Substitution of Met-38 to Ile in - synuclein found in two patients with amyotrophic lateral sclerosis induces aggregation into amyloid

alpha-, beta-, and gamma-Synuclein are intrinsically disordered proteins implicated in physiological processes in the nervous system of vertebrates. alpha-synuclein (alpha Syn) is the amyloido-genic protein associated with Parkinson's disease and certain other neurodegenerative disorders. Intensive research has focused on the mechanisms that cause alpha Syn to form amyloid structures, identifying its NAC region as being necessary and sufficient for amyloid assembly. Recent work has shown that a 7- residue sequence (P1) is necessary for alpha Syn amyloid formation. Although gamma-synuclein (gamma Syn) is 55% identical in sequence to alpha Syn and its pathological deposits are also observed in association with neurodegen-erative conditions, gamma Syn is resilient to amyloid formation in vitro. Here, we report a rare single nucleotide polymorphism (SNP) in the SNCG gene encoding gamma Syn, found in two patients with amyotrophic lateral sclerosis (ALS). The SNP results in the substi-tution of Met38 with Ile in the P1 region of the protein. These individuals also had a second, common and nonpathological, SNP in SNCG resulting in the substitution of Glu110 with Val. In vitro studies demonstrate that the Ile38 variant accelerates amyloid fibril assembly. Contrastingly, Val110 retards fibril assembly and mitigates the effect of Ile38. Substitution of residue 38 with Leu had little effect, while Val retards, and Ala increases the rate of amyloid formation. Ile38 gamma Syn also results in the formation of gamma Syn-containing inclusions in cells. The results show how a single point substitution can enhance amyloid formation of gamma Syn and highlight the P1 region in driving amyloid formation in another synuclein family member.