This chapter discusses receptor-activated tyrosine phosphatases, which are a diverse group of enzymes involved in signal–transduction pathways that are intimately associated with normal cellular physiology in most cell types and have been shown to be also involved in neuronal differentiation and neuroendocrine cell growth. This ill-defined group can be classified into receptor-linked phosphotyrosine phosphatases (PTPases) and receptor-coupled PTPases. Receptor-linked PTPases are analogous to receptor-linked tyrosine kinases (EGF receptor, NGF receptor/trk) and implicated in neuronal differentiation. Receptor-coupled PTPases are downstream effectors of G protein-coupled pathways primarily involved in the inhibition of cell growth. The discovery of both groups extends traditional notion of receptors and G protein-coupled pathways. Although a small percentage of total cellular phosphoproteins are phosphorylated on tyrosines, tyrosine phosphorylation appears to play an extremely important role in maintaining a balanced cellular environment. Tyrosine phosphorylation is now a well-accepted form of regulation of protein function, most clearly demonstrated by studying the signal–transduction pathways initiated by growth factor receptors that contain tyrosine kinase activity.
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