Protein phosphatase 5 modulates SMAD3 function in the transforming growth factor-β pathway

Protein phosphatases play a key role in balancing the cellular responses to the transforming growth factor-beta (TGF beta) signals. Several protein phosphatases have been attributed roles in the regulation of the TGF beta pathway. Among these. PPM1A is the only phosphatase reported to dephosphorylate SMAD2/3 in the nucleus. However we observed PPM1A exclusively in the cytoplasmic fractions independently of TOM treatment in all cells tested. These observations imply that a bona fide nuclear SMAD2/3 phosphatase remains elusive. In this study, we report a role for protein phosphatase 5 (PP5) in the TGF beta pathway. We identified PP5 as an interactor of SMAD2/3. Interestingly, in mouse embryonic fibroblast cells derived from PP5-null mice, TGF beta-induced transcriptional responses were significantly enhanced. Rather surprisingly, this enhancement is due to the increased levels of SMAD3 protein observed in PP5-null MEFs compared to the wild type. No differences in the levels of SMAD3 transcripts were observed between the wild-type and PPS-null MEFs. While PP5 is capable of dephosphoiylating SMAD3-tail in overexpression assays, we demonstrate that its activity is essential in controlling SMAD3 protein levels in MEFs. We propose that PP5 regulates the TGF beta pathway in MEFs by regulating the expression of SMAD3 protein levels. (C) 2012 Elsevier Inc. All rights reserved.