The cytoplasmic and transmembrane domains of the p75 and Trk a receptors regulate high affinity binding to nerve growth factor

Ligand-induced receptor oligomerization is an established mechanism for rec eptor-tyrosine kinase activation. However, numerous receptor-tyrosine kinas es are expressed in multicomponent complexes with other receptors that may signal independently or alter the binding characteristics of the receptor-t yrosine kinase. Nerve growth factor (NGF) interacts with two structurally u nrelated receptors, the Trk A receptor-tyrosine kinase and p75, a tumor nec rosis factor receptor family member. Each receptor binds independently to N GF with predominantly low affinity (K-d = 10(-9) M), but they produce high affinity binding sites (K-d = 10(-11) M) upon receptor co-expression. Here we provide evidence that the number of high affinity sites is regulated by the ratio of the two receptors and by specific domains of Trk A and p75. Co -expression of Trk A containing mutant transmembrane or cytoplasmic domains with p75 yielded reduced numbers of high affinity binding sites. Similarly , co-expression of mutant p75 containing altered transmembrane and cytoplas mic domains with Trk A also resulted in predominantly low affinity binding sites. Surprisingly, extracellular domain mutations of p75 that abolished N GF binding still generated high affinity binding with Trk A. These results indicate that the transmembrane and cytoplasmic domains of Trk A and p75 ar e responsible for high affinity site formation and suggest that p75 alters the conformation of Trk A to generate high affinity NGF binding.