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phycobiliprotein Any one of a class of pigments in the cyanobacteria, red algae, and photosynthetic cryptophytes that act as accessory pigments in photosynthesis, being attached to the thylakoid membranes. Each comprises a protein bound to a phycobilin, a coloured tetrapyrrole prosthetic group (see porphyrin). The main phycobiliproteins are phycocyanin and phycoerythrin. Within cells, phycobiliproteins form complex clusters called phycobilisomes.