SH2-B, a membrane-associated adapter, is phosphorylated on multiple serines/threonines in response to nerve growth factor by kinases within the MEK/ERK cascade

SH2-B has been shown to be required for nerve growth factor (NGF)-mediated neuronal differentiation and survival, associate with NGF receptor TrkA, and be tyrosylphosphorylated in response to NGF. In this work, we examined whether NGF stimulates phosphorylation of SH2-B on serines/threonines. NGF promotes a dramatic upward shift in mobility of SH2-B, resulting in multiple forms that cannot be attributed to tyrosyl phosphorylation. Treatment of SH2-B with protein phosphatase 2A, a serine/threonine phosphatase, reduces the many forms to two. PD98059, a MEK inhibitor, dramatically inhibits NGF-promoted phosphorylation of SH2-B on serines/threonines, whereas depletion of 4 beta-phorbol 12-myristate 13-acetate-sensitive protein kinase Cs does not. ERKs 1 and 2 phosphorylate SH2-B beta primarily on Ser-96 in vitro. However, NGF still stimulates serine/threonine phosphorylation of SH2-B beta(S96A). SH2-B beta(S96A), like wildtype SH2-B beta, enhances NGF-induced neurite outgrowth. In contrast, SH2-B beta(R555E) containing a defective SH2 domain blocks NGF-induced neurite outgrowth and displays greatly reduced phosphorylation on serines/threonines in response to NGF. SH2-B beta(R555E), like wild-type SH2-B beta, associates with the plasma membrane, suggesting that the dominant negative effect of SH2-B beta(R555E) cannot be explained by an abnormal subcellular distribution. In summary, NGF stimulates phosphorylation of SH2-B on serines/threonines by kinases downstream of MEK, which may be important for NGF-mediated neuronal differentiation and survival.