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

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 ca-rotenoids, 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 U1-loop that, in the apoform, is uniquely fixed on the a4-helix by an R173-D279 salt bridge. We exploit absorbance, Raman and dichroism spectros-copy, and calorimetry to describe how ZEA and BmCBP mutually affect each other in the complex. We iden-tify 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.

Automated summary

In this study, the crystal structures of a carotenoid-binding protein (BmCBP) from silkworm Bombyx mori were determined, revealing the interaction between BmCBP and zeaxanthin (ZEA). The research showed structural differences in the carotenoid-binding cavity between BmCBP and human STARD3, explaining their distinct carotenoid-binding capacities. These findings provide important insights into the function of xanthophyll-binding proteins in the eye.