SNAP-25 is required for a late postdocking step in Ca2+-dependent exocytosis

Area of Science:

• Neuroscience
• Cell Biology
• Biochemistry

Background:

• Ca2+-activated exocytosis of large dense core vesicles (LDCVs) is essential for neurotransmitter release.
• Botulinum neurotoxins (BoNTs) are potent neurotoxins that cleave proteins involved in exocytosis, including SNAP-25.
• Understanding the precise roles of SNAP-25 in membrane fusion is critical for neuroscience research.

Purpose of the Study:

• To investigate the differential effects of BoNT E and BoNT A on Ca2+-activated LDCV exocytosis.
• To determine the specific role of SNAP-25 in the late stages of LDCV fusion with the plasma membrane.

Main Methods:

• Reconstitution of Ca2+-activated LDCV exocytosis in mechanically permeabilized PC12 cells.
• Treatment with type E and type A botulinum neurotoxins (BoNTs).
• Analysis of detergent-extracted protein complexes and inhibition using C-terminal peptide antibodies.

Main Results:

• BoNT E completely inhibited Ca2+-activated LDCV exocytosis, while BoNT A had minimal effect, despite both cleaving SNAP-25.
• BoNT E caused greater destabilization of SNAP-25 containing protein complexes.
• The C-terminal region of SNAP-25 (Ile181-Gln197) was identified as essential for late post-docking fusion steps.

Conclusions:

• Plasma membrane-associated SNAP-25, particularly residues 181-197, is indispensable for Ca2+-regulated membrane fusion.
• This function of SNAP-25 occurs after LDCV docking and requires ATP.
• The differential inhibition by BoNT E and BoNT A highlights distinct functional domains within SNAP-25 for membrane fusion.