The adaptor protein Gads (Grb2-related adaptor downstream of Shc) is implicated in coupling hemopoietic progenitor kinase-1 to the activated TCR

The hemopoietic-specific Gads (Grb2-related adaptor downstream of Shc) adap tor protein possesses amino- and carboxyl-terminal Src homology 3 (SH3) dom ains banking a central SH2 domain and a unique region rich in glutamine and proline residues. Gads functions to couple the activated TCR to distal sig naling events through its interactions with the leukocyte-specific signalin g proteins SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and L AT (linker for activated T cells), Expression library screening for additio nal Gads-interacting molecules identified the hemopoietic progenitor kinase -1 (HPK1), and we investigated the HPK1-Gads interaction within the DO11.10 murine T cell hybridoma system. Our results demonstrate that HPK1 inducibl y associates with Gads and becomes tyrosine phosphorylated following TCR ac tivation. HPK1 kinase activity is up-regulated in response to activation of the TCR and requires the presence of its proline-rich motifs, Mapping expe riments have revealed that the carboxyl-terminal SH3 domain of Gads and the fourth proline-rich region of HPK1 are essential for their interaction. De letion of the fourth proline-rich region of HPK1 or expression of a Cads SH 2 mutant in T cells inhibits TCR-induced HPK1 tyrosine phosphorylation, Tog ether, these data suggest that HPK1 is involved in signaling downstream fro m the TCR, and that SH2/SH3 domain-containing adaptor proteins, such as Gad s, may function to recruit HPK1 to the activated TCR complex.