Backbone dynamics of a cbEGF domain pair in the presence of calcium

Calcium binding (cb) epidermal growth factor-like (EGF) domains are found i n a wide variety of extracellular proteins with diverse functions. In sever al proteins, including the fibrillins (1 and 2), the low-density lipoprotei n receptor, the Notch receptor and related molecules, these domains are org anised as multiple tandem repeats. The functional importance of calcium-bin ding by EGF domains has been underscored by the identification of missense mutations associated with defective calcium-binding, which have been linked to human diseases. Here, we present N-15 backbone relaxation data for a pa ir of cbEGF domains from fibrillin-1, the defective protein in the Marfan s yndrome. The data were best fit using a symmetric top model, confirming the extended conformation of the cbEGF domain pair. Our data demonstrate that calcium plays a key role in stabilising the rigidity of the domain pair on the pico-to millisecond time-scale. Strikingly, the most dynamically stable region of the construct is centred about the domain interface. These resul ts provide important insight into the properties of intact fibrillin-1, the consequences of Marfan syndrome causing mutations, and the ultrastructure of fibrillins and other extracellular ma h-ix proteins. (C) 2000 Academic P ress.