LIGATION OF MEMBRANE IGM STIMULATES A NOVEL C-JUN AMINO-TERMINAL DOMAIN KINASE-ACTIVITY IN DAUDI HUMAN B-CELLS

Stress-activated protein kinases (SAPK; also known as JNK for c-Jun N- terminal kinase) phosphorylate Ser63 and Ser73 in the amino-terminus o f the c-Jun protein and potentiate its transcriptional activity. We ha ve analysed phosphorylation of GST fusion proteins containing the c-Ju n N-terminal domain by lysates of Daudi human B lymphoblastoid cells s timulated with medium or anti-IgM. Crosslinking membrane IgM (mIgM) re sults in an increase in phosphorylation of GST-c-Jun (5-89) in an anti body dose-dependent manner. The kinase activity specifically phosphory lates the c-Jun N-terminal domain since it does not phosphorylate GST or GST-JunB. The activity preferentially phosphorylates the substrate that contains the sites for in vivo phosphorylation by SAPK/JNK and re quires the delta domain of c-Jun, which is also required for SAPK/JNK activity. However, the c-Jun N-terminal kinase activity induced by mIg M ligation is not precipitatable with anti-SAPK/JNK antibodies. In add ition, unlike SAPK/JNKs, the mIgM-dependent c-Jun N-terminal kinase ac tivity is not detectable in assays for renaturable kinase activity (in -gel assay) or in assays that test activities that bind to c-Jun (soli d-phase assay). The increased phosphorylation of c-Jun N-terminal doma in in response to mIgM ligation is unlikely to be due to mIgM-activate d ERKs as it was not suppressed by a selective MEK inhibitor. Thus, th e mIgM-induced activity is distinct from the known SAPK/JNKs and may r epresent a novel mechanism for c-Jun phosphorylation in response to mI gM engagement in human B cells. (C) 1997 Elsevier Science Ltd.