A variant alpha-allophycocyanin (AP) which forms a complex with beta-AP with maximum absorption at approximately 670 nanometers. It is an important phycobilisome terminal emitter involved in energy transfer to photosystem I (By similarity). Heterodimer of an alpha-B and a beta chain forming AP-B. Contains one covalently linked bilin chromophore. The chromophore is added by phycocyanobilin lyase CpcUS. Belongs to the phycobiliprotein family.