CALCIUM DETERMINES THE SUPRAMOLECULAR ORGANIZATION OF FIBRILLIN-RICH MICROFIBRILS

Microfibrils are ubiquitous fibrillin-rich polymers that are thought t o provide long-range elasticity to extracellular matrices, including t he zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks ind exing on a fundamental axial periodicity (D) of similar to 56 nm. A Ca 2+-induced reversible change in the intensities of the meridional Brag g peaks indicated that supramolecular rearrangements occurred in respo nse to altered concentrations of free Ca2+. In the presence of Ca2+, t he dominant diffracting subspecies were microfibrils aligned in an axi al 0.33-D stagger. The removal of Ca2+ caused an enhanced regularity i n molecular spacing of individual microfibrils, and the contribution f rom microfibrils not involved in staggered arrays became more dominant . Scanning transmission electron microscopy of isolated microfibrils r evealed that Ca2+ removal or addition caused significant, reversible c hanges in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated merid ional x-ray diffraction profiles and analyses of isolated Ca2+-contain ing, staggered microfibrillar arrays were used to interpret the effect s of Ca2+. These observations highlight the importance of Ca2+ to micr ofibrils and microfibrillar arrays in vivo.