DIFFERENTIAL EFFECTS OF PHOSPHOTYROSINE PHOSPHATASE EXPRESSION ON HORMONE-DEPENDENT AND INDEPENDENT PP60C-SRC ACTIVITY

pp60 c-src kinase activity can be increased by phosphotyrosine dephosphorylation or growth factor-dependent phosphorylation reactions. Expression of the transmembrane phosphotyrosine phosphatase (PTPase) CD45 has been shown to inhibit growth factor receptor signal transduction (Mooney, RA, Freund, GG, Way, BA and Bordwell, KL (1992) J Biol Chem 267, 23443–23446). Here it is shown that PTPase expression decreased platelet-derived growth factor (PDGF)-dependant activation of pp60 c-src but failed to increase hormone independent (basal) pp60 c-src activity. PDGF-dependent tyrosine phosphorylation of its receptor was reduced by approximately 60% in cells expressing the PTPase. In contrast, a change in phosphotyrosine content of pp60 c-src was not detected in response to PDGF or in PTPase+cells. PDGF increased the intrinsic tyrosine kinase activity of pp60 c-src in both control and PTPase+cells but the effect was smaller in PTPase+cells. In anin vitro assay, hormone-stimulate pp60 c-src autophosphorylation from PTPase+ cells was decreased 64±22%, and substrate phosphorylation by pp60 c-src was reduced 54±16% compared to controls. Hormone-independent pp60 c-src kinase activity was unchanged by expression of the PTPase. pp60 c-src was, however, anin vitro substrate for CD45, being dephosphorylated at both the regulatory (Tyr527) and kinase domain (Tyr416) residues. In addition,in vitro dephosphorylation by CD45 increased pp60 c-src activity. These findings suggest that the PDGF receptor was anin vivo substrate of CD45 but pp60 c-src was not. The lack of activation of pp60 c-src in the presence of expressed PTPase may demonstrate the importance of compartmentalization and/or accessory proteins to PTPase-substrate interactions.