Down-regulation of the Filamentous Actin Cross-linking Activity of Cortactin by Src-mediated Tyrosine Phosphorylation

Cortactin, a prominent substrate for pp60 c- src , is a filamentous actin (F-actin) binding protein. We show here that cortactin can promote sedimentation of F-actin at centrifugation forces under which F-actin is otherwise not able to be precipitated. Electron microscopic analysis after negative staining further revealed that actin filaments in the presence of cortactin are cross-linked into bundles of various degrees of thickness. Hence, cortactin is also an F-actin cross-linking protein. We also demonstrate that the optimal F-actin cross-linking activity of cortactin requires a physiological pH in a range of 7.3–7.5. Furthermore, pp60 c- src phosphorylates cortactin in vitro , resulting in a dramatic reduction of its F-actin cross-linking activity in a manner depending on levels of tyrosine phosphorylation. In addition, pp60 c- src moderately inhibits the F-actin binding activity of cortactin. This study presents the first evidence that pp60 c- src can directly regulate the activity of its substrate toward the cytoskeleton and implies a role of cortactin as an F-actin modulator in tyrosine kinase-regulated cytoskeleton reorganization.