SYK IS REQUIRED FOR BCR-MEDIATED ACTIVATION OF P90RSK, BUT NOT P70S6K, VIA A MITOGEN-ACTIVATED PROTEIN KINASE-INDEPENDENT PATHWAY IN B-CELLS

The tyrosine kinases Syk and Lyn are activated in B lymphocytes follow ing antibody induced cross-linking of the B cell receptor for antigen (BCR). It has been suggested that activation of Syk is dependent on Ly n. We tested this hypothesis by comparing the phosphorylation and acti vation of several downstream effector molecules in parental DT40, DT40 Syk(-) and DT40Lyn-B cells. The phosphorylation and activation of p90R sk was ablated in Syk-deficient B cells but unaffected in Lyn-deficien t B cells while the phosphorylation/activation of Ras GTPase activatin g protein (Ras GAP) and mitogen activated protein (MAP) kinase require d both Syk and Lyn. Thus, these data indicate that Syk can be activate d in the absence of Lyn after BCR cross-linking and results ire the ac tivation of p90Rsk via a MAP kinase-independent pathway in DT40Lyn(-) cells. We also demonstrated that BCR mediates the activation of p70S6k . However, activation of p70S6k in DT40Syk(-) and DT40Lyn(-) cells was comparable with that observed in parental cells. Thus, either Syk or Lyn may be sufficient for activation of p70S6k, or activation of p70S6 k occurs independently of both Syk and Lyn. The kinase activity of Syk was required for the phosphorylation/activation of each of these down stream effector molecules but only the phosphorylation of Ras GAP was affected in cells expressing a mutant of Syn in which tyrosines 525 an d 526 were substituted to phenlyalanines.