Lad, an adapter protein interacting with the SH2 domain of p56(lck), is required for T cell activation

T cell-specific Src family tyrosine kinase, p56(lck), plays crucial roles i n T cell differentiation, activation, and proliferation. These multiple fun ctions of p56(lck) are believed to be conducted through the protein-protein interactions with various cellular signaling proteins. To clarify the mech anisms through which p56(lck) contributes to T cell signaling, we identifie d the proteins binding to the Src homology 2 (SH2) domain of p56(lck) throu gh a tyrosine phosphorylation-dependent yeast two-hybrid screening, Subsequ ent characterization of positive clones revealed the presence of a protein of 366 aa named Lad (Lck-associated adapter protein), which is a potential murine homologue of previously reported TSAd, a T cell-specific adapter pro tein. Lad contains several protein-protein interaction domains including a zinc-finger motif, an SH2 domain, a proline-rich SH3 binding motif, and sev eral phosphotyrosine sites. Furthermore, Lad was tyrosine phosphorylated an d associated with p56(lck) in vivo and redistributed from cytoplasm to the plasma membrane in a T cell activation-dependent manner. Moreover in T cell s, IL-2 promoter activity was enhanced upon coexpression of Lad but was inh ibited by the coexpression of antisense Lad RNA. These characteristics of L ad suggest that Lad play an essential role as an adapter protein in p56(lck )-mediated T cell signaling.