Abstract
Cyanobacteria and red algae use light-harvesting pigments bound by proteins to capture solar radiation and to channel excitation energy into their reaction centres. In most cyanobacteria, a multi-megadalton soluble structure known as the phycobilisome is a major light-harvesting system. Allophycocyanin is the main component of the phycobilisome core, forming a link between the rest of the phycobilisome and the reaction-centre core. The crystal structure of allophycocyanin from Thermosynechococcus elongatus (TeAPC) has been determined and refined at 3.5 A resolution to a crystallographic R value of 26.0% (R(free) = 28.5%). The structure was solved by molecular replacement using the allophycocyanin structure from Spirulina platensis as the search model. The asymmetric unit contains an (alphabeta) monomer which is expanded by symmetry to a crystallographic trimer.