Calcium determines the shape of fibrillin.

Abstract Velocity sedimentation experiments using authentic fibrillin-1 demonstrated sedimentation coefficients of s20,w0 = 5.1 ± 0.1 in the Ca2+ form and s20,w0 = 6.2 ± 0.1 in the Ca2+-free form. Calculations based on these results and the corresponding molecular mass predicted a shortening of fibrillin by ∼25% and an increase in width of ∼13-17% upon removal of Ca2+. These observations were confirmed by analysis of Ca2+-loaded and Ca2+-free rotary shadowed fibrillin molecules. Analysis of recombinant fibrillin-1 subdomain rF17, consisting primarily of an array of 12 Ca2+-binding epidermal growth factor (cbEGF)-like repeats, by analytical ultracentrifugation and rotary shadowing further confirmed Ca2+-dependent structural changes in the tertiary structure of fibrillin-1. Based on these results, the contribution of a single cbEGF-like repeat to the length of tandem arrays is predicted to be ∼3 nm in the Ca2+ form. Ca2+-free forms demonstrated a decrease of 20-30% in length, indicating significant structural changes of these motifs when they occur in tandem. Circular dichroism measurements of rF17 in the presence and absence of Ca2+ indicated secondary structural changes within and adjacent to the interdomain regions that connect cbEGF-like repeats. The results presented here suggest a flexible structure for the Ca2+-free form of fibrillin which becomes stabilized, more extended, and rigid in the Ca2+ form.