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© The Rockefeller University Press,
0021-9525/2001//1045 $5.00
The Journal of Cell Biology, Volume 152, Number 5,
, 2001 1045-1056
Original Article |
The Supramolecular Organization of Fibrillin-Rich Microfibrils
clair.baldock{at}man.ac.uk
We propose a new model for the alignment of fibrillin molecules within fibrillin microfibrils. Automated electron tomography was used to generate three-dimensional microfibril reconstructions to 18.6-Å resolution, which revealed many new organizational details of untensioned microfibrils, including heart-shaped beads from which two arms emerge, and interbead diameter variation. Antibody epitope mapping of untensioned microfibrils revealed the juxtaposition of epitopes at the COOH terminus and near the proline-rich region, and of two internal epitopes that would be 42-nm apart in unfolded molecules, which infers intramolecular folding. Colloidal gold binds microfibrils in the absence of antibody. Comparison of colloidal gold and antibody binding sites in untensioned microfibrils and those extended in vitro, and immunofluorescence studies of fibrillin deposition in cell layers, indicate conformation changes and intramolecular folding. Mass mapping shows that, in solution, microfibrils with periodicities of <70 and >140 nm are stable, but periodicities of
100 nm are rare. Microfibrils comprise two in-register filaments with a longitudinal symmetry axis, with eight fibrillin molecules in cross section. We present a model of fibrillin alignment that fits all the data and indicates that microfibril extensibility follows conformation-dependent maturation from an initial head-to-tail alignment to a stable approximately one-third staggered arrangement.
Key Words: three-dimensional reconstruction • automated electron tomography • fibrillin microfibrils • molecular alignment • scanning transmission electron microscopy mass mapping
© 2001 The Rockefeller University Press
Abbreviations used in this paper: 3-D, three dimensional; AET, automated electron tomography; MUL, mass per unit length; STEM, scanning transmission electron microscopy; TB, TGF-β binding protein like.
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