p62(dok) negatively regulates CD2 signaling in Jurkat cells

p62(dok) belongs to a newly identified family of adaptor proteins. In T cel ls, the two members that are predominantly expressed, p56(dok) and p62(dok) , are tyrosine phosphorylated upon CD2 or CD28 stimulation, but not upon CD 3 ligation. Little is known about the biological role of Dok proteins in T cells. In this study, to evaluate the importance of p62(dok) in T cell func tion, we generated Jurkat clones overexpressing p62(dok). Our results demon strate that overexpression of p62(dok) in Jurkat cells has a dramatic negat ive effect on CD2-mediated signaling. The p62(dok)-mediated inhibition affe cts several biochemical events initiated by CD2 ligation, such as the incre ase of intracellular Ca2+, phospholipase C gamma1 activation, and extracell ular signal-regulated kinase 1/2 activation. Importantly, these cellular ev ents are not affected in the signaling cascade induced by engagement of the CD3/TCR complex. However, both CD3- and CD2-induced NF-AT activation and I L-2 secretion are impaired in p62(dok)-overexpressing cells. In addition, w e show that CD2 but not CD3 stimulation induces p62(dok) and Ras GTPase-act ivating protein recruitment to the plasma membrane. These results suggest t hat p62(dok) plays a negative role at multiple steps in the CD2 signaling p athway. We propose that p62(dok) may represent an important negative regula tor in the modulation of the response mediated by the TCR.