Fifth domain (immunoglobulin-like) of Trk receptors TrkA, TrkB, and TrkC; member of the I-set of Ig superfamily (IgSF) domains
The members here are composed of the fifth domain of Trk receptors TrkA, TrkB, and TrkC, an immunoglobulin (Ig)-like domain which binds to neurotrophin. The Trk family of receptors are tyrosine kinase receptors. They are activated by dimerization, leading to autophosphorylation of intracellular tyrosine residues, and triggering the signal transduction pathway. TrkA, TrkB, and TrkC share significant sequence homology and domain organization. The first three domains are leucine-rich domains while the fourth and fifth domains are Ig-like domains playing a part in ligand binding. TrkA, TrkB, and TrkC mediate the trophic effects of the neurotrophin Nerve Growth Factor (NGF) family. TrkA is recognized by NGF. TrkB is recognized by brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-4. TrkC is recognized by NT-3. NT-3 is promiscuous as in some cell systems it activates TrkA and TrkB receptors. TrkA is a receptor found in all major NGF targets, including the sympathetic, trigeminal, and dorsal root ganglia, cholinergic neurons of the basal forebrain, and the striatum. TrKB transcripts are found throughout multiple structures of the central and peripheral nervous systems. The TrkC gene is expressed throughout the mammalian nervous system. This group belongs to the I-set of IgSF domains.