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We present a structural description of a major conformational rearrangement mediated by metal ions. The metal induced disorder-->order transition observed in BLA leads to the formation of the extended substrate-binding site and explains on a structural level the calcium dependency of alpha-amylases. Sequence comparisons indicate that the unique Ca-Na-Ca metal triad and the additional calcium ion located between domains A and C might be found exclusively in bacterial alpha-amylases which show increased thermostability. The information presented here may help in the rational design of mutants with enhanced performance in biotechnological applications.

An electrophoretically purified allophycocyanin-linker complex, AP⅐L 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 sideto-side associated ''trimeric'' (␣␤) 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 ␤-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 ␣␤-subcomplexes, thereby bringing the ␤-chromophores closer together. These results will enable interpretations of energytransfer mechanisms within phycobiliproteins.