Structural analysis at 2.2 Å of orthorhombic crystals presents the asymmetry of the allophycocyanin–linker complex, AP⋅L(C)(7.8), from phycobilisomes of Mastigocladus laminosus

An electrophoretically purified allophycocyanin–linker complex, AP⋅L(C)(7.8), from phycobilisomes of Mastigocladus laminosus has been crystallized in the orthorhombic space group P2(1)2(1)2(1). Cryocrystallographic x-ray measurements enabled the structural analysis of the complex at a resolution of 2.2 Å. The asymmetric unit contains two side-to-side associated “trimeric” (αβ)(3) allophycocyanin complexes comprising the linker polypeptide in a defined orientation inside the trimer. The linker representing a protein fold related to the prosegment of procarboxypeptidase A is in contact with only two of the three β-subunits and directly interacts with the corresponding chromophores of these proteins. In addition to a modulation of the chromophores’ spectral properties, the linker polypeptide attracts the αβ-subcomplexes, thereby bringing the β-chromophores closer together. These results will enable interpretations of energy-transfer mechanisms within phycobiliproteins.