Caspase-mediated cleavage and functional changes of hematopoietic progenitor kinase 1 (HPK1)

Activation of c-Jun N-terminal kinase (JNK) by Fas ligation is caspase-depe ndent, suggesting that caspases may regulate activators of the JNK pathway. Here, we report that an upstream activator of JNK, hematopoietic progenito r kinase I (HPK1), was cleaved during apoptosis, Cleavage of HPK1 was block ed by peptide inhibitors for caspases. HPK1 was efficiently processed by re combinant caspase 3 ill vitro. A conserved caspase recognition site, DDVD ( amino acids 382-385), was found in the HPK1 protein sequence. By testing HP K1 proteins with ill vivo and ill vitro cleavage assays, we showed that asp artic acid residue 385 is the target for caspases. HPK1 cleavage separated the amino N-terminal kinase domain from the carboxyl C-terminal regulatory domain, and enhanced HPK1 kinase activity. Unlike the full-length HPK1, the N-terminal cleaved product failed to bind adaptor molecules Grb2 (growth f actor receptor-bound protein 2) and Crk (CT10 regulator of kinase). The C-t erminal fragment, although having three proline-rich domains, bound to Grb2 and Crk less efficiently than the full-length HPK1 protein. Taken together , the cleavage of HPK1 by caspase profoundly changed its biochemical proper ties.