Demonstrating that the defect in PLSCR1 BMN673 purchase phosphorylation was not due to aberrant maturation of these cells but to a kinase-related defect in FceRI signaling. After FceRI aggregation no significant increase in Lyn association with PLSCR1 was observed indicating that tyrosine phosphorylation of PLSCR1 by Lyn is not solely regulated by the interaction between both proteins but might be also regulated by Lyn activation. Previous studies have demonstrated that PLSCR1 also interacts with the prototypic kinase p60c-src within the EGF receptor signaling pathways, thus serving as a substrate for p60c-src and, in turn, amplifying the activation of this kinase. These data together with the ones collected herein, suggest a preferred connection between PLSCR1 and this family of tyrosine kinases that might be due to their common localization at the plasma membrane and particularly in lipid rafts. This preferred connection is also highlighted by the negative regulation of PLSCR1 tyrosine phosphorylation by Fyn, another member of the Src family. This was revealed by the more robust increase in FceRI-dependent phosphorylation of PLSCR1 in Fyndeficient BMMC. The increased phosphorylation of PLSCR1 was not due to an increased calcium signal because Fyn deficiency does not increase this signal. Neither was it due to an overall increase in tyrosine phosphorylation of cellular proteins since tyrosine phosphorylation of proteins in whole cell lysates from activated cells is significantly lower in Fyn-deficient BMMC compared to their wild-type counterparts. The data presented here extend our original observation that rat PLSCR1 is phosphorylated on tyrosine after FceRI aggregation in the RBL-2H3 mast cell line to non-tumoral mast cells derived in culture and to another species. Furthermore, the demonstration that it is initiated by the FcRc chain and by another FcRc-associated receptor suggests that other Fc receptors should be able to promote the tyrosine phosphorylation of PLSCR1. The complexity of the mechanisms regulating PLSCR1 tyrosine phosphorylation suggests that this phosphorylation might play an important role in the regulation of PLSCR1 amplifier function. Studies by others have reported that there is no correlation between mast cell degranulation, phosphatidylserine externalization and tyrosine phosphorylation of PLSCR1 when comparing mast cells stimulated through FceRI, Thy-1 and calcium ionophores. The evidence presented here suggesting that PLSCR1 is phosphorylated on multiple tyrosines raises the possibility that positive regulation of degranulation by PLSCR1 may be associated with its phosphorylation on particular tyrosines whereas phosphorylation of other tyrosines may be involved in down-regulation of PLSCR1 function as is known for many receptors. Studies are underway to clarify this question. Living cells constantly respond to both internal and external stimuli with the help of signalling systems. As the complexity of the organism increases, the complexity of signalling systems also increases. Complexity may be manifested by the introduction of new molecular players or inter-molecular interactions that constitute a network.