Structural and spectroscopic characterization of HCP2

Area of Science:

• Biochemistry
• Structural Biology
• Photobiology

Background:

• Helical Carotenoid Proteins (HCPs) represent a newly identified, diverse group of carotenoid-binding proteins in cyanobacteria.
• These proteins likely preceded the evolution of the Orange Carotenoid Protein (OCP) and may serve multiple functions.
• At least nine distinct HCP families suggest a broad functional repertoire beyond photoprotection.

Purpose of the Study:

• To determine the crystal structure of Helical Carotenoid Protein 2 (HCP2), a widespread HCP variant.
• To identify the native carotenoid bound by HCP2 from Tolypothrix sp. PCC 7601.
• To investigate the spectroscopic properties and carotenoid-protein interactions of HCP2.

Main Methods:

• Purification of HCP2 from the native cyanobacterium Tolypothrix sp. PCC 7601.
• X-ray crystallography to determine the 1.7 Å crystal structure of HCP2.
• Spectroscopic analysis (absorbance measurements) of purified HCP2 in solution and crystalline states.

Main Results:

• The crystal structure of HCP2 revealed its interaction with the carotenoid canthaxanthin.
• Natively purified HCP2 exclusively binds canthaxanthin.
• HCP2 exhibits distinct absorbance maxima in solution (530 nm) versus crystalline form (548 nm) due to carotenoid stacking.

Conclusions:

• HCPs, exemplified by HCP2, serve as a model system for studying diverse carotenoid-protein interactions.
• The structural and spectroscopic data provide a foundation for understanding the enigmatic functional roles of HCPs.
• Carotenoid stacking within the crystal lattice significantly influences HCP2's spectroscopic properties.