Leukocyte-specific adaptor protein Grap2 interacts with hematopoietic progenitor kinase 1 (HPK1) to activate JNK signaling pathway in T lymphocytes

Immune cell-specific adaptor proteins create various combinations of multip rotein complexes and integrate signals from cell surface receptors to the n ucleus, modulating the specificity and selectivity of intracellular signal transduction, Grap2 is a newly identified adaptor protein specifically expr essed in lymphoid tissues. This protein shares 40-50% sequence homology in the SH3 and the SH2 domain with Grb2 and Grap, However, the Grap2 protein h as a unique 120-amino acid glutamine-and proline-rich domain between the SH 2 and C-terminal SH3 domains, The expression of Grap2 is highly restricted to lymphoid organs and T lymphocytes. In order to understand the role of th is specific adaptor protein in immune cell signaling and activation, we sea rched for the Grap2 interacting protein in T lymphocytes. We found that Gra p2 interacted with the hematopoietic progenitor kinase 1 (HPK1) in vitro an d in Jurkat T cells. The interaction was mediated by the carboxyl-terminal SH3 domain of Grap2 with the second proline-rich motif of HPK1, Coexpressio n of Grap2 with HPK1 not only increased the kinase activity of HPK1 in the cell, but also had an additive effect on HPK1 mediated JNK activation. Furt hermore, over expression of Grap2 and HPK1 induced significant transcriptio nal activation of c-Jun in the JNK signaling pathway and IL-2 gene reporter activity in stimulated Jurkat T cells, Therefore, our data suggest that th e hematopoietic specific proteins Grap2 and HPK1 form a signaling complex t o mediate the c-Jun NH2-terminal kinase (JNK) signaling pathway in T cells.