Light-harvesting photosynthetic tetrapyrrole chromophore-protein from the phycobiliprotein complex. Heterododecamer of 6 alpha and 6 beta chains. The basic functional unit of phycobiliproteins is a ring-shaped hexamer formed from two back-to-back trimers contacting via the alpha chain subunits. The trimers are composed of alpha/beta subunit heterodimers arranged around a three-fold axis of symmetry. The phycoerythrins also contain a gamma subunit which is located in the center of the hexamer. Forms the core of the phycobilisome. Contains one covalently linked phycocyanobilin chromophore. The light-harvesting antenna system in red algae and cyanobacteria is formed of phycobilisomes. These are composed of the phycobiliproteins phycoerythrin (CPE), phycocyanin (CPC) and allophycocyanin (APC). Cyanobacteria also contain phycoerythrocyanin (PCC). The phycobiliproteins all share the same subunit composition and organization with variations in the covalently bound open-chain tetrapyrrole chromophores. The phycobiliprotein complexes are arranged sequentially in antenna complexes linked by linker proteins with CPE at the periphery, CPC in the middle and APC at the core feeding to the photosynthetic reaction center. Allophycocyanin has a maximum absorption at approximately 650 nanometers. Belongs to the phycobiliprotein family. Neither a protein sequence nor a nucleotide sequence had been determined for this protein when the crystallographic structure was reported. The sequence of a model fit to electron density plots at 2.2 Angstroms resolution was used. When a nucleotide sequence became available, it replaced the model sequence. There are 14 differences between the model and the nucleotide sequences.