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J. Biol. Chem., Vol. 283, Issue 5, 2773-2783, February 1, 2008

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Regulation of FynT Function by Dual Domain Docking on PAG/Cbp^*

Silje Anette Solheim¹, Knut Martin Torgersen, Kjetil Taskén, and Torunn Berge

From the Biotechnology Centre of Oslo and Centre for Molecular Medicine Norway, Nordic EMBL Partnership, University of Oslo, P.O. Box 1125, Blindern, NO-0317 Oslo, Norway

In resting T-cells, the transmembrane adaptor protein PAG (phosphoprotein associated with glycosphingolipid-enriched microdomains) is constitutively tyrosine-phosphorylated, a state maintained by the Src family kinase FynT. PAG has a role in negative regulation of Src family kinases in T-cells by recruitment of Csk (C-terminal Src kinase) to the membrane via binding to PAG phosphotyrosine 317. The interaction between FynT and PAG is essential for PAG function; however, so far the FynT binding mode has been unknown. Here, we demonstrate that the FynT-PAG complex formation is a dual domain docking process, involving SH2 domain binding to PAG phosphotyrosines as well as an SH3 domain interaction with the first proline-rich region of PAG. This binding mode affects FynT kinase activity, PAG phosphorylation, and recruitment of FynT and Csk, demonstrated in Jurkat TAg cells after antibody stimulation of the T cell receptor. Furthermore, we show that TCR-induced tyrosine phosphorylation is regulated by SH3 domain modulation of the FynT-PAG interaction in human primary T-cells. Although FynT SH3 domain association is shown to be crucial for efficiently initiating PAG phosphorylation, we suggest that engagement of the SH2 domain on PAG renders FynT insensitive to Csk negative regulation. Thus, in T-cells, PAG is involved in positive as well as negative regulation of FynT activity.

Received for publication, June 26, 2007 , and in revised form, December 4, 2007.

^* This work was supported by a Norwegian Research Council FUGE Career Fellowship (to T. B.) and by grants from the Norwegian Functional Genomics Programme, the Norwegian Research Council, and the Norwegian Cancer Society (to K. T.). 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.

¹ To whom correspondence should be addressed: The Biotechnology Centre of Oslo, Gaustadalléen 21, NO-0349 Oslo, Norway. Fax: 47-22840501; E-mail: Torunn.Berge{at}biotek.uio.no.

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