Structural Rearrangements in the C-Terminal Domain Homolog of Orange Carotenoid Protein are Crucial for Carotenoid Transfer

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. Dvir Harris et al. present the structure of a homolog of the orange carotenoid protein (OCP) C-terminal domain, elaborating on this protein family’s carotenoid transfer mechanism. They observed major structural shifts in the homolog compared to that of the OCP C-terminal domain, with a strong positive impact on carotenoid uptake and delivery.