Synergistic regulation of immunoreceptor signaling by SLP-76-related adaptor Clnk and serine/threonine protein kinase HPK-1

Recently, the identification of CInk, a third member of the SLP-76 family o f adaptors expressed exclusively in cytokine-stimulated hemopoietic cells, has been reported by us and by others. Like SLP-76 and BInk, CInk was shown to act as a positive regulator of immunoreceptor signaling. Interestingly, however, it did not detectably associate with known binding partners of SL P-76, including Vav, Nck, and GADS. In contrast, it became complexed in act ivated T cells and myeloid. cells with an as yet unknown tyrosine-phosphory lated polypeptide of similar to 92 kDa (p92). In order to understand better the function of CInk, we sought to identify the CInk-associated p92. Using a yeast two-hybrid screen and cotransfection experiments with Cos-1 cells, evidence was adduced that p92 is HPK-1, a serine/threonine-specific protei n kinase expressed in hemopoietic cells. Further studies showed that CInk a nd HPK-1 were also associated in hemopoietic cells and that their interacti on was augmented by immunoreceptor stimulation. A much weaker association w as detected between HPK-1 and SLP-76. Transient transfections in Jurkat T c ells revealed that CInk and HPK-1 cooperated to increase immunoreceptor-med iated activation of the interleukin 2 (IL-2) promoter. Moreover, the abilit y of CInk to stimulate IL-2 promoter activity could be blocked by expressio n of a kinase-defective version of HPK-1. Lastly we found that in spite of the differential ability of CInk and SLP-76 to bind cellular proteins, CInk was apt at rescuing immunoreceptor signaling in a Jurkat T-cell variant la cking SLP-76. Taken together, these results show that CInk physically and f unctionally interacts with HPK-1 in hemopoietic cells. Moreover, they sugge st that CInk is capable of functionally substituting for SLP-76 in immunore ceptor signaling, albeit by using a distinct set of intracellular effectors .