Closest packing of two-stranded coiled-coils as a model for the collagen fibril

Abstract

We propose that in the collagen fibril, the triple-helical molecules form two-stranded coiled-coils of period 5 × 670A˚. Coiled-coils are packed on a tetragonal lattice and are axially staggered with ten in the unit cell (observed side 55A˚) so that it carries the 670A˚periodicity of the fibril. When nearest neighbours have opposing supercoil hands, the observed tetragonal lattice represents closest packing of two-stranded coiled-coils. This proposal is consistent with the row line spacings measured from the low angle X-ray diffraction pattern of tendon and explains the systematic absences and the two undisputed equatorial reflections. Unlike explanations for the diffraction pattern which invoke a five-stranded microfibril, our interpretation is consistent with its equatorial intensity distribution.