Contribution of unfolding and intermolecular architecture to fibronectin fiber extensibility

The extracellular matrix contains components with remarkable mechanical properties, including fibronectin (Fn) fibers with extensibilities of >700% strain. We utilized what we consider a novel technique to quantify the extent of molecular unfolding that contributes to Fn fiber extension, and we compared this behavior with stochastic models of Fn fibers with different molecular arrangements. In vitro unfolding as a function of strain was measured by fluorescently labeling cysteines in modules FnIII7 and III15 in artificial Fn fibers. A calibration technique we also consider novel made it possible to demonstrate that 44% of cysteines in these modules were exposed in Fn fibers strained to 421% extension, up from 8% exposure without strain. In silico unfolding was measured by applying a constant strain rate to a fiber represented by a network of wormlike chain springs, each representing an individual Fn molecule. Unfolding rates were calculated with a tension-dependent stochastic model applied to FnIII modules in each molecule. A comparison of these approaches revealed that only a molecular arrangement permitting unequal mechanical loading of Fn molecules recapitulates in vitro unfolding. These data have implications for Fn-dependent mechanotransduction and give insight into how the molecular architecture of natural materials permits such remarkable extensibility.