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André Krammer, Hui Lu, Barry Isralewitz, Klaus Schulten, and Viola Vogel. Forced unfolding of the fibronectin type III module reveals a tensile molecular recognition switch. Proceedings of the National Academy of Sciences, USA, 96:1351-1356, 1999. The tenth type III module of fibronectin, $FnIII_{10}$, mediates cell adhesion to surfaces. It posesses a $\beta$-sandwich structure containing seven $\beta$-strands (A through G) that are arranged in two anti-parallel sheets where the cell binding motif, Arg78-Gly79-Asp80 (RGD), is placed at the apex of the loop connecting $\beta$-strands F and G. Steered molecular dynamics (SMD) simulations in which tension is applied to the protein's terminal ends reveal that the module can act as a tensile molecular recognition switch. Analysis of the forced unfolding process of $FnIII_{10}$ shows that the G-strand is the first to be released while the remaining module maintains its structural integrity. This leads to a gradual shortening of the distance between the apex of the RGD-containing loop and th surface of the remaining module followed by a straightening of this loop from a $\beta$-turn into a linear conformation. Experimental data have previously shown that shortening the RGD-containing loop reduces its accessibility to membrane-bound integrins, and that the loop's affinity and selectivity to integrins decreases when linearized. Full Text

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