivery. These results have allowed us to suggest a detailed model for carotenoid transfer via these soluble proteins.