Structural basis for the carotenoid binding and transport function of a START domain

• Published in: Structure (London) 2022-12, Vol.30 (12), p.1647-1659.e4
• Main Authors: Sluchanko, Nikolai N., Slonimskiy, Yury B., Egorkin, Nikita A., Varfolomeeva, Larisa A., Kleymenov, Sergey Yu, Minyaev, Mikhail E., Faletrov, Yaroslav V., Moysenovich, Anastasia M., Parshina, Evgenia Yu, Friedrich, Thomas, Maksimov, Eugene G., Boyko, Konstantin M., Popov, Vladimir O.
• Format: Article
• Language: English
• Subjects: Animals; antioxidants; Bombyx - metabolism; carotenoid transfer; Carotenoids - metabolism; carotenoprotein; Carrier Proteins - chemistry; circular dichroism of carotenoids; crystallography; etinal degeneration; Humans; ligand-induced protein stabilization; lipid-exchange; Lutein - chemistry; Lutein - metabolism; oligomeric state; oxidative stress; Zeaxanthins - metabolism
• ISSN: 0969-2126; 1878-4186
• DOI: 10.1016/j.str.2022.10.007

STARD3, a steroidogenic acute regulatory lipid transfer protein, was identified as a key xanthophyll-binding protein in the human retina. STARD3 and its homologs in invertebrates are known to bind and transport carotenoids, but this lacks structural elucidation. Here, we report high-resolution crystal structures of the apo- and zeaxanthin (ZEA)-bound carotenoid-binding protein from silkworm Bombyx mori (BmCBP). Having a STARD3-like fold, BmCBP features novel elements, including the Ω1-loop that, in the apoform, is uniquely fixed on the α4-helix by an R173-D279 salt bridge. We exploit absorbance, Raman and dichroism spectroscopy, and calorimetry to describe how ZEA and BmCBP mutually affect each other in the complex. We identify key carotenoid-binding residues, confirm their roles by ZEA-binding capacity and X-ray structures of BmCBP mutants, and also demonstrate that markedly different carotenoid-binding capacities of BmCBP and human STARD3 stem from differences in the structural organization of their carotenoid-binding cavity. [Display omitted] •Crystal structures of the BmCBP START domain in the apo- and ZEA-bound form•The conformation of BmCBP monomer observed in the crystal is preserved in solution•Binding of one ZEA molecule in the cavity of BmCBP raises its thermostability by 30°C•Mutagenesis explains differences in carotenoid binding by BmCBP and human STARD3 START domains bind and transfer different lipids including carotenoids in many species. Sluchanko et al. determined the crystal structures of the silkworm Carotenoid-Binding Protein in the apo- and zeaxanthin-bound forms and used mutagenesis, spectroscopy and calorimetry measurements to provide further insights into the carotenoid binding and transport mechanism of this protein domain.