Key determinants of the dual clamp/activator function of Complexin

Area of Science:

• Neurobiology
• Cell Biology
• Molecular Mechanisms of Exocytosis

Background:

• Complexin proteins regulate the magnitude and kinetics of synchronized secretion.
• The precise molecular mechanisms by which complexins control exocytosis remain incompletely understood.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying complexin's role in regulating synchronized secretion.
• To investigate the distinct functions of the N-terminal domain (NTD) and C-terminal domain (CTD) of Complexin II (CpxII).

Main Methods:

• Functional analysis of Complexin II mutants in mouse chromaffin cells.
• Investigating the interaction of CpxII domains with SNARE proteins and synaptotagmin I.
• Utilizing calcium-binding mutants of synaptotagmin I to assess CpxII function.

Main Results:

• The hydrophobic face of the CpxII C-terminal amphipathic helix binds SNAREs, arresting the pre-fusion state and preventing premature secretion.
• An unrelated amphipathic helix can functionally substitute for the CpxII CTD.
• The CpxII NTD specifically interacts with synaptotagmin I (SytI) to promote synchronous fusion.
• CpxII expression rescues slow release kinetics in SytI mutants, while N-terminally truncated CpxII exacerbates delays.

Conclusions:

• Complexin II's C-terminal domain acts as a clamp, preventing spontaneous and premature vesicle fusion.
• Complexin II's N-terminal domain facilitates rapid, calcium-triggered synchronous release by cooperating with synaptotagmin I.
• These findings clarify the dual role of Complexin II in controlling both the arrest and acceleration of exocytosis.