Immuno-detection of aluminium and aluminium induced conformational changes in calmodulin-implications in Alzheimer's disease

Binding of calcium to calmodulin (CAM) induces specific structural rearrangements in the whole protein molecule. Ca²⁺ organizes and stabilizes the four-domains structure of calmodulin in a helical, active conformation that can bind to its target proteins; the central helix remaining flexible is an essential condition for their bio-recognition. The conformation of calmodulin, and its efficacy to interact with target proteins, is profoundly altered when bound to metal ions other than calcium. As recently reported, the local structural changes of CaM, which occur upon aluminium binding, lead to the impairment of protein flexibility and to the loss of its ability to interact with several other proteins, which may decrease or inhibit the regulatory character of calmodulin. In this study we followed conformational changes occurring in the calmodulin molecule after aluminium binding using highly specific monoclonal antibodies (mAbs) able to differentiate between the conformational states of calmodulin, as well as mAbs which recognize aluminium free or bound to proteins. Under the same experimental conditions, mAb CAM-1, a Ca²⁺ conformation sensitive antibody raised against calmodulin, fails to recognize the calmodulin-aluminium complex, despite the presence of Ca²⁺, while the anti-Al antibodies show a maximal binding pattern towards their antigen. These data suggest that Al³⁺ ions bind to calmodulin in the presence of Ca²⁺ ions, leading to an inactive, reversible conformation, instead of its physiological active form. Alteration of the conformation of calmodulin imposed by Al binding may have possible implications in the neurotoxicity mechanism related to Alzheimer's disease.