These data suggest that the phosphorylation of PLSCR1 is pivotal to the crossinteraction of the Lyn- and Fyn-initiated signaling pathways. Whereas initial analyses of FceRI-dependent signaling pathways concentrated on defining specific effectors for each pathway, it becomes increasingly evident that multiple signals converge and that cross-talk of signals is key for an integrated cellular response. Example of this is provided by the tyrosine kinase Csk and by the adaptor Cbp that are recruited by Lyn and that negatively regulate the Fyn signaling in FceRI-mediated mast cell activation. Therefore, whereas Lyn can negatively regulate Fyn-initiated signals, herein we report that, conversely, Fyn can negatively regulate at least some Lyn-initiated signals, demonstrating that both pathways have the capacity to control each other. Thus, particular substrates could function at such crossroads promoting integration of signaling pathways and allowing fine-tuning and regulation. Interestingly, we previously demonstrated that PLSCR1 acts as an amplifier of the LAT-PLCc-calcium axis thus modulating degranulation and VEGF production. This axis depends on the Lyn-initiated signaling pathway. The association between Lyn and PLSCR1 is reminiscent of the association between PLSCR1 and Src in EGF receptor signaling that potentiates Src kinase activity. Our present study demonstrates that whereas PLSCR1 can modulate the Lyn initiated pathway, this MG132 pathway controls PLSCR1 tyrosine phosphorylation, revealing a particular partnership between PLSCR1 and Lyn in FceRI-induced mast cell activation. The absolute requirement of Lyn and Syk is demonstrated by the absence of FceRI-dependent increase in PLSCR1 phosphorylation in mast cells deficient for either kinase. Of note, absence of either kinase abolishes the mobilization of calcium. Yet, we observed a residual, but significant, increase in tyrosine phosphorylation of PLSCR1 in the absence of calcium after FceRI aggregation. This indicates that PLSCR1 could be phosphorylated on multiple tyrosines, which would be in agreement with the heterogeneous molecular weight observed for phospho-PLSCR1 in mast cells, although the extent of this heterogeneity varies somehow from experiment to experiment for reasons that remain so far unclear. The increase in PLSCR1 tyrosine phosphorylation observed in the presence of calcium may be due to its phosphorylation on additional phosphorylation sites accessible after the conformational change induced by calcium. Alternatively, new molecules of PLSCR1 could be recruited for tyrosine phosphorylation by the same or other kinases. That Lyn and Syk could be directly involved in the phosphorylation of PLSCR1 is supported by several observations. Lyn and Syk were found to physically interact with PLSCR1. In addition, the FceRI-dependent tyrosine phosphorylation of PLSCR1 was fully ablated in the absence of FcRc, and partially in the absence of calcium, mapping the occurrence of the residual increase in the phosphorylation of PLSCR1 upstream of the calcium signal but downstream of the FcRc chain, i.e. where Lyn and Syk are thought to function in IgE-mediated signaling.