The supramolecular organization of fibrillin-rich microfibrils

We propose a new model for the alignment of fibrillin molecules within fibr illin microfibrils. Automated electron tomography was used to generate thre e-dimensional microfibril reconstructions to 18.6-Angstrom resolution, whic h revealed many new organizational details of untensioned microfibrils, inc luding heart-shaped beads from which two arms emerge, and inter-bead diamet er variation. Antibody epitope mapping of untensioned microfibrils revealed the juxtaposition of epitopes at the COOH terminus and near the proline-ri ch region, and of two internal epitopes that would be 42-nm apart in unfold ed molecules, which infers intramolecular folding. Colloidal gold binds mic rofibrils in the absence of antibody. Comparison of colloidal gold and anti body binding sites in untensioned microfibrils and those extended in vitro, and immunofluorescence studies of fibrillin deposition in cell layers, ind icate conformation changes and intramolecular folding. Mass mapping shows t hat, in solution, microfibrils with periodicities of <70 and >140 nm are st able, but periodicities of similar to 100 nm are rare. Microfibrils compris e two in-register filaments with a longitudinal symmetry axis, with eight f ibrillin molecules in cross section. We present a model of fibrillin alignm ent that fits all the data and indicates that microfibril extensibility fol lows conformation-dependent maturation from an initial head-to-tail alignme nt to a stable approximately one-third staggered arrangement.