Alteration of NH2-terminal residues of nerve growth factor affects activity and Trk binding without affecting stability or conformation.

The role of the NH2-terminal region of nerve growth factor (NGF) was studied with an NGF delta 9/13 deletion mutant, overexpressed in a baculovirus system, and mouse NGF truncated at Met-9 by cleavage with CNBr (des-(1-9)-NGF). Structural studies have been performed on the purified proteins, in addition to biological activity assessment, in order to determine effects of such modifications on global conformation and stability. The activity of NGF delta 9/13 was reduced below detectable levels, and the activity of the des-(1-9)-NGF form was decreased by at least a 50-fold in a PC12 bioassay. Competitive binding of NGF delta 9/13 to low affinity receptors on PC12 cells was not impaired; however, the mutant was not capable of competing for the cold chase-stable, high affinity binding of NGF to the cells. The binding of NGF delta 9/13 to Sf21 cells ectopically expressing the TrkA NGF receptor was also abolished. Thus, deletion of residues 9-13 significantly altered the binding affinity for the high affinity receptors on PC12 cells and for the TrkA receptor, but not for the low affinity receptor. Neither the secondary structure, determined by circular dichroism, nor the conformational stability determined by equilibrium denaturation of NGF delta 9/13 was altered as compared with wild type NGF. Slight conformational and stability perturbations of des-(1-9)-NGF were revealed by the same analysis; however, these changes were found to reflect the influence of the formic acid treatment, not the truncation of 9 residues. Our results support the conclusion that the NH2-terminal domain encompassing residues 1-9 and 9-13 is essential for maintaining the binding capability of NGF for high affinity TrkA receptors. Moreover, conformational and stability data show that the functional results of these modifications of the NH2-terminal region are directly due to receptor binding and not to secondary effects of improper folding or other indirect structural changes.