Structural characteristics of phycobiliproteins from dulse

When water is added to a dried dulse sample, the red component is easily extracted. The color of the extract is bright red in sunlight and fluoresces strongly when illuminated by green light. These characteristics are attributed to “phycobiliprotein”, the major protein in the dulse phloem. In red algae, phycobiliprotein acts as an auxiliary pigment for photosynthesis, efficiently absorbing light at wavelengths that chlorophyll cannot.

In general, phycobiliproteins of red algae are mainly composed of two types of subunits, α- and β-chains, which form circular doughnut-like assemblies consisting of three each of them. These doughnut-like structures are then further assembled to form a large aggregate called a phycobilisome. Phycobilisomes are arranged in large numbers on the surface of the chloroplast thylakoid membrane. The major phycobiliproteins of red algae are phycoerythrin (red), phycocyanin (blue), and allophycocyanin (purple). Their coloration is determined by the type of the pigment (phycoerythrobilin, phycocyanobilin, or phycourobilin) that bound apoprotein of α- and β-chains and number.

Protein extracts prepared from dulse were subjected to spectral analysis, which revealed that the main component of phycobiliproteins from dulse is red phycoerythrin, followed by phycocyanin and allophycocyanin. Therefore, dulse phycoerythrin was purified and crystals were prepared, and its three-dimensional structure was determined by X-ray crystallography. As a result, dulse phycoerythrin had the same three-dimensional structural characteristics as those of other red algae. In addition, to elucidate the genetic structure of phycobiliproteins from dulse, chloroplast DNA encoding the genes of phycobiliproteins was analyzed by a next-generation sequencer. As a result, the gene structures and primary structures of three phycobiliproteins (phycoerythrin, phycocyanin, and allophycocyanin) were clarified.