Cryptomonad biliproteins: Bilin types and locations

Area of Science:

• Biochemistry
• Phycology
• Spectroscopy

Background:

• Cryptophycean biliproteins are light-harvesting pigments crucial for photosynthesis in marine environments.
• Understanding the bilin attachment sites is key to elucidating their spectral properties and energy transfer mechanisms.

Purpose of the Study:

• To purify and characterize two cryptophycean phycocyanins (Cr-PCs): Hemiselmis strain HP9001 Cr-PC 612 and Falcomonas daucoides Cr-PC 69.
• To determine the number, type, and location of bilin chromophores within these Cr-PCs.
• To compare bilin organization with previously studied cryptophycean biliproteins to identify conserved features.

Main Methods:

• Purification of Cr-PC 612 and Cr-PC 69 from respective cryptophycean strains.
• Spectroscopic analysis to identify bilin types and spectral properties.
• Peptide mapping and mass spectrometry to determine bilin attachment sites (Cys residues) on α and β subunits.

Main Results:

• Each Cr-PC contained one bilin on the α subunit and three on the β subunit.
• Cr-PC 612 had phycocyanobilin at α-Cys-18, β-Cys-82, β-Cys-158, and a doubly-linked 15,16-dihydrobiliverdin at β-DiCys-50,61.
• Cr-PC 569 had phycocyanobilin at α-Cys-18 and β-Cys-82, a singly-linked Bilin 584 at β-Cys-158, and a doubly-linked Bilin 584 at β-DiCys-50,61.
• Varied bilin locations, particularly for the longest wavelength absorbing bilin, were observed across different cryptophycean biliproteins.

Conclusions:

• There is no conserved location for the bilin responsible for the longest visible absorption wavelength among the studied cryptophycean biliproteins.
• Consequently, a conserved energy transfer pathway common to all native cryptophycean biliproteins cannot be assumed.
• While phycocyanobilin or phycoerythrobilin is conserved at β-Cys-82, other attachment sites exhibit greater bilin variability.