Integrins glycoproteins

Collagen is a major component of connective tissue that becomes exposed at the subendothelium of injured blood vessels. It contributes to platelet adhesion and also plays a role in platelet activation by binding to several receptors on platelets such as integrin a 2(3 1 or glycoprotein VI (GP VI). 

Fibronectin receptor is a two-chain glycoprotein of the integrin family that serves as a transmembrane linker by binding to talin on the cytoplasmic side and to fibronectin on the external side of the membrane. The pull exerted by stress fibers on attached structures may be produced by bipolar assemblies of nonmuscle myosin molecules producing a sliding of actin filaments of opposite polarity. 

Thus, while glycoprotein Ia/IIa (a2Pj) binds to collagen, glycoprotein Ic/IIa (a ) binds to fibronectin (5). The other integrin belonging to the beta 1 family that is involved in platelet adhesion is ct6P, which binds laminin. 

Cadherins Transmembrane glycoproteins (352) promoting cellcell adhesion (353,354) linked to integrins (355) and via catenins to muscle action (356)... 

ICAM-1 and -2 are constitutively expressed on endothelial cells ICAM-1 may be further up-regulated by exposure to cytokines. ICAM-3 has recently been described (its identity based on the unique specificity of a monoclonal antibody) and is a 124-kDa glycoprotein present on the surfaces of T cells, monocytes and neutrophils its expression may be up-regulated by stimulants such as mitogens. VCAM-1, which is expressed on the luminal surface of cytokine-exposed endothelial cells, binds T cells via VLA-4. It is also expressed on the surface of some leukaemic cell lines, on rheumatoid synovial cells and on some tumours.On the other hand, ICAM-1 is found on all endothelial surfaces, and its interaction with neutrophil integrins is the major mechanism that results in the stimulation of transendothelial migration. 

Macromolecular complexes of proteins and carbohydrate present in the ECM serve not only as adhesive keeping cells in their correct positions but also facilitate control of cell activity by signalling through membrane proteins such as the integrin family of receptors. Glycoproteins are mainly protein with covalently attached carbohydrate whereas proteoglycans are predominantly complex carbohydrates secured on a protein framework. 

Simon DI, Chen Z, Xu H, et al. Platelet glycoprotein ibalpha is a counterreceptor for the leukocyte integrin Mac-1 (CDllb/CD18). JExp Med 2000 192 193-204. 

---