The Lipid-binding Domain of Wild Type and Mutant α-Synuclein

Abstract

␣-Synuclein (␣S) is linked to Parkinson disease through its deposition in an amyloid fibril form within Lewy Body deposits, and by the existence of three ␣S point mutations that lead to early onset autosomal dominant Parkinsonism. The normal function of ␣S is thought to be linked to the ability of the protein to bind to the surface of synaptic vesicles. Upon binding to vesicles, ␣S undergoes a structural reorganization from a dynamic and disordered ensemble to a conformation consisting of a long extended helix. In the presence of small spheroidal detergent micelles, however, this extended helix conformation can convert into a broken helix state, in which a region near the middle of the helix unwinds to form a linker between the two resulting separated helices. Membrane-bound conformations of ␣S likely mediate the function of the protein, but may also play a role in the aggregation and toxicity of the protein. Here we have undertaken a study of the effects of the three known PD-linked mutations on the detergent-and membrane-bound conformations of ␣S, as well as factors that govern the transition of the protein between the extended helix and broken helix states. Using pulsed dipolar ESR measurements of distances up to 8.7 nm, we show that all three PD-linked ␣S mutants retain the ability to transition from the broken helix to the extended helix conformation. In addition, we find that the ratio of protein to detergent, rather than just the absolute detergent concentration, determines whether the protein adopts the broken or extended helix conformation.Since its initial discovery as a protein enriched in the electric organ of the Pacific electric ray, Torpedo californica (1), the protein ␣-synuclein has been progressively associated with a role in neurodegeneration in humans, starting with its identification as the precursor protein of a non-amyloid-␤ peptide component of amyloid plaques (the so-called NAC peptide) (2) and culminating in its discovery as the first gene product to be linked to familial Parkinson disease (3) and its contemporaneous identification as the primary component of the fibrillary aggregates found in the hallmark Lewy body deposits associated with Parkinson (4).Subsequently, a great deal of effort has been made to clarify and understand the mechanisms by which ␣S causes neuronal degeneration, as well as to understand the normal functional roles of ␣S and how these are related to its role in disease (5, 6). Although aggregation of ␣S remains a key focus in these efforts, it remains possible that other, or additional, effects beyond the presence of various aggregated species of ␣S may be involved in the etiology of synuclein-associated PD. A key piece of the puzzle that remains unanswered is which of the various forms of ␣S that have been observed in vitro are involved in mediating the toxicity of the protein in vivo. In particular, although ␣S is highly disordered when isolated in solution in vitro (7-9) it is not clear to what extent this intrinsically disordered state is populated in vivo. Be...

Dynamic structural flexibility of α-synuclein

“…These helices are stabilized by interaction with a variety of phospholipid bilayers, though α-synuclein interacts preferentially with membranes of high curvature and an abundance of acidic phospholipids, properties consistent with those of synaptic vesicles (Davidson et al, 1998; Zhu et al, 2003). Upon interaction with membranes of low curvature α-synuclein adopts a distinct secondary structure characterized by a single extended helix that includes both previously described helical domains and the linker region (amino acids 38–44) (Ferreon et al, 2009; Georgieva et al, 2010; Trexler and Rhoades, 2009). All known mutations associated with familial PD (A30P, E46K, H50Q, G51D, A53E, and A53T) are found in the N-terminal domain (Krüger et al, 2008; Lesage et al, 2013; Pasanen et al, 2014; Polymeropoulos et al, 1997; Proukakis et al, 2013; Zarranz et al, 2004).…”

Efficient Exploration of Membrane-Associated Phenomena at Atomic Resolution

“…Experiments have identified two predominant states for monomeric α S bound to anionic membranes, a horseshoe-like broken helix identified with NMR and EPR (Ulmer et al, 2005; Drescher et al, 2008; Rao et al, 2010) where a kink brings together the two helical segments, and an extended kink-free helix conformation without a kink postulated by ESR and DEER experiments (Jao et al, 2004; Georgieva et al, 2008; Jao et al, 2008; Trexler and Rhoades, 2009). It has been suggested that the extended and broken-helix conformations could interconvert (Georgieva et al, 2010; Robotta et al, 2011; Lokappa and Ulmer, 2011), potentially in a lipid-dependent fashion (Borbat et al, 2006; Middleton and Rhoades, 2010). Outside of coarse-grained simulations (Braun et al, 2012; Braun and Sachs, 2015), this conformational diversity had not been sampled, and presented a prime candidate for using the HMMM model to investigate the specific lipid interactions that determine the membrane-bound structure of α S.…”

Effects of impaired membrane interactions on α-synuclein aggregation and neurotoxicity

“…1A). The protein adopts a predominantly broken or continuous α-helical structure upon binding high- or low-curvature membranes, respectively (Chandra et al, 2003; Ferreon et al, 2009; Georgieva et al, 2010; Trexler and Rhoades, 2009; Ulmer et al, 2005). Interactions involving an N-terminal segment spanning residues 1–25 are critical for membrane binding and for the adoption of an α-helical structure (Bartels et al, 2010).…”