PREFERENTIAL LOCALIZATION OF TYROSINE-PHOSPHORYLATED PAXILLIN IN FOCAL ADHESIONS

Focal adhesions are sites for integrin-mediated attachment of cultured cells to the extracellular matrix. Localization studies have shown th at focal adhesions can be stained by antiphosphotyrosine antibodies, b ut the role of tyrosine-phosphorylated proteins in focal adhesions is not known. By using Ventral plasma membranes prepared from chicken emb ryo fibroblasts spread on the substrate, we present evidence for the p referential localization of a minor poor of tyrosine-phosphorylated pa xillin in focal adhesions. Ventral plasma membranes showed an enrichme nt in beta 1-integrins, and in several tyrosine-phosphorylated polypep tides, while focal adhesion proteins like vinculin and paxillin, altho ugh localized to focal adhesions in ventral plasma membranes, were not particularly enriched in these preparations compared to whole cell ly sates. Biochemical and morphological analysis of ventral plasma membra nes showed a dramatic increase in the level of tyrosine-phosphorylatio n of the pool of paxillin localized to the adhesive sites, when compar ed to the paxillin present in whole cell lysates. The observed prefere ntial localization of tyrosine-phosphorylated paxillin to focal adhesi ons may represent a general mechanism to compartmentalize focal adhesi on components from large non-phosphorylated, cytosolic pools.