Dimerization in vivo and inhibition of the nonreceptor form of protein tyrosine phosphatase epsilon

cyt-PTPε is a naturally occurring nonreceptor form of the receptor-type protein tyrosine phosphatase (PTP) epsilon. As such, cyt-PTPε enables analysis of phosphatase regulation in the absence of extracellular domains, which participate in dimerization and inactivation of the receptor-type phosphatases receptor-type protein tyrosine phosphatase alpha (RPTPα) and CD45. Using immunoprecipitation and gel filtration, we show that cyt-PTPε forms dimers and higher-order associations in vivo, the first such demonstration among nonreceptor phosphatases. Although cyt-PTPε readily dimerizes in the absence of exogenous stabilization, dimerization is increased by oxidative stress. Epidermal growth factor receptor stimulation can affect cyt-PTPε dimerization and tyrosine phosphorylation in either direction, suggesting that cell surface receptors can relay extracellular signals to cyt-PTPε, which lacks extracellular domains of its own. The inactive, membrane-distal (D2) phosphatase domain of cyt-PTPε is a major contributor to intermolecular binding and strongly interacts in a homotypic manner; the presence of D2 and the interactions that it mediates inhibit cyt-PTPε activity. Intermolecular binding is inhibited by the extreme C and N termini of D2. cyt-PTPε lacking these regions constitutively dimerizes, and its activities in vitro towards paranitrophenyl phosphate and in vivo towards the Kv2.1 potassium channel are markedly reduced. We conclude that physiological signals can regulate dimerization and phosphorylation of cyt-PTPε in the absence of direct interaction between the PTP and extracellular molecules. Furthermore, dimerization can be mediated by the D2 domain and does not strictly require the presence of PTP extracellular domains.