Activation of the Lck tyrosine protein kinase by the Herpesvirus saimiri tip protein involves two binding interactions

The Tip protein of Herpesvirus saimiri strain 484C binds to and activates t he Lck tyrosine protein kinase. Two sequences in the Tip protein were previ ously shown to be involved in binding to Lck. A proline-rich region, residu es 132-141, binds to the SH3 domain of the Lck protein. We show here that t he other Lck-binding domain, residues 104-113, binds to the carboxyl-termin al half of Lck and that this binding does not require the Lck SH3 domain. M utated Tip containing only one functional Lck-binding domain can bind stabl y to Lck, although not as strongly as wild-type Tip. Interaction of Tip wit h Lck through either Lck-binding domain increases the activity of Lck in vi vo. Simultaneous binding of both domains is required for maximal activation of Lck. The transient expression of Tip in T cells was found to stimulate both Stat3-dependent and NF-AT-dependent transcription. Mutant forms of Tip lacking one or the other of the two Lck-binding domains retained the abili ty to stimulate Stat3-dependent transcription. Tip lacking the proline-rich Lck-binding domain exhibited almost wild-type activity in this assay. In c ontrast, ablation of either Lck-binding domain abolished the ability of Tip to stimulate NF-AT dependent transcription. Full biological activity of Ti p, therefore, appears to require both Lck-binding domains. (C) 2000 Academi c Press.