@article {546746,
	title = {Metal ion and ligand binding of integrin α5β1.},
	journal = {Proc Natl Acad Sci USA},
	volume = {111},
	number = {50},
	year = {2014},
	pages = {17863-8},
	abstract = {Integrin\&nbsp;α5β1 binds to an Arg-Gly-Asp (RGD) motif in its\&nbsp;ligand\&nbsp;fibronectin. We report high-resolution crystal structures of a four-domain α5β1 headpiece fragment, alone or with RGD peptides soaked into crystals, and RGD peptide affinity measurements. The headpiece crystallizes in a closed conformation essentially identical to that seen previously for α5β1 complexed with a Fab that allosterically inhibits\&nbsp;ligand\&nbsp;binding\&nbsp;by stabilizing the closed conformation. Soaking experiments show that\&nbsp;binding\&nbsp;of cyclic RGD peptide with 20-fold higher affinity than a linear RGD peptide induces conformational change in the β1-subunit βI domain to a state that is intermediate between closed (low affinity) and open (high affinity). In contrast,\&nbsp;binding\&nbsp;of a linear RGD peptide induces no shape shifting. However, linear peptide\&nbsp;binding\&nbsp;induces shape shifting when Ca(2+) is depleted during soaking. Ca(2+) bound to the adjacent to\&nbsp;metal\&nbsp;ion-dependent adhesion site (ADMIDAS), at the locus of shape shifting, moves and decreases in occupancy, correlating with an increase in affinity for RGD measured when Ca(2+) is depleted. The results directly demonstrate that Ca(2+)binding\&nbsp;to the ADMIDAS stabilizes integrins in the low-affinity, closed conformation. Comparisons in affinity between four-domain and six-domain headpiece constructs suggest that flexible\&nbsp;integrin\&nbsp;leg domains contribute to conformational equilibria. High-resolution views of the hybrid domain interface with the plexin-semaphorin-integrin\&nbsp;(PSI) domain in different orientations show a ball-and-socket joint with a hybrid domain Arg side chain that rocks in a PSI domain socket lined with carbonyl oxygens.},
	author = {Xia W and Springer TA}
}
