UCSD Molecule Pages

Integrins are heterodimeric transmembrane (TM) glycoproteins containing one each of α and β subunit, which are held together by non-covalent forces. Integrin β2 (CD18) is the β subunit for four heterodimers: αDβ2, αXβ2, αMβ2 and αLβ2. Integrin β2 family plays an essential role in leukocyte recruitment and activation during inflammation. Structurally, while most part of the αβ dimer is extracellular, both the subunits traverse the plasma membrane and terminate as short cytoplasmic domains. Each heterodimeric integrin exists on the cell surface mainly in an inactive (bent) form until they receive stimulating signals from other receptors (via inside-out signaling), and the end result of integrin activation is a shift in integrin conformation from a bent to an extended one. The binding of cytoplasmic proteins to α- and/or β-subunit carboxy-terminal tails is an essential part of the activation process, as these interactions stabilize the extended integrin conformation and provide connections to the cytoskeleton. The binding of extracellular ligand to the extended form of integrin (via outside-in signaling) triggers a large variety of signal transduction events that modulate cell behaviors such as adhesion, proliferation, survival or apoptosis, shape, polarity, motility, and differentiation, mostly through effects on the cytoskeleton. The receptors αMβ2 (Complement Receptor type 3, CR3) and αXβ2 (Complement Receptor type 4, CR4) are regarded to be the most important mediators for complement-driven phagocytosis.