THE GERMINAL CENTER KINASE (GCK)-RELATED PROTEIN-KINASES HPK1 AND KHSARE CANDIDATES FOR HIGHLY SELECTIVE SIGNAL TRANSDUCERS OF CRK FAMILY ADAPTER PROTEINS

Adapter proteins function by mediating the rapid and specific assembly of multi-protein complexes during the signal transduction which guard s proliferation, differentiation and many functions of higher eukaryot ic cells, To understand their functional roles in different cells it i s important to identify the selectively interacting proteins in these cells. Two novel candidates for signalling partners of Crk family adap ter proteins, the hematopoietic progenitor kinase 1 (HPK1) and the kin ase homologous to SPS1/STE20 (KHS), were found to bind with great sele ctivity to the first SH3 domains of c-Crk and CRKL. While KHS bound ex clusively to Crk family proteins, HPK1 also interacted with both SH3 d omains of Grb2 and weakly with Nck, but not with more than 25 other SH 3 domains tested. The interaction of HPK1 with c-Crk and CRKL was stud ied in more detail. HPK1-binding to the first SH3 domain of CRKL is di rect and occurs via proline-rich motifs in the C-terminal, non-catalyt ic portion of HPK1. In vitro complexes were highly stable and in vivo complexes of c-Crk and CRKL with HPK1 were detectable by co-immnnoprec ipitation with transiently transfected cells but also with endogenous proteins. Furthermore, c-Crk II and, to a lesser extent, CRKL were sub strates for HPK1, These results make it likely that HPK1 and KHS parti cipate in the signal transduction of Crk family adapter proteins in ce rtain cell types.