α-Synuclein (α-Syn) is an abundant presynaptic protein of unknown function, which has been implicated in the pathogenesis of Parkinson’s disease. α-Syn has been suggested to play a role in lipid transport and synaptogenesis, and growing evidence suggests that α-Syn interactions with cellular membranes are physiologically important. In the current study, we demonstrate that the familial Parkinson’s disease-linked A30P mutant α-Syn is defective in binding to phospholipid vesicles in vitro as determined by vesicle ultracentrifugation, circular dichroism spectroscopy, and low-angle X-ray diffraction. Interestingly, our data also suggest that α-Syn may bind to the lipid vesicles as a dimer, which suggest that this species could be a physiologically relevant and functional entity. In contrast, the naturally occurring murine A53T substitution, which is also linked to Parkinson’s disease, displayed a normal membrane-binding activity that was comparable to wild-type α-Syn. A double mutant A53T/A30P α-Syn showed defective membrane binding similar to the A30P protein, indicating that the proline mutation is dominant in terms of impairing the membrane-binding activity. With these observations, we suggest that the A53T and A30P mutants may have different physiological consequences in vivo and could possibly contribute to early onset Parkinson’s disease via unique mechanisms.