Tomosyns attenuate SNARE assembly and synaptic depression by binding to VAMP2-containing template complexes

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Tomosyns are regulators of membrane fusion, particularly in synaptic vesicle exocytosis.
• Their proposed mechanism involves competing with synaptobrevin-2/VAMP2 for SNARE complex assembly.

Purpose of the Study:

• To investigate the precise mechanism by which tomosyns regulate membrane fusion and synaptic transmission.
• To challenge the prevailing model of tomosyn function in SNARE complex assembly.

Main Methods:

• Generation and analysis of a novel tomosyn-1/2 deficient mouse model.
• Rescue experiments using wild-type and mutated tomosyn-1m.
• Single-molecule force measurements to assess protein interactions.
• Structure-function analyses of tomosyn domains.

Main Results:

• Tomosyn-1/2 deficiency enhances synaptic vesicle fusion probability and synaptic strength.
• Tomosyn's SNARE motif cannot substitute for synaptobrevin-2/VAMP2 in forming essential SNARE assembly intermediates.
• Tomosyns bind to synaptobrevin-2/VAMP2-containing complexes and inhibit SNAP-25 association.
• The C-terminal polybasic region of tomosyn contributes to its inhibitory function.

Conclusions:

• Tomosyns do not compete with synaptobrevin-2/VAMP2 for initial SNARE complex formation.
• Tomosyns regulate synaptic transmission by preventing SNAP-25 binding to synaptobrevin-2/VAMP2-containing complexes.
• This mechanism limits initial synaptic strength and ensures equalization during sustained stimulation.