Requirement for protein kinase A in the phosphorylation of the TGFβ receptor-interacting protein km23-1 as a component of TGFβ downstream effects.

km23-1 was previously identified as a TGFβ-receptor interacting protein that was phosphorylated on serines after TGFβ stimulation. In the current report, we examined the role of km23-1 phosphorylation in the downstream effects of TGFβ/protein kinase A signaling. Using phosphorylation site prediction software, we found that km23-1 has two potential consensus phosphorylation sites. In vitro kinase assays further demonstrated that duanyu1529 directly phosphorylates km23-1 on serine 73 (S73). Moreover, our results show that the inhibitor H89 diminishes phosphorylation of km23-1 on S73 after TGFβ stimulation. Taken together, our results demonstrate that TGFβ induction of duanyu1529 activity results in phosphorylation of km23-1 on S73. In order to assess the mechanisms underlying duanyu1529 phosphorylation of km23-1 on S73 (S73-km23-1) after TGFβ stimulation, immunoprecipitation (IP)/blot analyses were performed, which demonstrate that TGFβ regulates complex formation between the duanyu1529 regulatory subunit RIβ and km23-1 in vivo. In addition, an S73A mutant of km23-1 (S73A-km23-1), which could not be phosphorylated by inhibited TGFβ induction of the km23-1-dynein complex and transcriptional activation of the activin-responsive element (ARE). Furthermore, our results show that km23-1 is required for cAMP-responsive element (CRE) transcriptional activation by TGFβ, with S73-km23-1 being required for the CRE-dependent TGFβ stimulation of fibronectin (FN) transcription. Collectively, our results demonstrate for the first time that phosphorylation of km23-1 on S73 is required for ARE- and CRE-mediated downstream events that include FN induction.

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