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J. Biol. Chem., Vol. 277, Issue 47, 44996-45003, November 22, 2002

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Phosphorylation of ³³⁸SSYY³⁴¹ Regulates Specific Interaction between Raf-1 and MEK1^*

Xiaoqin Xiang, Mengwei Zang, Christine A. Waelde, Rong Wen^§, and Zhijun Luo^¶

From the  Diabetes and Metabolism Research Unit, Endocrinology Section, Evans Department of Medicine and the ^¶ Department of Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118 and the ^§ Department of Ophthalmology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104

The present study characterizes the interaction between the Raf-1 kinase domain and MEK1 and examines whether the magnitude of their interaction correlates to the ability of Raf to phosphorylate MEK1. Here we show that the minimal domain required for the Raf kinase activity starts from tryptophan 342. Maximal binding of the Raf kinase domain to MEK1 and its kinase activity are achieved upon phosphorylation of the region ³³⁸SSYY³⁴¹ in response to 4-12-O-tetradecanoylphorbol-13-acetate (TPA), or mutation of Y340Y341 to aspartic acids. Conversely, the TPA-stimulated MEK binding and kinase activity are diminished when this region is deleted or Ser³³⁸ and Ser³³⁹ are mutated to alanines. We also show that the integrity of the Raf ATP-binding site is necessary for the interaction between Raf-1 and MEK1. Furthermore, two MEK-binding sites are identified; the first is localized between amino acids 325 and 349, and the second is within the region between amino acids 350 and 648. Separately, the binding of each site to MEK1 is weak, but in a cis context, they give rise to a much stronger association, which can be further stimulated by TPA. Finally, we find that tryptophan 342, which is conserved among the Raf family and other protein kinases, is essential for the Ser³³⁸ phosphorylation of the full-length Raf and its binding to MEK1. Taken together, our results indicate that the phosphorylation of Ser³³⁸ and Tyr³⁴¹ on Raf exerts an important effect on reconfiguring the two MEK-binding sites. As a result, these two sites coordinate to form a high affinity MEK-binding epitope, leading to a marked increase in Raf kinase activity.

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