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Papers In Press, published online ahead of print July 7, 2004
Department of Medicine, University of Liverpool, Liverpool, Liverpool L69 3GA
Corresponding Author: lgyu{at}liv.ac.uk
Protein phosphatase 2A (PP2A) is a family of mammalian serine/threonine phosphatases that is involved in the control of many cellular functions including those mediated by extracellular signal-regulated kinase (ERK) signaling. Whilst investigating the reversible anti-proliferative effect of the dietary lectin, jacalin, which binds the Thomsen-Friedenreich antigen (galactose b1-3 N-acetylgalactosaminea-, TF), we have found that this lectin (30ug/ml) induces rapid, transient, tyrosine phosphorylation of putative human HLA-DR associated protein I (PHAPI, also known as the tumour suppressor pp32) in HT29 human colon cancer cells. This is accompanied by release of PP2A from association with PHAPI, allowing increased phosphatase activity of PP2A (by 42 ±10% at 10 minutes), and consequent complete de-phosphorylation of the ERK kinase, MEK1/2 by 10 minutes, and of ERK1/2 by 60 minutes. PHAPI knockdown by RNA interference abolished the effects of jacalin on PP2A activation and MEK inhibition. Thus phosphorylation of PHAPI/pp32 is a critical regulatory step in PP2A activation and ERK signalling.
J. Biol. Chem, 10.1074/jbc.M400017200
Submitted on January 5, 2004
Revised on July 7, 2004
Accepted on July 7, 2004
Protein phosphatase 2A, a negative regulator of the ERK signaling pathway, is activated by tyrosine phosphorylation of PHAPI/pp32 in response to the anti-proliferative lectin, jacalin
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