Structural Basis for the Prawn Blue Pigment Formed by the Ternary Lipocalin-Astaxanthin-Phospholipid Coassemblies.

Reconstituting natural pigment complexes in vitro enables dissection of their formation and color-tuning mechanisms, but remains challenging. Here we reconstructed the thermochromic pigment of the prawn by a ternary coassembly of recombinant lipocalin rMrLC, astaxanthin and phospholipid. The complex is blue at room temperature with a bathochromic shift of 130 nm relative to free astaxanthin and turns orange-red upon heating, a property matching the natural pigment. The crystal structures of both apo rMrLC and the ternary complex reveal that the binding of astaxanthin to the protein causes a closed-to-open state transition, producing the ternary complex where a homodimeric lipocalin scaffold clamps a sandwich-like astaxanthin-phospholipid-astaxanthin core. Structural analyses, coupled with site-directed mutagenesis, spectroscopy and quantum mechanics/molecular mechanics calculations elucidate the formation and color-tuning mechanism of the blue complex. This work establishes design principles for engineering tunable protein-based pigments and color-changing biomaterials.