INTERACTION WITH TRKA IMMOBILIZES GP75 IN THE HIGH-AFFINITY NERVE GROWTH-FACTOR RECEPTOR COMPLEX

It has been proposed that the high affinity nerve growth factor (NGF) receptor required for NGF response is a complex of two receptor protei ns, gp75 and the tyrosine kinase TrkA, but direct biochemical or bioph ysical evidence has been lacking, We have previously shown using fluor escence recovery after photobleaching that gp75 is highly mobile on NG F-nonresponsive cells, but relatively immobile on NGF-responsive cells . In this report, we show that a physical interaction with TrkA causes gp75 immobilization We found that gp75 is relatively mobile on TrkA n egative nnr5 cells, a PC12 variant which is nonresponsive to NGF. In c ontrast, on T14 nnr5 cells (which bear a TrkA expression vector) gp75 is relatively immobile, Similarly, using baculoviruses to express gp75 and TrkA on Sf9 insect cells, we found that TrkA immobilizes gp75 mol ecules, The related receptor, TrkB, caused a more modest immobilizatio n of gp75. Immobilization was found to require intact TrkA kinase and gp75 cytoplasmic domains, paralleling the requirements of high affinit y binding of NGF, Analysis of gp75 diffusion coefficients indicates th at mutated gp75 and TrkA molecules may form a complex, even in the abs ence of the ability to bind NGF with high affinity.