Previous studies of receptor coupled tyrosine kinases and tyrosine kinase oncogene products have conclusively demonstrated that a broad variety of cellular processes; but particularly cell growth, are regulated via tyrosine phosphorylation. The fact that activation of cellular tyrosine kinases does not normally result in deregulated cell growth suggests that enzymes must exist that counteract tyrosine phosphorylation. Although likely candidates are phosphatases which mediate tyrosine dephosphorylation, this possibility has not been analyzed carefully, primarily because of a lack of appropriate model systems. In recent studies which employed B lymphoid cells, we have observed that expression of the major membrane tyrosine phosphatase CD45 is essential for signal transduction through the antigen receptor (mIg). This was accomplished using a powerful cell system, the J558Lmu-m3 myeloma, which lacks expression of endogenous CD45 and does not transduce Ca++ mobilizing signals through mIg. Antigen receptor mediated signal transduction could be rescued in these cells by transfection of wildtype CD45, implicating CD45 in antigen receptor function. We further observed that CD45 interacts with mIg in the plasma membrane and dephosphorylates tyrosine phosphorylated components of the antigen receptor complex. These findings suggest that CD45 is a component of the receptor complex and may function in dynamic equilibrium with the receptor-associated tyrosine kinase to regulate receptor function and/or to transduce signals following receptor ligation. In this project, we propose to address this hypothesis using the J558Lmu- m3 model system by conducting mutational analyses of CD45 structural requirements for maintenance of receptor signaling function, interaction with mIg, and dephosphorylation of receptor components. We will identify tyrosine phosphorylation sites on the operative tyrosine kinase and on receptor alpha, beta and gamma subunits. We will then utilize mutational analysis to determine the role of phosphorylation and dephosphorylation of these residues in receptor function. The proposed studies should provide important novel insight regarding the role of tyrosine phosphatases in signal transduction.
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