Figure 2.

Structure of the α_IIbβ3 headpiece. a, Ribbon diagram of the α_IIbβ3:10E5 complex. Calcium and magnesium ions are shown as gold and silver spheres, respectively. Tirofiban is shown in cpk. The Cα atom of HPA-1a alloantigenic determinant Leu 33 in the PSI domain is shown as a blue sphere. Glycan chains are displayed as black sticks. Integrin disulphides are shown as yellow Cα –Ca bonds. b, Superimposed on the basis of the β I domain are the three independent α_IIbβ3 heterodimers in crystal form B (magenta, green and yellow), and one in form A (cyan). c, The hybrid and PSI domains of the four independent α_IIbβ3 structures are superimposed. d, Liganded-open α_IIbβ3 (crystal form A) and unliganded-closeda α_Vβ₃ headpieces⁷ are superimposed using the β I domainb -sheet. The α and β subunits are coloured magenta and cyan in α_IIbβ3 and grey and yellow in α_Vβ₃. Calcium and magnesium ions inα_IIbβ3 only are gold and silver spheres, respectively. Yellow cylinders in the β I/hybrid interface show positions of residues where introduction of N-glycosylation sites induces high affinity for ligand and LIBS epitope exposure^9,42. e, Superposition of an α_IIbβ3 structure from crystal form B (red Cα-trace) on the three-dimensional electron microscopy density (grey chickenwire) of the fibronectin-bounda β₅β₁ headpiece⁶. Domains are labelled and those only in the α₅b₁ structure including the tenth FN3 domain of fibronectin are in parentheses. Figures in this paper utilize crystal form A unless stated otherwise and were prepared with programs Bobscript⁴⁶, Povray (The Povray Team, http://www.povray.org), Raster3D⁴⁷ and Ribbons⁴⁸.