NATURALLY-OCCURRING TYROSINE KINASE INSERTS BLOCK HIGH-AFFINITY BINDING OF PHOSPHOLIPASE C-GAMMA AND SHC TO TRKC AND NEUROTROPHIN-3 SIGNALING

Neurotrophin-3 binds to the receptor tyrosine kinase, TrkC. Several na turally occurring splice variants of TrkC exist including those with 1 4- and 39 amino acid inserts within the tyrosine kinase homology regio n. When expressed in fibroblasts, full-length TrkC, but not the kinase insert variants, mediated neurotrophin-3-stimulated cell proliferatio n. We investigated the molecular basis of this signaling defect. The k inase inserts blocked the ability of TrkC to mediate neurotrophin-3 st imulated c-myc and c-fos transcription and activation of the AP-1 tran scriptional complex. In cells expressing full-length TrkC, neurotrophi n-3 promoted a sustained activation of mitogen-activated protein kinas e; TrkC containing kinase inserts only mediated transient activation o f mitogen-activated protein kinase. The kinase inserts specifically bl ocked neurotrophin-3-stimulated autophosphorylation of the phospholipa se C gamma binding site on TrkC (tyrosine 789) resulting in a severe r eduction in phospholipase C gamma association with TrkC and its tyrosi ne phosphorylation. Neurotrophin-3-stimulated phosphorylation of the S he binding site (tyrosine 485) on TrkC, and tyrosine phosphorylation o f She itself, was unaffected by the kinase inserts; however, the kinas e inserts blocked high affinity She association with TrkC. It is propo sed that the lack of high affinity binding of Shc and/or phospholipase C gamma to the TrkC kinase insert variants may be responsible for the inability of these variants to bring about a full biological response in fibroblasts.