Nerve growth factor receptors: analysis of the interaction of ßNGF with membranes of chick embryo dorsal root ganglia

Abstract

The binding of the beta subunit of Nerve Growth Factor (beta NGF) to membrane preparations of 8-day chick embryo dorsal root ganglia (DRG) has been investigated under conditions similar to those used to study the binding of beta NGF to intact single cell dissociates of DRG. The equilibrium binding data reveal heterogeneityy of binding that is more complex than that seen with intact cells. Binding is not saturable up to 125I beta NGF concentrations of 10(-8) M. Steady-state and kinetic binding data show two sites with dissociation constants similar to those found on DRG cells. In addition, displacement data reveal a binding component with lower affinity (Kd = 10(-6) M) which is not found on intact cells. As with intact cells, the difference in the affinities of the two high affinity sites has been shown to be due to different rate constants of dissociation. The kinetics of dissociation of NGF are slower with membranes than with cells, and dissociation characteristics of 125I beta NGF change with increasing time of exposure to membranes. Degradation of 125I beta NGF during incubation with membranes is minimal and does not complicate the analysis of steady-state binding. Insulin does not bind to either of the two high affinity sites. Heterogeneity of the 125I beta NGF preparation and cooperativity of binding as a cause for the heterogeneity of the binding of NGF has been ruled out. Although there was an apparent increase in the rate of dissociation of 125I beta NGF in the presence of unlabelled NGF, a finding previously interpreted as evidence for negative cooperativity, this was shown to be independent of receptor site occupancy by NGF, and in part due to isotopic dilution within a diffusion barrier around the membranes.