The role of phosphorylation and limited proteolytic cleavage of talin and vinculin in the disruption of focal adhesion integrity

Chemical agents which activate specific kinases were employed to disrupt the stress fiber and focal adhesion organization of cells spread on a substratum. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C, promoted a rapid loss of stress fibers and focal adhesions from African green monkey kidney (BSC-1) cells. This was paralleled by an increase in the level of talin phosphorylation suggesting that this may play a role in the removal of talin from focal adhesions. Similar morphological changes were produced in the rat embryo fibroblast line (REF 52) by dibutyryl-cAMP, which stimulates protein kinase A. In contrast, however, the phosphorylation of talin was reduced in REF 52 cells when treated with dibutyryl cAMP. In untreated cells we found that the levels of vinculin phosphorylation were very low relative to the levels of talin phosphorylation and did not change following drug treatment in either cell line. Although limited proteolytic cleavage of cytoskeletal proteins represents a potential mechanism for focal adhesion disruption, we observed no proteolysis of talin or vinculin in response to either drug treatment.