Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites - The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown pre viously that shrinkage induces Fyn-dependent tyrosine phosphorylation of th e cortical actin-binding protein, cortactin, Because FER kinase was implica ted in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation, These effects were ab olished by the Src family inhibitor PP2 and strongly mitigated in Fyn-defic ient but not in Src-deficient cells. FER overexpression caused cortactin ph osphorylation that was further enhanced by hypertonicity. Exchange of tyros ine residues 421, 466, and 482 for phenylalanine prevented cortactin phosph orylation by hypertonicity and strongly decreased it upon FER overexpressio n, suggesting that FER targets primarily the same osmo-sensitive tyrosines, Because constituents of the cell-cell contacts are substrates of Fyn and F ER, we investigated the effect of shrinkage on the adherens junctions. Hype rtonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin fr om the contacts, This process was delayed in Fyn-deficient or PP2-treated c ells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the F yn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.