The Amino-terminal One-third of αIIb Defines the Ligand Recognition Specificity of Integrin αIIbβ3

Joseph C Loftus,Carol E Halloran,Mark H Ginsberg,Larry P Feigen,Jeffery A Zablocki,Jeffrey W Smith

doi:10.1074/jbc.271.4.2033

Abstract

The integrin alpha subunits play a major role in the regulation of ligand binding specificity. To gain further insight into the regions of the alpha subunits that regulate ligand specificity, we have utilized alpha v / alpha IIb chimeras to identify regions of alpha IIb that when substituted for the homologous regions of alpha v switched the ligand binding phenotype of alpha v beta 3 to that of alpha IIb beta 3. We report that the ligand recognition specificity of beta 3 integrins is regulated by the amino-terminal one-third of the alpha subunit. Substitution of the amino-terminal portion of alpha v with the corresponding 334 residues of alpha IIb reconstituted reactivity with both alpha IIb beta 3-specific activation-dependent (PAC1) and -independent (OPG2) ligand mimetic antibodies in addition to small highly specific activation-independent ligands. In contrast, substitution of the amino-terminal portion alone or the divalent cation repeats alone were not sufficient to change ligand binding specificity. These data in combination with previous studies demonstrate that integrin ligand recognition requires cooperation between elements in both the alpha and beta subunits and indicate that the ligand binding pocket is a structure assembled from elements of both the alpha and beta subunits.

Highlights

Integrins are heterodimeric adhesion receptors composed of noncovalently associated ␣ and ␤ subunits
A major difficulty in determining the role of integrin ␣ subunits in the regulation of ligand binding specificity is that the binding of most macromolecular ligands is activation-dependent, i.e. the binding of these ligands is highly regulated by the conformational state of the receptor [30, 31]
The Divalent Cation Repeats and the NH2-terminal Region of ␣IIb Are Required for an ␣IIb␤3 Ligand Binding Specificity—To begin to investigate the regions of the ␣IIb␤3 subunit that regulate ligand recognition specificity of ␣IIb␤3, ␣v/␣IIb chimeric ␣ subunits were generated (Fig. 1)

Summary

Introduction

Integrins are heterodimeric adhesion receptors composed of noncovalently associated ␣ and ␤ subunits. To gain further insight into the structures in the ␣ subunits that regulate ligand recognition specificity, we exploited the unique tools available for the integrins ␣IIb␤3 and ␣v␤3. These two integrins share the common ␤3 subunit, and the two ␣ subunits are 36% identical in primary sequence [34]. The ligand mimetic property of both mAbs is linked to the tripeptide sequence RYD within the third complementarity-determining region that appears to mimic the RGD recognition sequence [4, 5]

Methods

Results

Conclusion