Activation of Epstein-Baff virus/C3d receptor (gp140, CR2, CD21) on human B lymphoma cell surface triggers Cb1 tyrosine phosphorylation, its association with p85 subunit, Crk-L and Syk and its dissociation with Vav

it is well established that CD21 activation on human B cell surface triggers B cell proliferation. We previously demonstrated that CD21 activation also triggers tyrosine phosphorylation of two components, p95 and p 120, both interacting with SH2 domains of the p85 subunit of PI 3-kinase. We successively identified p95 as the nucleolin and the first signal transduction pathway specifically triggered by CD21 activation, i.e.: pp60Src activation, tyrosine phosphorylation of p95 nucleolin, its interaction with SH2 domains of p85 subunit and PI 3-kinase activation, followed by AKT-GSK-3 activations. We herein identified the p 120 component as the protooncoprotein Cb1 and the first steps associated to its activation. First, CD21 activation triggered Cb1 tyrosine phosphorylation, which required c-Src kinase but not PI 3-kinase or Syk kinase activities. Involvement of Src kinase in this step was supported by inhibition of Cb1 phosphorylation and its interactions with other components when cells were either preincubated with specific Src inhibitor or transfected with dominant-negative c-Src form. Second, once tyrosine phosphorylated, Cb1 interacts with SH2 domains of p85 subunit, SH2 domains of Crk-L and with tyrosine phosphorylated Syk kinase. The third and unexpected feature was to found that, at the contrary of BCR or of CD19 (herein also analyzed for the first time), CD21 activation triggers dissociation of Cb1-Vav complex. Thus, these results provide the first molecular basis of a new signal transduction pathway specifically triggered by CD21 activation. (c) 2005 Elsevier Inc. All rights reserved.