Characterization of the recombinant extracellular domain of the neurotrophin receptor TrkA and its interaction with nerve growth factor (NGF).

Nerve growth factor (NGF) is the prototype of a family of neurotrophins that support important neuronal programs such as differentiation and survival of a subset of sympathetic, sensory, and brain neurons. NGF binds to two classes of cell surface receptors: p75LANR and p140TrkA. NGF binding to p140TrkA initiates the neuronal signaling pathway through activation of the tyrosine kinase activity, which subsequently results in a rapid signal transduction through a phosphorylation cascade. To examine this crucial signaling step in more detail, the TrkA extracellular domain polypeptide (TrkA-RED) was overexpressed in Sf21 insect cells and purified to homogeneity. The recombinant TrkA-RED is a 70 kDa acidic glycoprotein with a pI of 5.1, and mimics the intact TrkA receptor for NGF binding with a dissociation constant, Kd, of 2.9 nM. Thus, the recombinant TrkA-RED is functionally competent and can be used to elucidate the interaction of NGF and TrkA receptor. Circular dichroism difference spectra indicated that, upon association of NGF with TrkA-RED, a minor conformational change occurred to form a complex with decreased ordered secondary structure. Interaction between NGF and TrkA-RED was also demonstrated by size exclusion chromatography, light scattering, and chemical crosslinking with evidence for formation of a higher molecular weight complex consistent with a (TrkA-RED)2-(NGF dimer) complex. Association and dissociation rates of 5.6 x 10(5) M(-1) s(-1) and 1.6 x 10(-3) s(-1), respectively, were determined by biosensor technology. Thus, initiation of signaling may stem from NGF-induced receptor dimerization concomitant with a small conformational change.