The C-terminus of α-Synuclein Regulates its Dynamic Cellular Internalization by Neurexin 1β

The aggregation of the disordered neuronal protein, alpha-Synuclein (alpha S), is the primary pathological feature of Parkinson's disease. Current hypotheses favor cell-to-cell spread of alpha S species as underlying disease progression, driving interest in identifying the molecular species and cellular processes involved in cellular internalization of alpha S. Prior work from our lab identified the chemically specific interaction between alpha S and the presynaptic adhesion protein neurexin-1 beta (N1 beta) to be capable of driving cellular internalization of both monomer and aggregated forms of alpha S. Here we explore the physical basis of N1 beta-driven internalization of alpha S. Specifically, we show that spontaneous internalization of alpha S by SH-SY5Y and HEK293 cells expressing N1 beta requires essentially all of the membrane-binding domain of alpha S; alpha S constructs truncated beyond residue 90 bind to N1 beta in the plasma membrane of HEK cells, but are not internalized. Interestingly, before internalization, alpha S and N1 beta codiffuse rapidly in the plasma membrane. alpha S constructs that are not internalized show very slow mobility themselves, as well as slow N1 beta diffusion. Finally, we find that truncated alpha S is capable of blocking internalization of full-length alpha S. Our results draw attention to the potential therapeutic value of blocking alpha S-N1 beta interactions.