γ-1-syntrophin mediates trafficking of γ-enolase towards the plasma membrane and enhances its neurotrophic activity.

Syntrophins are scaffold proteins that can bind several signaling molecules and localize them to the plasma membrane. We demonstrate here that in neuroblastoma SH-SY5Y cells, brain-specific γ1-syntrophin binds the neurotrophic factor γ-enolase through its PDZ domain, and translocates it to the plasma membrane, as shown by immunoprecipitation, surface plasmon resonance, fluorescence colocalization and flow cytometry. Extensive colocalization of γ1-syntrophin and γ-enolase was observed in neurite growth cones in differentiated SH-SY5Y cells. Silencing of the γ1-syntrophin gene by RNA interference significantly reduced the re-distribution of γ-enolase to the plasma membrane and impaired its neurotrophic effects. We demonstrated that an intact C-terminal end of γ-enolase is essential for its γ1-syntrophin-assisted trafficking. The cleavage of two amino acids at the C-terminal end of γ-enolase by the carboxypeptidase cathepsin X prevents binding with the γ1-syntrophin PDZ domain. Collectively, these data demonstrate that γ1-syntrophin participates in γ-enolase translocation towards the plasma membrane, a pre-requisite for its neurotrophic activity. By disrupting this γ1-syntrophin-guided subcellular distribution, cathepsin X reduces γ-enolase-induced neurotrophic signaling.