Not surprisingly, fluorescence recovery after photobleaching in a C. elegans this website model of inclusion formation by synuclein in muscle has shown a substantial immobile fraction ( van Ham et al., 2008). The results in neurons with more physiological levels of expression thus indicate that synuclein
interacts weakly with elements of the nerve terminal. Despite its weak interaction with cellular membranes, synuclein nonetheless recovers more slowly after photobleaching than GFP (Fortin et al., 2004), and the N-terminal membrane-binding domain of synuclein seems likely to mediate the interaction. The A30P mutation associated with familial PD in fact disrupts both the association of synuclein with brain membranes and the presynaptic location of synuclein in cultured neurons and accelerates the rate of recovery after photobleaching to that of GFP (Fortin et al., 2004 and Jensen et al., 1998). The A30P mutation also impairs the interaction of purified, recombinant synuclein with artificial membranes (Jo et al., 2002). Although less dramatic in vitro than in cells, the effect of the A30P mutation strongly supports a role for membrane binding
by the N terminus in presynaptic localization. How then does synuclein localize specifically to presynaptic boutons rather than other cell membranes? Acidic headgroups are found on the cytoplasmic leaflet of many intracellular membranes, but synuclein has a preference for membranes with high curvature Neratinib manufacturer (Jensen et al., 2011 and Middleton and Rhoades, 2010), and synaptic vesicles are among the smallest biological membranes described. Rolziracetam Consistent with this, synuclein disperses from presynaptic boutons with stimulation (Fortin et al., 2005), suggesting that it dissociates from the membrane upon delivery to the relatively flat plasma membrane by synaptic vesicle exocytosis. What confers the specificity of synuclein
for membranes with high curvature? Interestingly, the hydrophobic face of the N-terminal α-helix contains a series of threonines at position 3 in the repeat (Figure 1). Although this polar residue might be expected to disrupt hydrophobic interactions with the membrane, threonine is in fact less polar than serine, and the precise positioning of this residue in repeats 2–5 and 7 is highly conserved among all synuclein isoforms. It is therefore possible that threonine at these positions weakens the interaction of synuclein with membranes precisely so that it can acquire specificity for high curvature. To test this possibility, the threonines were replaced by large, nonpolar residues (leucine and phenylalanine) and the recombinant mutant protein indeed loses its specificity for both acidic membranes and small vesicles (Pranke et al., 2011). When expressed in yeast, the mutant also localizes to the plasma membrane rather than to intracellular vesicles, consistent with stronger membrane interaction interfering with the preference for high curvature.