Changes in integrin structure in response to cellular signals regulate its binding to extracellular matrix (ECM) proteins like fibrinogen during processes such as platelet aggregation. Integrin is composed of α and β subunits, each of which is a transmembrane protein with a short cytoplasmic tail and several large extracellular domains. The binding sites for extracellular ligands lie far from the transmembrane domain, so how an intracellular signal is transmitted through so many extracellular domains has been difficult to determine.
The Cleveland Clinic group examined how the cytoplasmic tails respond to internal signals. Their studies revealed that the α and β tails of inactive integrin interact at a region adjacent to the plasma membrane. Activation of integrin, either by known constitutive mutations or by binding of the cytoskeletal protein talin, disrupted the cytoplasmic interaction and allowed the extracellular portion to bind fibrinogen.
The extracellular structural consequences of cytoplasmic uncoupling was then examined by the Harvard group. Their electron micrographs of linked soluble extracellular α and β domains confirmed a previous crystal structure of integrin in a condensed shape, like a “V” that points back toward the cell. A cell surface version held in this bent conformation by a disulfide bond did not bind fibrinogen unless the disulfide was broken. Based on the EM of the soluble protein, disrupting a membrane-proximal link between integrins causes the integrin to extend upwards like an opening switchblade. The extended form places the ligand-binding domain atop the dimer, where it is more accessible to physiological substrates. Thus, says Takagi, “we show that extension is at least partly responsible for making integrin high affinity.” However, two extended forms were found, which differed in the angle of the ligand-binding region. Takagi is now examining how these two conformers affect ligand binding. ▪