pp125FAK in the focal adhesion.

Integrins are a large superfamily of transmembrane adhesion molecules. In many types of cultured cells, integrins are concentrated in specialized sites called focal adhesions. Integrins are capable of transducing signals to the inside of the cell, which can effect cell migration, differentiation and growth, but the signaling mechanism of integrins has been obscure because their short cytoplasmic domains do not possess endogenous catalytic activity. The recent discovery of a tyrosine kinase called pp125FAK (for focal adhesion kinase) has led to a proposed model in which the binding of integrins to extracellular ligands activates FAK, which then generates a tyrosine phosphorylation cascade within the cell. Data both for and against this model have been obtained, and the precise function of FAK in cultured cells and organized tissues is still not clear. However, many interesting features (its unusual molecular structure, its functional and physical association with integrins, and its potential for participating in multiple signaling pathways) suggest that FAK may play a pivotal role in conveying information from the membrane to the inside of the cell.