InterPro domain: IPR012128

Cyanobacteria and red algae harvest light through water-soluble complexes, called phycobilisomes, which are attached to the outer face of the thylakoid membrane [ 1 ]. These complexes are capable of transferring the absorbed energy to the photosynthetic reaction centre with greater than 95% efficiency. Phycobilisomes contain various photosynthetic light harvesting proteins known as biliproteins, and linker proteins which help assemble the structure. The two main structural elements of the complex are a core located near the photosynthetic reaction centre, and rods attached to this core. Allophycocyanin is the major component of the core, while the rods contain phycocyanins, phycoerythrins and linker proteins. The rod biliproteins harvest photons, with the excitation energy being passed through the rods into the allophycocyanin in the core. Other core biliproteins subsequently pass this energy to chlorophyll within the thylakoid membrane.

This entry represents the alpha and beta subunits found in biliproteins from cyanobacteria and red algae. Structural studies indicate that the basic structural unit of most biliproteins is a heterodimer composed of these alpha and beta subunits [ 2 , 3 , 4 , 5 ]. The full protein is a ring-like trimer assembly of these heterodimers. Each subunit of the heterodimer has eight helices and binds chromophores through thioester bonds formed at particular cysteine residues. These chromophores, also known as bilins, are open-chain tetrapyrroles whose number and type vary with the particular biliprotein eg R-phyocerythrin binds five phycoerythrobilins per heterodimer, while allophycocyanin binds two phycocyanobilins per heterodimer.

1. Allophycocyanin and energy transfer. Biochim. Biophys. Acta 1657, 73-81
2. Crystal structure of a phycourobilin-containing phycoerythrin at 1.90-A resolution. J. Struct. Biol. 126, 86-97
3. Crystallization and 2.2 A resolution structure of R-phycoerythrin from Gracilaria chilensis: a case of perfect hemihedral twinning. Acta Crystallogr. D Biol. Crystallogr. 57, 52-60
4. Crystal structure of R-phycocyanin and possible energy transfer pathways in the phycobilisome. Biophys. J. 81, 1171-9
5. Isolation, crystallization, crystal structure analysis and refinement of allophycocyanin from the cyanobacterium Spirulina platensis at 2.3 A resolution. J. Mol. Biol. 249, 424-40