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Association of the Src Family Tyrosine Kinase Fyn with TrkB

Abstract: Fyn tyrosine kinase, a member of the Src family, was recently reported to be present in neurons and glia cells. We investigated whether Fyn is involved in the Trk‐dependent signal transduction pathways of neurotrophin. The Fyn‐Src homology domain 2 (SH2) was observed to associate in vitro with the intracellular domain of TrkB (ICD‐TrkB). This association was dependent on the autophosphorylation of ICD‐TrkB. The Fyn‐SH2 domains bound to phosphorylated ICD‐TrkB (plCD‐TrkB) with an affinity similar to the binding of phospholipase Cγ (PLCγ)‐SH2 domains to its autophosphorylation site in TrkB. The Src‐SH2 domains showed substantially lower affinity with plCD‐TrkB, suggesting that the association between Fyn‐SH2 and plCD‐TrkB is not due to non‐specific interactions of SH2 domains with phosphorylated tyrosine residues. This is further supported by the observation that Fyn‐SH2 was able to trap phosphorylated TrkB in cell lysate prepared from primary rat cortical neurons stimulated with brain‐derived neurotrophic factor (BDNF). In contrast, endogenous Fyn was coprecipitated with TrkB from cortical neurons without BDNF stimulation. This basal association showed a threefold increase on BDNF stimulation, probably due to the SH2/phosphotyrosine interaction that was observed in the cell‐free system. All these data suggest the involvement of Fyn in the neurotrophin signal transduction pathways downstream of TrkB.