Vesicle docking and fusion at the presynaptic terminal requires:

Answer: SNARE proteins (synaptobrevin on vesicle, syntaxin and SNAP-25 on plasma membrane) forming a complex when Ca 2+ triggers synaptotagmin.

• A Calcium ions alone, requiring little additional dedicated protein fusion machinery at the presynaptic membrane region as frequently described
• B SNARE proteins (synaptobrevin on vesicle, syntaxin and SNAP-25 on plasma membrane) forming a complex when Ca²⁺ triggers synaptotagmin
• C ATP hydrolysis alone, occurring largely independent of any incoming calcium signal at the synaptic terminal in most textbook accounts
• D Calmodulin protein acting largely alone, without much involvement of a SNARE protein complex at the membrane during normal conditions

Structure of a neuron. Image: LadyofHats (Mariana Ruiz), Public Domain, via Wikimedia Commons.