Microtubule binding to Smads may regulate TGF beta activity

Smad proteins are intracellular signaling effecters of the TGF beta superfa mily. We show that endogenous Smad2, 3, and 4 bind microtubules (MTs) in se veral cell lines. Binding of Smads to MTs does not require TGF beta stimula tion. TGF beta triggers dissociation from MTs, phosphorylation, and nuclear translocation of Smad2 and 3, with consequent activation of transcription in CCL64 cells. Destabilization of the MT network by nocodazole, colchicine , or a tubulin mutant disrupts the complex between Smads and MTs and increa ses TGF beta-induced Smad2 phosphorylation and transcriptional response in CCL64 cells. These data demonstrate that MTs may serve as a cytoplasmic seq uestering network for Smads, controlling Smad2 association with and phospho rylation by activated TGF beta receptor I, and suggest a novel mechanism fo r the MT network to negatively regulate TGF beta function.