Role of initial protein phosphorylation events and localized release‐activated calcium influx in B cell antigen receptor signaling

An increase in intracellular calcium concentration is one of the major initial steps in B cell activation following antigen receptor (BCR) ligation. We show herein that in C57BL/6 murine B lymphocytes and in model cell lines, BCR-mediated calcium ion (Ca2+) influx occurs via highly selective Ca2+ release-activated channels, and stromal interaction molecule 1 (STIM1) plays an important role in this pathway. We also demonstrate the temporal relation between Ca2+-dependent signaling events and formation of the immune synapse. Our data indicate that cytoplasmic Ca2+ levels in areas adjacent to the immune synapse differ from those in the rest of the cytoplasm. Finally, a comparison of phosphorylation patterns of BCR-triggered signaling proteins in the presence or absence of Ca2+ revealed the unanticipated finding that initial BCR-triggered, Ca2+-dependent tyrosine phosphorylation events involve predominantly Ca2+ released from intracellular stores and that influx-derived Ca2+ is not essential. This suggests a different role for this phase of Ca2+ influx.