The Effects of Transmembrane Sequence and Dimerization on Cleavage of the p75 Neurotrophin Receptor by γ-Secretase

Cleavage of transmembrane receptors by γ-secretase is the final step in the process of regulated intramembrane proteolysis (RIP) and has a significant impact on receptor function. Although relatively little is known about the molecular mechanism of γ-secretase enzymatic activity, it is becoming clear that substrate dimerization and/or the α-helical structure of the substrate can regulate the site and rate of γ-secretase activity. Here we show that the transmembrane domain of the pan-neurotrophin receptor p75NTR, best known for regulating neuronal death, is sufficient for its homodimerization. Although the p75NTR ligands NGF and pro-NGF do not induce homerdimerization or RIP, homodimers of p75NTR are γ-secretase substrates. However, dimerization is not a requirement for p75NTR cleavage, suggesting that γ-secretase has the ability to recognize and cleave each receptor molecule independently. The transmembrane cysteine 257, which mediates covalent p75NTR interactions, is not crucial for homodimerization, but this residue is required for normal rates of γ-secretase cleavage. Similarly, mutation of the residues alanine 262 and glycine 266 of an AXXXG dimerization motif flanking the γ-secretase cleavage site within the p75NTR transmembrane domain alters the orientation of the domain and inhibits γ-secretase cleavage of p75NTR. Nonetheless, heteromer interactions of p75NTR with TrkA increase full-length p75NTR homodimerization, which in turn potentiates the rate of γ-cleavage following TrkA activation independently of rates of α-cleavage. These results provide support for the idea that the helical structure of the p75NTR transmembrane domain, which may be affected by co-receptor interactions, is a key element in γ-secretase-catalyzed cleavage.