A transcription function for the T cell-specific adapter (TSAd) protein inT cells: Critical role of the TSAd Src homology 2 domain

T cell-specific adapter (TSAd) protein is an Src homology 2 (SH2) domain-co ntaining adapter molecule implicated in T cell receptor for antigen (TCR)-m ediated interleukin 2 (IL-2) secretion in T cells. Here, we demonstrate tha t a substantial fi-action of TSAd is found in the T cell nucleus. Nuclear i mport of TSAd is an active process that depends on TSAd SH2 domain recognit ion of a phosphotyrosine-containing ligand. Importantly, we show that TSAd can act as a potent transcriptional activator in T cells. Furthermore, the TSAd SH2 domain appears to be essential for this transcription-activating f unction independent of its role in nuclear import. Biochemical analyses sug gest that a single TSAd SH2 domain ligand of 95-100 kD may be involved in t hese processes. Consistent with a role as a transcription activator, cotran sfection of TSAd with an IL-2 promoter-reporter gene construct results in a considerable upregulation of IL-2 promoter activity. Further, we show that this augmentation requires a functional TSAd SH2 domain. However, TSAd doe s not appear to modulate the activity of-the major recognized IL-2 gene tra nscription factors, nuclear factor kappaB (NF-kappaB), nuclear factor of ac tivated T cells (NFAT), or activator protein 1 (AP-1). These findings point to the function of TSAd as a novel transcription-regulatory protein in T c ells and illustrate the importance of the TSAd SH2 domain in this role.