Ring-like oligomers of Synaptotagmins and related C2 domain proteins.

Maria N Zanetti,Oscar D Bello,James E Rothman,Charles V Sindelar,Yiying Cai,Jeff Coleman,Shyam S Krishnakumar,Jing Wang

doi:10.7554/elife.17262

Abstract

We recently reported that the C2AB portion of Synaptotagmin 1 (Syt1) could self-assemble into Ca(2+)-sensitive ring-like oligomers on membranes, which could potentially regulate neurotransmitter release. Here we report that analogous ring-like oligomers assemble from the C2AB domains of other Syt isoforms (Syt2, Syt7, Syt9) as well as related C2 domain containing protein, Doc2B and extended Synaptotagmins (E-Syts). Evidently, circular oligomerization is a general and conserved structural aspect of many C2 domain proteins, including Synaptotagmins. Further, using electron microscopy combined with targeted mutations, we show that under physiologically relevant conditions, both the Syt1 ring assembly and its rapid disruption by Ca(2+) involve the well-established functional surfaces on the C2B domain that are important for synaptic transmission. Our data suggests that ring formation may be triggered at an early step in synaptic vesicle docking and positions Syt1 to synchronize neurotransmitter release to Ca(2+) influx.

Highlights

Synchronized rapid release of neurotransmitters at the synapse is a highly orchestrated cellular process
We analyzed the organization of membrane bound C2AB domains of other neuronal isoforms of Synaptotagmin (Syt2, Syt7 and Syt9) on lipid surface under Ca2+-free conditions by negative stain electron microscopy (EM)
In support of a functional role for the Synaptotagmin 1 (Syt1) ring-oligomers, we find that the molecular basis of the Syt1 ring oligomer assembly and its reversal are coupled to well-established mechanisms of Syt1 action

Summary

Introduction

Synchronized rapid release of neurotransmitters at the synapse is a highly orchestrated cellular process This involves maintaining a pool of synaptic vesicles (SV) containing neurotransmitters docked at the pre-synaptic membrane, ready to fuse and release their contents upon the influx of calcium ions (Ca2+) following an action potential, while preventing the spontaneous fusion of SVs in absence of the appropriate cue (Sudhof and Rothman, 2009; Jahn and Fasshauer, 2012; Sudhof, 2013; Rizo and Xu, 2015). Syt is needed for the initial stage of close docking of SVs to the plasma membrane (PM), requiring in particular the interaction of the polybasic region on C2B domain with the anionic lipid, phosphatidylinositol 4, 5-bisphosphate (PIP2)

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Results

Conclusion