Structural basis for spectral red shift and UVA absorption in the microalgal water-soluble astaxanthin-binding...

AstaPs are water-soluble, photooxidative stress-inducible astaxanthin (AXT)-binding proteins found only in Scenedesmaceae microalgae, where they play a central role in survival under severe photooxidative stress. Here, we focused on the unique function of AstaP-pink1, which converts orange AXT into a pink form and generates a UVA absorption spectrum upon protein binding. AstaP-pink1 was expressed in genetically engineered Escherichia coli strains capable of synthesizing AXT. The host strain harboring pAC-Asta produced adonixanthin, AXT, and zeaxanthin in an approximate ratio of 5:3:2, whereas the strain carrying pMF573 predominantly produced AXT (∼90 % of total carotenoid). Co-expression of the gene encoding AstaP-pink1 in these strains resulted in moderate and selective AXT binding, accompanied by a spectral red shift and UVA absorption, thereby generating pink coloration. Crystal structure analysis of AXT-bound recombinant AstaP-pink1 (rAstaP-pink1) revealed both similarities and differences in AXT binding compared with rAstaP-orange1. Density functional theory (DFT) calculations based on the crystal structure suggested that the larger red shift than that of AstaP-orange1 and the distinct UVA absorption are derived from the conformation of AXT that is compelled by binding to AstaP-pink1. This study suggests that AXT binding by AstaP-pink1 not only facilitates the water solubilization of AXT but also generates the observed spectral properties.