Structural role of compensatory amino acid replacements in the α-synuclein protein

A subset of familial Parkinson's disease (PD) cases is associated with the presence of disease-causing point mutations in human α-synuclein [huAS(wt)], including A53T. Surprisingly, the human neurotoxic amino acid 53T is present in non-primate, wild-type sequences of α-synucleins, including that expressed by mice [mAS(wt)]. Because huAS(A53T) causes neurodegeneration when expressed in rodents, the amino acid changes between the wild-type human protein [huAS(wt)] and mAS(wt) might act as intramolecular suppressors of A53T toxicity in the mouse protein, restoring its physiological structure and function. The lack of structural information for mAS(wt) in aqueous solution has prompted us to conduct a comparative molecular dynamics study of huAS(wt), huAS(A53T), and mAS(wt) in water at 300 K. The calculations are based on an ensemble of nuclear magnetic resonance-derived huAS(wt) structures. huAS(A53T) turns out to be more flexible and less compact than huAS(wt). Its central (NAC) region, involved in fibril formation by the protein, is more solvent-exposed than that of the wild-type protein, in agreement with nuclear magnetic resonance data. The compactness of mAS(wt) is similar to that of the human protein. In addition, its NAC region is less solvent-exposed and more rigid than that of huAS(A53T). All of these features may be caused by an increase in the level of intramolecular interactions on passing from huAS(A53T) to mAS(wt). We conclude that the presence of "compensatory replacements" in the mouse protein causes a significant change in the protein relative to huAS(A53T), restoring features not too dissimilar to those of the human protein.