A Novel Transforming Growth Factor-β Receptor-interacting Protein That Is Also a Light Chain of the Motor Protein Dynein

The phosphorylated, activated cytoplasmic domains of the transforming growth factor-β (TGFβ) receptors were used as probes to screen an expression library that was prepared from a highly TGFβ-responsive intestinal epithelial cell line. One of the TGFβ receptor-interacting proteins isolated was identified to be the mammalian homologue of the LC7 family (mLC7) of dynein light chains (DLCs). This 11-kDa cytoplasmic protein interacts with the TGFβ receptor complex intracellularly and is phosphorylated on serine residues after ligand-receptor engagement. Forced expression of mLC7-1 induces specific TGFβ responses, including an activation of Jun N-terminal kinase (JNK), a phosphorylation of c-Jun, and an inhibition of cell growth. Furthermore, TGFβ induces the recruitment of mLC7-1 to the intermediate chain of dynein. A kinase-deficient form of TGFβ RII prevents both mLC7-1 phosphorylation and interaction with the dynein intermediate chain (DIC). This is the first demonstration of a link between cytoplasmic dynein and a natural growth inhibitory cytokine. Furthermore, our results suggest that TGFβ pathway components may use a motor protein light chain as a receptor for the recruitment and transport of specific cargo along microtublules