Extracellular domain of neurotrophin receptor trkB: disulfide structure, N-glycosylation sites, and ligand binding.

An extracellular domain of a human neurotrophin receptor trkB was expressed in Chinese hamster ovary cells and isolated as a glycoprotein possessing binding activity for brain-derived neurotrophic factor. The extracellular domain contains 398 amino acids and has a molecular weight of 60.6 kDa according to laser desorption mass spectrometry, indicating that the extracellular domain of trkB contains 33.3% carbohydrate moieties. Six disulfide linkages were determined to be Cys1-Cys7, Cys5-Cys14, Cys121-Cys145, Cys123-Cys163, Cys187-Cys235, and Cys271-Cys314, respectively. Cys300 was detected as a free sulfhydryl residue. Cysteine clusters 1 and 2 located in the N-terminal domain possess a similar type of disulfide structure and two other disulfide bonds in the C-terminal region are homologous to that of the Ig-like C2 domain. Among 12 potential N-linked glycosylation sites proposed in the soluble domain of trkB, 10 sites are actually glycosylated.