Evidence for the Intramolecular Pleating Model of Fibrillin Microfibril Organisation from Single Particle Image Analysis

Fibrillin microfibrils endow mammalian connective tissues with elasticity and are fundamental for the deposition of elastin. The microfibrils are 57 nm periodic supramolecular protein polymers with a mass of 2.4 MDa per repeat. The detailed structure and organisation of most matrix assemblies is poorly understood due to their large size and complexity and it has proved a major challenge to define their structural organisation. Therefore, we have used low dose electron microscopy and single particle image analysis to study the structure of fibrillin microfibrils. Three novel features were detected: a globular feature that bridges the “arm” region, a double band of density crossing the microfibril and stain penetrating holes present in the interbead region, possibly produced by the removal of microfibril associated proteins in the purification procedure. Fine filaments of approximately 2.4 nm diameter are resolved in the interbead region, which correspond to the reported diameter of the fibrillin molecule. Comparison of the stain exclusion pattern of microfibrils with the theoretical stain exclusion pattern of fibrillin packing models indicates that the intramolecular pleating model, where each fibrillin molecule is pleated within one microfibril period allowing extensibility by unpleating, has the best fit to the data.