Calcium Determines the Supramolecular Organization of Fibrillin-rich Microfibrils

Microfibrils are ubiquitous fibrillin-rich polymers that are thought to provide long-range elasticity to extracellular matrices, including the zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks indexing on a fundamental axial periodicity (D) of ∼56 nm. A Ca²⁺-induced reversible change in the intensities of the meridional Bragg peaks indicated that supramolecular rearrangements occurred in response to altered concentrations of free Ca²⁺. In the presence of Ca²⁺, the dominant diffracting subspecies were microfibrils aligned in an axial 0.33-D stagger. The removal of Ca²⁺ caused an enhanced regularity in molecular spacing of individual microfibrils, and the contribution from microfibrils not involved in staggered arrays became more dominant. Scanning transmission electron microscopy of isolated microfibrils revealed that Ca²⁺ removal or addition caused significant, reversible changes in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated meridional x-ray diffraction profiles and analyses of isolated Ca²⁺-containing, staggered microfibrillar arrays were used to interpret the effects of Ca²⁺. These observations highlight the importance of Ca²⁺ to microfibrils and microfibrillar arrays in vivo.