ACTIVATION LOOP TYROSINES CONTRIBUTE VARYING ROLES TO TRKB AUTOPHOSPHORYLATION AND SIGNAL-TRANSDUCTION

The TrkB receptor tyrosine kinase (RTK) is a high affinity receptor fo r the neurotrophins brain derived neurotrophic factor (BDNF) and neuro trophin-4/5 (NT-4/5), Following exposure to BDNF or NT-4/5, TrkB is au tophosphorylated on five cytoplasmic tyrosines: Y384, Y670, Y674, Y675 , and Y785. Based on crystallographic analyses for others RTKs, TrkB t yrosines Y670, Y674, and Y675 are expected to lie within a putative ki nase activation loop, Phosphorylation of these activation loop tyrosin es is postulated to be a conserved event required for complete RTK act ivation, Here, we have assessed the importance these activation loop t yrosines play in regulating TrkB autophosphorylation, cytoplasmic sign al transduction, and cell proliferation, We show that while tyrosine 6 70 is dispensable for BDNF-inducible TrkB autophosphorylation and the activation of certain signal transduction events, it is required for c omplete TrkB-mediated cellular proliferation, Combinatorial mutagenesi s of tyrosines 674 and 675 only moderately affects TrkB autophosphoryl ation, but significantly impairs the BDNF-inducible stimulation of cyt oplasmic signaling events and cellular proliferation, The combined mut ation of all three activation loop tyrosines results in an inactive re ceptor, which is unable to autophosphorylate, stimulate signaling even ts, or induce mitogenesis, The data highlight the varying degrees of i mportance of the three activation loop tyrosines in TrkB mediated biol ogical responses.