| Note |
Three TrkB isoforms are reported by UniProt/Swiss-Prot:
1. The long isoform TrkB, including the tyrosine kinase domain (ID Q16620-1; variant c).
2. The truncated isoform TrkB-T1 lacking the tyrosine kinase domain (ID Q16620-2; variant b).
3. The truncated isoform TrkB-T-Shc lacking the tyrosine kinase domain but retaining the Shc site (ID Q16620-3; variant e). |
| Description | The unprocessed precursor of the full-length TrkB (a) consists of 838 AA. Variant (c) excludes 16 AA of unknown function, located downstream of the transmembrane segment.
The N-terminal portion (AA 32-430) is potentially extracellular and includes several N-glycosylation sites (AA 67, 121, 254). It follows a single transmembrane segment (AA 432-454). The C-terminal portion is cytosolic (AA 455-822) and comprises the Protein Kinase domain. This region includes the ATP binding site (AA 544-552) and several sites of autophosphorylation such as Tyr-516/702/706/707/817 (AA position refers to variant c).
The truncated Trkb-T1 (b) is composed of 477 AA. TrkB-T-Shc variants d and e consist of 553 AA and 537 AA, respectively. Truncated isoforms TrkB-T1 and TrkB-T-Shc include C-terminal sequence variations of 10 and 9 AA, respectively. |
| Expression | NTRK2 gene is preferentially expressed in brain, spinal cord, cranial and spinal ganglia. Expression is most prominent in the following brain regions: amygdale, caudate nucleus, cerebellum, choroid plexus, corpus callosum, cortex, hippocampus, hypothalamus and thalamus. In addition, a variety of cranial structures such as eyes, ophthalmic nerves, various facial districts and vestibular system indicate significant expression. Lower expression is described in several other tissues such as heart, kidney, lung, ovaries, pancreas, pituitary gland, prostate, salivary glands, skeletal muscle, spleen, thymus and thyroid.
Isoforms TrkB and TrkB-T1 are expressed in brain as well as in several peripheral areas, whereas TrkB-T-Shc is primarily expressed in brain.
AceView (NCBI) analysis of cDNA clones supports the expression pattern suggested by the evaluation of mRNA described above. In addition suggests elevated expression in several tumor tissues. |
| Localisation | Neuronal activity promotes TrkB translocation from intracellular vesicles to the plasma membrane where it becomes available for neurotrophins. The N-terminal segment is extracellular and is involved in neurotrophin binding and cell adhesion. A single transmembrane segment is located in the central portion of the polypeptide. The C-terminal segment is intracellular and comprises the protein kinase domain. |
| Function | TrkB specifically binds brain-derived neurotrophic factor (BDNF) and neurotrophin4/5. It can also bind neurotrophin-3 with low affinity but it excludes nerve growth factor (NGF). Neurotrophin binding triggers receptor dimerization and consequent trans-phosphorylation of tyrosine residues of the TyrKc domain. Phosphorylated receptor undergoes conformational changes, which promote the recruitment of intracellular substrates such SHC1, PI-3 kinase, and PLC-gamma-1. The signaling cascades consequently activated support neuronal survival during development and following injuries, promote neuronal differentiation and maintenance, control short-term and long-term synaptic activity. TrkB can also form heterodimers with the pan-neurotrophin receptor p75NTR or with truncated TrkB. This influences the establishment of specific connections with signaling pathways. |
| Homology | TrkB belongs to the large family of protein kinase comprising a conserved kinase domain. It is included in the subfamily of tyrosine protein kinase. For the presence of a highly conserved intracellular TyrKc domain it is most related to growth factor receptors, and particularly to the neurotrophic factor receptors TrkA and TrkC. The homology with tyrosine kinase receptors is extended to the IGC-2 and LRRs domains, however, these are also present in cell-adhesion molecules. |
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