Structural analysis at 2.2 Å of orthorhombic crystals presents the asymmetry of the allophycocyanin-linker complex, AP·L7.8C, from phycobilisomes of Mastigocladus laminosus

Abstract

An electrophoretically purified allophycocyanin-linker complex, AP·L7.8C, from phycobilisomes of Mastigocladus laminosus has been crystallized in the orthorhombic space group P212121. 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.