The transmembranal and cytoplasmic forms of protein tyrosine phosphatase epsilon physically associate with the adaptor molecule Grb2

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Abstract
The protein tyrosine phosphatase Epsilon (PTPε) gene gives rise to two physiologically-distinct protein products – a transmembranal, receptor-like form and a cytoplasmic, non-receptor form. Previous studies have suggested a link between expression of transmembranal PTPε and transformation of mouse mammary epithelium specifically by ras or neu, although little is known about the underlying molecular mechanisms; cytoplasmic PTPε is believed to function mainly in hematopoietic tissues. As part of our efforts to understand PTPε function at the molecular level, we demonstrate here that both forms of PTPε associate with the adaptor molecule Grb2 in vivo. Binding is mediated by the SH2 domain of Grb2; this domain binds exclusively to the carboxy-terminal phosphotyrosine of cytoplasmic PTPε(Y638), and probably to additional phosphotyrosine residues in transmembranal PTPε. Through its SH2 domain, Grb2 can constitutively associate with transmembranal PTPε in mammary tumors initiated by ras or neu, and can be induced to associate with cytoplasmic PTPε in Jurkat T-cells following stimulation of T-cell receptor signaling by pervanadate. These findings indicate that tyrosine phosphorylation of PTPε and subsequent binding to Grb may link this phosphatase to downstream events which transduce signals from the cell membrane to its interior.