DELETIONS IN THE EXTRACELLULAR DOMAIN OF RAT TRKA LEAD TO AN ALTERED DIFFERENTIATIVE PHENOTYPE IN NEUROTROPHIN RESPONSIVE CELLS

We have investigated the role(s) of conserved sequences in the extrace llular domain of rat trkA by generating 5' and 3' deletions and assayi ng changes in neurotrophin binding, tyrosine kinase activity, and neur ite outgrowth. Essential sequences required for both nerve growth fact or (NGF) and neurotrophin-3 (NT-3) binding were mapped to the immunogl obulin-like domains. Small deletions in the second immunoglobulin-like domain and in the juxtamembrane region abolished neurotrophin binding . Dose-response curves on cells expressing full-length trkA were ident ical for Nor and NT-3 (0.1 ng/ml-50 ng/ml) while cells expressing leuc ine rich motif (LRM) minus receptors required high concentrations of N T-3 (50 ng/ml). Scatchard analysis revealed a loss of high-affinity NT -3, but not NGF, binding to the LRM minus receptor consistent with the neurite dose-response curves. Moreover, cells expressing the LRM minu s receptors failed to fasciculate and showed delayed arborization in c omparison to cells expressing wild-type trkA, suggesting a possible ro le for the LRM's in neurotrophin-induced differentiation and in high-a ffinity NT-3 binding.