A novel mutation within the extracellular domain of TrkA causes constitutive receptor activation

The TrkA NGF receptor extracellular region contains three leucine repeats f lanked by cysteine clusters and two immunoglobulin-like domains that are re quired for specific ligand binding. Deletion of the immunoglobulinlike doma ins abolishes NGF binding and causes ligand independent activation of the r eceptor. Here we report a specific mutation that increases the binding affi nity of the TrkA receptor for NGF, A change of proline 203 to alanine (P203 A) in the linker region between the leucine repeats and the first Ig-like d omain increased NGF binding by decreasing the ligand rate of dissociation, This mutated receptor was appropriately expressed on the cell surface and p romoted ligand-independent neurite outgrowth in PC12nnr5 cells. The mutant receptor was capable of spontaneous dimerization and was constitutively pho sphorylated in the absence of ligand, Moreover, expression of TrkA-P203A re ceptor in fibroblasts induced DNA synthesis and transformation and generate d tumours in nude mice. These data suggest that domains outside of the immu noglobulin-like structure contribute to ligand binding and constitutive act ivation of Trk receptors.