Requirement of km23 for TGFβ-mediated growth inhibition and induction of fibronectin expression

We previously identified km23 as a novel TGFβ receptor-interacting protein. Here we show that km23 is ubiquitously expressed in human tissues and that cell-type specific differences in endogenous km23 protein expression exist. In addition, we demonstrate that the phosphorylation of km23 is TGFβ-dependent, in that EGF was unable to phosphorylate km23. Further, the kinase activity of both TGFβ receptors appears to play a role in the TGFβ-mediated phosphorylation of km23, although TGFβ RII kinase activity is absolutely required for km23 phosphorylation. Blockade of km23 using small interfering RNAs significantly decreased key TGFβ responses, including induction of fibronectin expression and inhibition of cell growth. Thus, our results demonstrate that km23 is required for TGFβ induction of fibronectin expression and is necessary, but not sufficient, for TGFβ-mediated growth inhibition.