Structural and spectroscopic characterization of HCP2

The Helical Carotenoid Proteins (HCPs) are a large group of newly identified carotenoid
 binding proteins found in ecophysiologically diverse cyanobacteria. They likely evolved before
 becoming the effector (quenching) domain of the modular Orange Carotenoid Protein (OCP). The
 number of discrete HCP families—at least nine—suggests they are involved in multiple distinct
 functions. Here we report the 1.7 Å crystal structure of HCP2, one of the most widespread HCPs
 found in nature, from the chromatically acclimating cyanobacterium Tolypothrix sp. PCC 7601.
 By purifying HCP2 from the native source we are able to identify its natively-bound carotenoid,
 which is exclusively canthaxanthin. In solution, HCP2 is a monomer with an absorbance maximum
 of 530 nm. However, the HCP2 crystals have a maximum absorbance at 548 nm, which is
 accounted by the stacking of the β1 rings of the carotenoid in the two molecules in the asymmetric
 unit. Our results demonstrate how HCPs provide a valuable system to study carotenoid-protein
 interactions and their spectroscopic implications, and contribute to efforts to understand the
 functional roles of this large, newly discovered family of pigment proteins, which to-date remain
 enigmatic.