Dynamic Formation of the Protein-Lipid Pre-fusion Complex

Area of Science:

• Neuroscience
• Biophysics
• Molecular Biology

Background:

• Synaptic vesicle (SV) fusion with the presynaptic membrane (PM) releases neurotransmitters, a process critical for neuronal communication.
• Synaptotagmin 1 (Syt1) acts as the primary calcium (Ca2+) sensor, initiating SV fusion.
• The precise mechanism by which Syt1 triggers fusion, particularly its interaction with the SNARE complex and Complexin (Cpx), remains under investigation.

Approach:

• Utilized all-atom molecular dynamics (MD) simulations to model the interactions within the Syt1-SNARE-Cpx complex at the SV and PM interface.
• Investigated the conformational dynamics of Syt1, SNARE, and Cpx during the fusion process in response to Ca2+ binding.
• Analyzed the lipid bilayer interactions and the formation of distinct protein-lipid complex states.

Key Points:

• Simulations identified a critical pre-fusion state where Ca2+-bound Syt1 domains penetrate the PM, facilitated by simultaneous Syt1 docking to the SNARE-Cpx bundle and PM.
• Direct interactions between Syt1 and Cpx were found to be essential for promoting the conformational transitions leading to the pre-fusion state.
• An alternative "dead-end" state was observed, characterized by tight Syt1 attachment to the PM without immersion, suggesting a potential disruption in the fusion pathway.

Conclusions:

• Developed an all-atom dynamic model elucidating the conformational transitions driving the formation of the pre-fusion PM-Syt1-SNARE-Cpx complex.
• Highlighted the indispensable role of Syt1-Cpx interactions in orchestrating Ca2+-dependent synaptic vesicle fusion.
• Provided mechanistic insights into how disruptions in these interactions can lead to non-productive "dead-end" states, impacting neurotransmitter release.