Crystal structure and functional characterization of the microalgal carotenoprotein AstaP-pink1.

The recently discovered family of microalgal water-soluble astaxanthin-binding proteins (AstaPs) functions in carotenoid sequestration and transfer. We present the crystal structure and characterization of AstaP-pink1, a homolog from Scenedesmus sp. We show that despite low sequence identity with the known AstaP-orange1, AstaP-pink1 also binds various xanthophylls, but in contrast to AstaP-orange1, induces a pronounced bathochromic shift and a near-UV spectral bump upon binding ketocarotenoids, which is reminiscent of photoactivated Orange Carotenoid Protein (OCP). Mutagenesis and domain-swap experiments indicate that its unique spectral tuning likely results from carotenoid isomerization, forced by the interplay between its N-terminal jaw and body subdomains connected by the hinge loop of different sequence and length. This study attempts to rationalize the spectral and functional diversification of AstaPs and highlights their potential as engineered modules for carotenoid delivery.