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Crystal structure of the complete integrin Vβ3 ectodomain plus an /β transmembrane fragment

¹ Program in Leukocyte Biology and Inflammation and ² Program in Structural Biology, Nephrology Division, Department of Medicine and ³ Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129
⁴ Biologicals: Protein and Cell Science, ⁵ Biologicals: Protein Purification, and ⁶ Therapeutic Area Oncology: Biochemistry and Cellular Pharmacology, Merck-Serono Research, 64293 Darmstadt, Germany

Correspondence to M. Amin Arnaout: arnaout{at}receptor.mgh.harvard.edu

We determined the crystal structure of 1TM-Vβ3, which represents the complete unconstrained ectodomain plus short C-terminal transmembrane stretches of the V and β3 subunits. 1TM-Vβ3 is more compact and less active in solution when compared with TM-Vβ3, which lacks the short C-terminal stretches. The structure reveals a bent conformation and defines the –β interface between IE2 (EGF-like 2) and the thigh domains. Modifying this interface by site-directed mutagenesis leads to robust integrin activation. Fluorescent lifetime imaging microscopy of inactive full-length Vβ3 on live cells yields a donor–membrane acceptor distance, which is consistent with the bent conformation and does not change in the activated integrin. These data are the first direct demonstration of conformational coupling of the integrin leg and head domains, identify the IE2–thigh interface as a critical steric barrier in integrin activation, and suggest that inside-out activation in intact cells may involve conformational changes other than the postulated switch to a genu-linear state.

Abbreviations used in this paper: βTD, β-tail domain; FLIM, fluorescent lifetime imaging microscopy; FN, fibronectin; FRET, fluorescence resonance energy transfer; LIMBS, ligand-associated metal-binding site; MIDAS, metal ion–dependent adhesion site; NMR, nuclear magnetic resonance; PSI, plexin-semaphorin-integrin; TM, transmembrane; WT, wild type.

© 2009 Xiong et al.
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