The Solution and Crystal Structures of a Module Pair from the Staphylococcus aureus-Binding Site of Human Fibronectin—A Tale with a Twist

An important goal of structural studies of modular proteins is to determine the inter-module orientation, which often influences biological function. The N-terminal domain of human fibronectin (Fn) is composed of a string of five type 1 modules (F1). Despite their small size, to date F1 modules have proved intractable to X-ray structure solution, although there are several NMR structures available. Here, we present the first structures (two X-ray models and an NMR-derived model) of the 2F13F1 module pair, which forms part of the binding site for Fn-binding proteins from pathogenic bacteria. The crystallographic structure determination was aided by the novel technique of UV radiation damage-induced phasing. The individual module structures are very similar in all three models. In the NMR structure and one of the X-ray structures, a similar but smaller interdomain interface than that observed previously for 4F15F1 is seen. The other X-ray structure has a different interdomain orientation. This work underlines the benefits of combining X-ray and NMR data in the studies of multi-domain proteins.