Locking the β\u3csub\u3e3\u3c/sub\u3e Integrin I-like Domain into High and Low Affinity Conformations with Disulfides

Although integrin α subunit I domains exist in multiple conformations, it is controversial whether integrin β subunit I-like domains undergo structurally analogous movements of the α7-helix that are linked to affinity for ligand. Disulfide bonds were introduced into the β3 integrin I-like domain to lock its β6-α7 loop and α7-helix in two distinct conformations. Soluble ligand binding, ligand mimetic mAb binding and cell adhesion studies showed that disulfide-bonded receptor αIIbβ3T329C/A347C was locked in a low affinity state, and dithiothreitol treatment restored the capability of being activated to high affinity binding; by contrast, disulfide-bonded αIIbβ3V332C/M335C was locked in a high affinity state. The results suggest that activation of the β subunit I-like domain is analogous to that of the α subunit I domain, i.e. that axial movement in the C-terminal direction of the α7-helix is linked to rearrangement of the I-like domain metal ion-dependent adhesion site into a high affinity conformation

A novel monoclonal antibody recognizing a cation-dependent epitope within the regulatory loop of human beta(1) integrin (CD29)

Cell adhesion receptors of the integrin superfamily can be expressed in different affinity states towards their ligands. It has been previously demonstrated that beta(1), integrins alpha4beta(1) and alpha5beta(1) are expressed in a nonligand binding form by human hemopoietic progenitor cells but can be activated into a ligand binding form by a variety of stimuli including intracellular stimuli generated by cytokine receptors and extracellular stimuli generated by function-activating anti-beta(1) integrin monoclonal antibodies (MAbs). In both instances, the activation of beta(1) integrins is believed to be the result of conformational,changes propagating along the beta(1) integrin chain which in turn increase accessibility to the ligand. A cluster of either function-activating or function-inhibiting anti-beta(1) integrin MAbs have been shown to bind within a 12 amino acid long regulatory loop between residues 207 and 218 of the human beta(1) integrin chain. We describe in this report the first MAb (96.9H9) specific for this regulatory loop whose binding is cation-dependent and requires either Ca2+ or Mn2+ but not Mg2+. In addition, the activation of alpha4beta(1) and alpha5beta(1) integrins by 96.9H9 is a two-step process with distinct cation requirements. Whereas Ca2+ is sufficient to promote binding of the antibody to the beta(1) integrin chain, Mg2+ is necessary for activating function following 96.9H9 binding. Our data therefore suggest that the regulatory epitope of the human beta(1) integrin chain is flexible with multiple conformations according to the cationic environment

Structural Basis of Dimeric Rasip1 RA Domain Recognition of the Ras Subfamily of GTP-Binding Proteins

Ras-interacting protein 1 (Rasip1) is an endothelial-specific Rap1 and Ras effector, important for vascular development and angiogenesis. Here, we report the crystal structure of the Rasip1 RA domain (RRA) alone, revealing the basis of dimerization, and in complex with Rap1 at 2.8 Å resolution. In contrast to most RA domains, RRA formed a dimer that can bind two Rap1 (KD = 0.9 μM) or Ras (KD = 2.2 μM) molecules. We solved the Rap1-RRA complex and found that Rasip1 binds Rap1 in the Switch I region, and Rap1 binding induces few conformation changes to Rasip1 stabilizing a β strand and an unstructured loop. Our data explain how Rasip1 can act as a Rap1 and Ras effector and show that Rasip1 defines a subgroup of dimeric RA domains that could mediate cooperative binding to membrane-associated Ras superfamily members

Regulation of integrin function: evidence that bivalent-cation-induced conformational changes lead to the unmasking of ligand-binding sites within integrin alpha5 beta1.

The molecular mechanisms that regulate integrin-ligand binding are unknown; however, bivalent cations are essential for integrin activity. According to recent models of integrin tertiary structure, sites involved in ligand recognition are located on the upper face of the seven-bladed beta-propeller formed by the N-terminal repeats of the alpha subunit and on the von Willebrand factor A-domain-like region of the beta subunit. The epitopes of function-altering monoclonal antibodies (mAbs) cluster in these regions of the alpha and beta subunits; hence these mAbs can be used as probes to detect changes in the exposure or shape of the ligand-binding sites. Bivalent cations were found to alter the apparent affinity of binding of the inhibitory anti-alpha5 mAbs JBS5 and 16, the inhibitory anti-beta1 mAb 13, and the stimulatory anti-beta1 mAb 12G10 to alpha5 beta1. Analysis of the binding of these mAbs to alpha5beta1 over a range of Mn2+, Mg2+ or Ca2+ concentrations demonstrated that there was a concordance between the ability of cations to elicit conformational changes and the ligand-binding potential of alpha5 beta1. Competitive ELISA experiments provided evidence that the domains of the alpha5 and beta1 subunits recognized by mAbs JBS5/16 and 13/12G10 are spatially close, and that the distance between these two domains is increased when alpha5 beta1 is occupied by bivalent cations. Taken together, our findings suggest that bivalent cations induce a conformational relaxation in the integrin that results in exposure of ligand-binding sites, and that these sites lie near an interface between the alpha subunit beta-propeller and the beta subunit putative A-domain

Critical cysteine residues for regulation of integrin alphaIIbbeta3 are clustered in the epidermal growth factor domains of the beta3 subunit.

Chemical or enzymic reduction/oxidation of integrin cysteine residues (e.g. by reducing agents and protein disulphide isomerase) may be a mechanism for regulating integrin function. It has also been proposed that unique cysteine residues in the integrin beta3 subunit are involved in the regulation of alphaIIbbeta3. In the present study, we studied systematically the role of disulphide bonds in beta3 on the ligand-binding function of alphaIIbbeta3 by mutating individual cysteine residues of beta3 to serine. We found that the disulphide bonds that are critical for alphaIIbbeta3 regulation are clustered within the EGF (epidermal growth factor) domains. Interestingly, disrupting only a single disulphide bond in the EGF domains was enough to activate alphaIIbbeta3 fully. In contrast, only two (of 13) disulphide bonds tested outside the EGF domains activated alphaIIbbeta3. These results suggest that the disulphide bonds in the EGF domains should be intact to keep alphaIIbbeta3 in an inactive state, and that there is no unique cysteine residue in the EGF domain critical for regulating the receptor. The cysteine residues in the EGF domains are potential targets for chemical or enzymic reduction