News and Publications

Integrin Structure and Function

Y. Takada, T. Kamata, T. Tarui, K. Yokoyama, W. Puzon-McLaughlin, X. Zhang

Integrins are a family of /ß heterodimers of cell adhesion receptors that mediate interactions between cells and between cells and the extracellular matrix. Interactions between integrins and their ligands are critical in the pathogenesis of many diseases and thus are therapeutic targets.

STRUCTURE AND FUNCTION OF THE ß I DOMAIN

The N-terminal region of the ß subunit is currently the subject of much interest because this region is critical for ligand binding and the regulation of binding. The I or A domain, a set of inserted sequences consisting of about 200 amino acid residues, which is present in several integrin subunits, is important in ligand binding and receptor activation. The N-terminal region of the ß subunit may have a similar structure. We recently detected 8 conserved oxygenated residues critical for ligand binding in several separate predicted loop structures in the ß I domain (Fig. 1). These conserved residues probably constitute multiple binding sites for ligands and cations within the ß I domain and thus are ubiquitously involved in ligand binding regardless of the ligand or the integrin.

We observed that a large predicted loop region (residues 176--199 of ß₁) is diverse among the ß subunits. We hypothesized that this predicted loop (especially the short disulfide-linked sequence CTSEQNC 187--193 of ß₁) is involved in determining the ligand specificity of integrins. We designed experiments with _vß₁ and _vß₃ integrins to ascertain whether this diverse sequence is involved in determining the ligand specificity of integrins. The integrin _vß₃ recognizes a wide variety of ligands, including fibronectin, fibrinogen, von Willebrand factor, and vitronectin; _vß₁ is specific to fibronectin.

We found that when the sequence CTSEQNC 187--193 of ß₁ is replaced with the corresponding CYDMKTTC sequence of ß₃, the ligand specificity of _vß₁ is altered. The mutant integrin (_vß_1-3-1), like _vß₃, recognizes fibrinogen, von Willebrand factor, and vitronectin (a gain-of-function effect). The mutant is recruited to focal contacts on fibrinogen and vitronectin, a finding that suggests that it transduces intracellular signals on adhesion. The reciprocal ß_3-1-3 mutation blocks binding of _vß₃ to these multiple ligands and to a function-blocking antibody to _vß₃. These results suggest that this highly divergent sequence is a key determinant of integrin-ligand specificity. The data also support a recently proposed model of the ß I domain in which this short disulfide-linked sequence (CTSEQNC 187--193 in ß₁, and CYDMKTTC in ß₃) is located in the ligand-binding site.

A NEW LIGAND FOR INTEGRIN /alphasb/_Vß₃

Metalloprotease/disintegrin/cysteine-rich proteins (MDCs) are membrane-anchored proteins that have a metalloprotease domain and a disintegrin-like domain. MDCs are potential ligands for integrins, because most snake venom disintegrins interact with integrins _IIbß₃ and _vß₃. MDC-15 (metargidin) has an arginine--glycine--aspartic acid (RGD) sequence in its disintegrin-like domain and is widely expressed in various tissues and cells, including endothelial cells and smooth muscle cells.

We used the recombinant disintegrin-like domain of MDC-15 to determine the receptor specificities of MDC-15. We discovered that this recombinant disintegrin-like domain specifically interacts with integrin _vß₃ but not with other RGD-dependent or RGD-independent integrins tested, including _IIbß₃. In contrast, most snake venom disintegrins bind to both _IIbß₃ and _vß₃. We also found that the integrin specificity of the recombinant disintegrin-like domain is mediated by the RGD tripeptide and the flanking sequence of the tripeptide, as is the case in snake venom disintegrins. These results suggest that MDC-15 may be involved in _vß₃-mediated cell-cell adhesion.

PUBLICATIONS

King, S.L., Kamata, T., Cunningham, J.A., Emsley, J., Liddington, R.C., Takada, Y., Bergelson, J.M. Echovirus 1 interaction with the human very late antigen-2 (integrin ₂ß₁) I domain: Identification of two independent virus contact sites distinct from the metal ion-dependent adhesion site residues. J. Biol. Chem. 272:28518, 1997.

Mould, A.P., Garratt, A.N., Puzon-McLaughlin, W., Takada, Y., Humphries, M.J. Allosteric regulation of integrin function by divalent cations: Evidence that divalent-cation induced conformational changes lead to the exposure of ligand binding sites within integrin ₅ß₁. Biochem. J. 331:821, 1998.

Takada, Y., Kamata, T., Irie, A., Puzon-McLaughlin, W., Zhang, X.-P. Structural basis of integrin-mediated signal transduction (a review). Matrix Biol. 16:143, 1997.

Tsuchida, J., Ueki, S., Takada, Y., Saito, Y., Takagi, J. The 'ligand-induced conformational change' of ₅ß₁ integrin. J. Cell Sci. 111:1759, 1998.

Zhang, X.-P., Kamata, T., Yokoyama, K., Puzon-McLaughlin, W., Takada, Y. Specific interaction of the recombinant disintegrin-like domain of MDC-15 (metargidin, ADAM-15) with integrin _vß₃. J. Biol. Chem. 273:7345, 1998.