Oligomers of Parkinson’s disease-related alpha-synuclein mutants have similar structures but distinctive membrane permeabilization properties

Single-amino acid mutations in the human alpha-synuclein (alpha S) protein are related to early onset Parkinson's disease (PD). In addition to the well-known A30P, A53T, and E46K mutants, recently a number of new familial disease-related alpha S mutations have been discovered. How these mutations affect the putative physiological function of alpha S and the disease pathology is still unknown. Here we focus on the H50Q and G51D familial mutants and show that like wild-type alpha S, H50Q and G51D monomers bind to negatively charged membranes, form soluble partially folded oligomers with an aggregation number of similar to 30 monomers under specific conditions, and can aggregate into amyloid fibrils. We systematically studied the ability of these isolated oligomers to permeabilize membranes composed of anionic phospholipids (DOPG) and membranes mimicking the mitochondrial phospholipid composition (CL:POPE:POPC) using a calcein release assay. Small-angle X-ray scattering studies of isolated oligomers show that oligomers formed from wild-type alpha S and the A30P, E46K, HS0Q G51D, and A53T disease-related mutants are composed of a similar number of monomers. However, although the binding affinity of the monomeric protein and the aggregation number of the oligomers formed under our specific protocol are comparable for wild-type alpha S and HSOQ and G51D alpha S, GS1D oligomers cannot disrupt negatively charged and physiologically relevant model membranes. Replacement of the membrane-immersed glycine with a negatively charged aspartic acid at position 51 apparently abrogates membrane destabilization, whereas a mutation in the proximal but solvent-exposed part of the membrane-bound alpha-helix such as that found in the HS0Q mutant has little effect on the bilayer disrupting properties of oligomers.

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