Evidence for phosphatase activity of p27SJ and its impact on the cell cycle

J Cell Biochem. 2009 Jun 1;107(3):400-7. doi: 10.1002/jcb.22135.


p27SJ, a novel protein isolated from St John's wort (Hypericum perforatum), belongs to an emerging family of DING proteins that are related to a prokaryotic phosphate-binding protein superfamily. Here we demonstrate that p27SJ exhibits phosphatase activity and that its expression in cells decreases the level of phosphorylated Erk1/2, a key protein of several signaling pathways. Treatment of p27SJ-expressing cells with phosphatase inhibitors including okadaic acid, maintained Erk1/2 in its phosphorylated form, suggesting that dephosphorylation of Erk1/2 is mediated by p27SJ. Further, expression of p27SJ affects Erk1/2 downstream regulatory targets such as STAT3 and CREB. Moreover, the level of expression of cyclin A that associates with active ERK1/2 and is regulated by CREB, was modestly reduced in p27SJ-expressing cells. Accordingly, results from in vitro kinase assays revealed a noticeable decrease in the activity of cyclin A in cells expressing p27SJ. Cell cycle analysis demonstrated dysregulation at S and G2/M phases in cells expressing p27SJ, supporting the notion that a decline in cyclin A activity by p27SJ has a biological impact on cell growth. These observations provide evidence that p27SJ alters the state of Erk1/2 phosphorylation, and impacts several biological events associated with cell growth and function.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Cell Cycle*
  • Cell Line, Tumor
  • DNA-Binding Proteins / metabolism
  • Humans
  • Hypericum / chemistry
  • Hypericum / enzymology*
  • Mitogen-Activated Protein Kinase 1 / metabolism
  • Models, Biological
  • Phosphoprotein Phosphatases / metabolism*
  • Phosphorylation
  • Plant Proteins / metabolism*
  • Protein Conformation


  • DNA-Binding Proteins
  • Plant Proteins
  • Mitogen-Activated Protein Kinase 1
  • Phosphoprotein Phosphatases