integrins

Superfamily of cell surface proteins that are involved in binding to extracellular matrix components in some cases. Most are heterodimeric with a b subunit of 95 kD that is conserved through the superfamily, and a more variable a subunit of 150-170 kD. The first examples described were fibronectin and vitronectin receptors of fibroblasts, which bind to an RGD (Arg-Gly-Asp) sequence in the ligand protein, though the "context" of the RGD seems important and there is also a divalent cation-dependence. Subsequently the platelet IIb/IIIa surface glycoprotein (fibronectin and fibrinogen receptor) and the LFA-1 class of leucocyte surface protein were recognized as integrins, together with the VLA surface protein. The requirement for the RGD sequence in the ligand does not seem to be invariable.