Structural basis for the ligand promiscuity of the neofunctionalized, carotenoid-binding fasciclin domain protein AstaP

Fasciclins (FAS1) are ancient adhesion protein domains with no common small ligand binding reported. A unique microalgal FAS1-containing astaxanthin (AXT)-binding protein (AstaP) binds a broad repertoire of carotenoids by a largely unknown mechanism. Here, we explain the ligand promiscuity of AstaP-orange1 (AstaPo1) by determining its NMR structure in complex with AXT and validating this structure by SAXS, calorimetry, optical spectroscopy and mutagenesis. alpha 1-alpha 2 helices of the AstaPo1 FAS1 domain embrace the carotenoid polyene like a jaw, forming a hydrophobic tunnel, too short to cap the AXT beta-ionone rings and dictate specificity. AXT-contacting AstaPo1 residues exhibit different conservation in AstaPs with the tentative carotenoid-binding function and in FAS1 proteins generally, which supports the idea of AstaP neofunctionalization within green algae. Intriguingly, a cyanobacterial homolog with a similar domain structure cannot bind carotenoids under identical conditions. These structure-activity relationships provide the first step towards the sequence-based prediction of the carotenoid-binding FAS1 members. FAS1 domains, a family of cell adhesion molecules, have a carotenoid-binding function in an astaxanthin-binding protein AstaP from green algae, suggesting neofunctionalization of FAS1 in a subset of AstaP-like proteins in green algae

Automated summary

This study explains the ligand promiscuity of AstaPo1 by determining its NMR structure and validating it through various experiments. The FAS1 domain of AstaPo1 binds carotenoids by embracing their polyene structure, but cannot cap the beta-ionone rings to dictate specificity. This research also suggests the neofunctionalization of FAS1 in a subset of AstaP-like proteins in green algae.