Identification of a Linear Epitope in Sortilin That Partakes in Pro-neurotrophin Binding *

Sortilin acts as a cell surface receptor for pro-neurotrophins (pro-NT) that upon complex formation with the p75 neurotrophin receptor (p75 NTR ) is able to signal neuronal cell death. Here we screened a sortilin peptide library comprising 16-mer overlapping sequences for binding of the pro-domains of nerve growth factor and brain-derived neurotrophic factor. We find that a linear surface-exposed sequence, 163 RIFRSSDFAKNF 174, constitutes an important pro-NT binding epitope in sortilin. Systematic mutational analysis revealed residues Arg 163, Phe 165, Arg 166, and Phe 170 to be critical for the interaction. Expression of a sortilin mutant in which these four amino acids were substituted by alanines disrupted pro-NT binding without affecting receptor heterodimerization with p75 NTR or binding of ligands that selectively engages the centrally located tunnel in the β-propeller of sortilin. We furthermore demonstrate that a peptide comprising the ligand-binding epitope can prevent pro-NT-induced apoptosis in RN22 schwannoma cells.