X-RAY DIFFRACTION STUDIES OF FIBRILLIN-RICH ICROFIBRILS
T.J.Wess,1 P.P. Purslow2 and C.M. Kielty3
1Department
of Biological and Molecular Sciences, University of Stirling,
Stirling FK9 4LA.
2The
Royal Veterinary and Agricultural University, Rolighedsvej 30,
1958 Fredriksberg C Denmark.
3School of Biological Sciences, 2.205 Stopford
Building, University of Manchester, Manchester M13 9PT.
Keywords
fibrillin-rich microfibrils, X-ray small angle scattering,
diffraction
Microfibrils are
ubiquitous fibrillin-rich polymers which are thought to provide
long-range elasticity to extracellular matrices, including the
zonular filaments of mammalian eyes. The microfibrils appear in
electron microscopy as beaded structures several hundred nm in
length with a fundamental bead periodicity of around 56nm. The
possible basis for elasticity of the tissue is based on the
reversible changes in the beaded periodicity. X-ray diffraction of hydrated bovine
zonular filaments (at the Daresbury Synchrotron) demonstrated
meridional diffraction peaks indexing on a fundamental
periodicity of ~56nm[1]. The effect of stretching the tissue up to 50% of
the rest length had little effect on the position of meridional
Bragg reflections, indicating a static population of bead
lengths. The effect of extension up to 100% causes a
deterioration of the diffraction signal and a weak fundamental
peak can be observed at 84nm[2]. 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 overall enhanced
regularity in molecular spacing, and the contribution from
microfibrils not involved in staggered arrays became more
dominant[3]. Simulated diffraction profiles and analyses of the
staggered arrays of isolated microfibrils formed in vitro
in the presence of Ca2+ were used to interpret the
effects of Ca2+. These observations indicate how Ca2+
could modulate the organisation of individual microfibrils and
three-dimensional arrays in vivo. This data allows a
mechanism for elasticity at the molecular level to be related to
the macroscopic tissue elasticity. Under the usual conditions of
extension in the eye, bead periodicity changes in a discrete
population along fibrils facilitate elasticity. Molecular
junctions of specifically staggered microfibrils maintain the
integrity of the tissue and serve as anchors connecting the
stretched microfibrils and allows the force to be transmitted
through the tissue. the removal of Ca2+ is to enhance
the regularity of the bead regions where stretching occurs and
also allows the diffraction from this population to be observed.