TGFβ‐stimulated Smad1/5 phosphorylation requires the ALK5 L45 loop and mediates the pro‐migratory TGFβ switch

During the course of breast cancer progression, normally dormant tumour‐promoting effects of transforming growth factor β (TGFβ), including migration, invasion, and metastasis are unmasked. In an effort to identify mechanisms that regulate the pro‐migratory TGFβ ‘switch’ in mammary epithelial cells in vitro, we found that TGFβ stimulates the phosphorylation of Smad1 and Smad5, which are typically associated with bone morphogenetic protein signalling. Mechanistically, this phosphorylation event requires the kinase activity and, unexpectedly, the L45 loop motif of the type I TGFβ receptor, ALK5, as evidenced by studies using short hairpin RNA‐resistant ALK5 mutants in ALK5‐depleted cells and in vitro kinase assays. Functionally, Smad1/5 co‐depletion studies demonstrate that this phosphorylation event is essential to the initiation and promotion of TGFβ‐stimulated migration. Moreover, this phosphorylation event is preferentially detected in permissive environments such as those created by tumorigenic cells or oncogene activation. Taken together, our data provide evidence that TGFβ‐stimulated Smad1/5 phosphorylation, which occurs through a non‐canonical mechanism that challenges the notion of selective Smad phosphorylation by ALK5, mediates the pro‐migratory TGFβ switch in mammary epithelial cells.