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J. Biol. Chem., Vol. 280, Issue 12, 11590-11598, March 25, 2005

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Protein Phosphatase 2A Activity Associated with Golgi Membranes during the G₂/M Phase May Regulate Phosphorylation of ERK2^*

Chad N. Hancock, Surabhi Dangi, and Paul Shapiro

From the Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, Baltimore, Maryland 21201

The extracellular signal-regulated kinase (ERK) 1 and 2 proteins are mitogen-activated protein kinase (MAPK) members that regulate cell proliferation and differentiation. ERK proteins are activated exclusively by MAPK kinase 1 and 2 phosphorylation of threonine and tyrosine residues located within the conserved TXY MAPK activation motif. Although dual phosphorylation of Thr and Tyr residues confers full activation of ERK, in vitro studies suggest that a single phosphorylation on either Thr or Tyr may yield partial ERK activity. Previously, we have demonstrated that phosphorylation of the tyrosine residue (Tyr(P) ERK) may be involved in regulating the Golgi complex structure during the G₂ and M phases of the cell cycle (Cha, H., and Shapiro, P. (2001) J. Cell Biol. 153, 1355–1368). In the present study, we examined mechanisms for generating Tyr(P) ERK by determining cell cycle-dependent changes in localized phosphatase activity. Using fractionated nuclei-free cell lysates, we find increased serine/threonine phosphatase activity associated with Golgi-enriched membranes in cells synchronized in the late G₂/early M phase as compared with G₁ phase cells. The addition of phosphatase inhibitors in combination with immunodepletion assays identified this activity to be related to protein phosphatase 2A (PP2A). The increased activity was accounted for by elevated PP2A association with mitotic Golgi membranes as well as increased catalytic activity after normalization of PP2A protein levels in the phosphatase assays. These data indicate that localized changes in PP2A activity may be involved in regulating proteins involved in Golgi disassembly as cells enter mitosis.

Received for publication, July 21, 2004 , and in revised form, December 20, 2004.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Pharmaceutical Sciences, University of Maryland School of Pharmacy, 20 N. Pine St., Baltimore, MD 21201. Tel.: 410-706-8522; Fax: 410-706-0346; E-mail: pshapiro{at}rx.umaryland.edu.

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