Covalent attachment of 125I-beta nerve growth factor to its receptors on sympathetic neurons

When 125I-beta nerve growth factor binds to sympathetic and sensory neurons, some labeled ligand is sequestered (becomes inaccessible to the external milieu) in a time- and energy-dependent manner. It would appear that the higher affinity receptor (type I) participates in this process to a greater extent than does the lower affinity receptor (type II) [ Olender and Stach , 1980; Olender et al., 1981]. A small portion of the sequestered 125I-beta nerve growth factor is found as part of a high molecular weight complex. When cells, which have been incubated with 125I-beta nerve growth factor, are solubilized with Triton X-100 and subjected to sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, a complex with an apparent molecular weight of approximately 240,000 is obtained. The formation of the covalent complex can be prevented by the prior addition of excess unlabeled beta nerve growth factor or sodium fluoride and dinitrophenol. The covalent 125I-beta nerve growth factor-receptor complex is dissociated in 50 mM dithiothreitol indicating that disulfide linkages are involved. At concentrations of beta nerve growth factor (3.8 X 10(-11) -3.8 X 10(-10) M) where maximal fiber outgrowth occurs in vitro, approximately 50-266 attomoles (0.3-1.6% of the type I receptors) of the covalent complex are formed per 10(7) nerve cells. These data suggest that a small portion of the 125I-beta nerve growth sequestered by sympathetic neurons becomes covalently attached to its receptor subsequent to its sequestration in a manner which appears to involve type I receptors.