Structural analysis at 2.2 angstrom of orthorhombic crystals presents the asymmetry of the allophycocyanin-linker complex, AP center dot 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 t he orthorhombic space group P2(1)2(1)2(1). Cryocrystallographic x-ray measu rements enabled the structural analysis of the complex at a resolution of 2 .2 Angstrom. The asymmetric unit contains two side-to-side associated "trim eric" (alpha beta)(3) allophycocyanin complexes comprising the linker polyp eptide in a defined orientation inside the trimer. The linker representing a protein fold related to the prosegment of procarboxypeptidase A is in con tact with only two of the three beta-subunits and directly interacts with t he corresponding chromophores of these proteins. In addition to a modulatio n of the chromophores' spectral properties, the linker polypeptide attracts the alpha beta-subcomplexes, thereby bringing the beta-chromophores closer together. These results will enable interpretations of energy-transfer mec hanisms within phycobiliproteins.