Intersubunit signal transmission in integrins by a receptor-like interaction with a pull spring

摘要

The function of some multidomain proteins is regulated by interdomain communication. We use second-site suppressor cysteine mutations to test a hypothesis on how the inserted (I)-like domain in the integrin β-subunit regulates ligand binding by the neighboring I domain in the integrin α-subunit [Huth, J. R., Olejniczak, E. T., Mendoza, R., Liang, H., Harris, E. A., et al. (2000) Proc. Natl. Acad. Sci. USA 97, 5231–5236; and Alonso, J. L., Essafi, M., Xiong, J. P., Stehle, T. & Arnaout, M. A. (2002) Curr. Biol. 12, R340–R342]. The hypothesis is that an interaction between the β I-like metal ion-dependent adhesion site (MIDAS) and an intrinsic ligand in the linker following the α I domain, Glu-310, exerts a pull that activates the α I domain. Individual mutation of αL linker residue Glu-310 or β2 MIDAS residues Ala-210 or Tyr-115 to cysteine abolishes I domain activation, whereas the double mutation of αL-E310C with either β2-A210C or β2-Y115C forms a disulfide bond that constitutively activates ligand binding. The disulfide-bonded mutant is resistant to small molecule antagonists that bind to the β I-like domain near its interface with the α I domain and inhibit communication between these domains but remains susceptible to small molecule antagonists that bind underneath the I domain α7-helix and certain allosteric antagonistic antibodies. Thus, the α7-helix and its linker are better modeled as a pull spring than a bell rope. The results suggest that αL residue Glu-310, which is universally conserved in all I domain-containing integrins, functions as an intrinsic ligand for the β I-like domain, and that when integrins are activated, the β I-like MIDAS binds to Glu-310, pulls the spring, and thereby activates the α I domain.