5 Jul 2018

Abstract

Neurotransmitter containing synaptic vesicles (SVs) form tight clusters at synapses. These clusters act as a reservoir from which SVs are drawn for exocytosis during sustained activity. Several components associated with synaptic vesicles likely to help forming such clusters have been reported, including synapsin. Here we found that synapsin can form a distinct liquid phase in an aqueous environment. Other scaffolding proteins could co-assemble into this condensate, but were not necessary for its formation. Importantly, the synapsin phase could capture small lipid vesicles. The synapsin phase rapidly disassembled upon phosphorylation by calcium/calmodulin-dependent protein kinase II (CaMKII), mimicking the dispersion of synapsin 1 that occurs at presynaptic sites upon stimulation. Thus, principles of liquid-liquid phase separation may apply to the clustering of SVs at synapses.

[Image]

Synapsin 1 drives phase separation of SH3 domain containing binding partners.

(A and B) Full-length synapsin 1 (10 μM) and either Grb2 (10 μM), or the SH3 domain containing region of intersectin (10 μM) form droplets under physiological conditions. Top: domain organization of the proteins. Bottom: fluorescence images of the protein mixtures at 30 and 45 min. (C and D) Fluorescence images of the solution immediately (within one min) after mixing of the two proteins in the presence of the crowding reagent (3% PEG 8,000). (E and F) Fluorescence recovery of synapsin 1 after photobleaching a region within a synapsin 1-Grb2 droplet, or a synapsin 1-(SH3)5-intersectindroplet (see insets). Error bars represent s.e.m., red is the fit with a hyperbolic function.