The short cytoplasmic tails regulate activation of integrin adhesion receptors via clasping/unclasping of their membrane-proximal helices. Using integrin alpha(IIb)beta(3) as a model, we show that a previously reported activating mutation alpha(IIb)(R(995)D) that perturbs the electrostatic interface in the clasp only partially activates alpha(IIb)beta(3) and that extensive activation of the receptor is achieved by complete deletion of alpha(IIb) CT or triple mutations in alpha(IIb)(V(990)A/F(992)A/R(995)D) that disrupt both electrostatic and hydrophobic interfaces in the clasp. The results provide quantitative evidence for an equilibrium-based integrin activation process where shifting the equilibrium to the fully activated state requires total unclasping of the cytoplasmic tails. We further demonstrate that while the C-terminal region of the alpha(IIb) tail minimally influences alpha(IIb)beta(3) activation, the C-terminal region of the beta(3) tail is critically involved. A disease-causing mutation of S(752)P in this region, but not S(752)A, suppressed partial activation induced by R(995)D or the talin head domain but did not affect activation induced by alpha(IIb) truncation. NMR spectroscopy revealed that S(752)P but not the S(752)A mutation disrupted a C-terminal helix within the beta(3) tail, suggesting that the C-terminal helix may regulate the equilibrium-based clasping/unclasping process. Together, these data provide molecular insights into how distinct regions of the cytoplasmic tails differentially and cooperatively regulate integrin activation.

The A-domains within integrin ␤ subunits contain three metal sites termed the metal ion-dependent adhesion site (MIDAS), site adjacent to the metal ion-dependent adhesion site (ADMIDAS), and ligand-induced metal-binding site (LIMBS), and these sites are involved in ligand engagement. The ␤ 3 integrin subfamily consists of two members, ␣ v ␤ 3 and ␣ llb ␤ 3 . The two receptors share the same ␤ 3 subunit, and the amino acid sequences of their ␣ subunits are 36% identical (1). Both are typical integrin ␣/␤ heterodimers, in which each subunit is composed of a large extracellular region, a transmembrane region, and a short cytoplasmic tail. ␣ llb ␤ 3 expression is restricted primarily to platelets and their megakaryocyte precursors and plays an indispensable role in thrombus formation (2-4). ␣ v ␤ 3 is expressed by a variety of vascular cells, including endothelial cells, where it influences the adhesive and migratory properties of these cells (5-7). Both ␤ 3 integrins bind multiple adhesive ligands, and many of the ligands bind to both receptors, including fibrinogen (8 -10). However, there is a fundamental difference in the specificity involved in fibrinogen recognition by the two integrins; ␣ llb ␤ 3 binds fibrinogen through the sequence at the carboxyl terminus of its ␥ chain 406 KQAGDV 411 (11,12), and ␣ v ␤ 3 binds to one of the two 572 RGD 574 sequences in its A␣ chain (13). In addition, sites within the ␥ chain, distinct from the KQAGDV, have been implicated in recognition of fibrinogen by ␣ v ␤ 3 (14). According to recently published crystal structures of the extracellular regions of ␣ v ␤ 3 (15) and ␣ llb ␤ 3 (16) with bound ligand mimetics, the main area of ligand binding lies between the ␤-propeller of the ␣ subunit and the A-domain in the ␤ 3 subunit. This direct demonstration of ␤ 3 A-domain involvement in ligand engagement was preceded by numerous studies using mutational, immunological, and biochemical approaches (17-21), which indicated that this region played a critical role in ligand binding to the two ␤ 3 integrins.The binding of most ligands to integrins requires metal ions. This is also true for the ␤ 3 integrins and the cellular responses arising from ligand engagement (e.g. platelet aggregation (22-24)). The ␤-propeller domain of the ␣ subunit contains four divalent ion-binding sites within the loops of blades 4 -7 near the bottom of the propeller, but these are not directly involved in ligand engagement (25). The ␤ 3 A-domain contains three metal sites for binding ligands, which are more directly involved in ligand binding. These are referred to as the metal ion-dependent adhesion site (MIDAS), 4 a site adjacent to the MIDAS (ADMIDAS), and a ligand-induced metal-binding site (LIMBS). The ␣ v ␤ 3 and ␣ llb ␤ 3 crystal structures establish that cation bound in the MIDAS is involved directly in ligand engagement; the ADMIDAS cation is not directly involved in contacting ligand but may help to regulate ligand binding; and the LIMBS cation may aid in stabilizing the receptor-ligand complex (26). ...

No abstract

No abstract