Figure 3. Proposed mechanisms for facilitation.

Illustration of the proposed mechanisms for facilitation. Voltage-gated Ca²⁺ channels (VGCC) open during the first action potential and allow Ca²⁺ influx (I_Ca). During the first action potential (AP) Ca²⁺ influx fails to fully activate Syt1 and trigger fusion.

A) The residual Ca²⁺ hypothesis: Ca_res from the first AP adds to Ca_local during the second AP. Because the fusion sensor binds Ca²⁺ in a supralinear manner, a small increase in the Ca²⁺ signal drives a large increase in p.

B) Residual Ca²⁺ binds to a facilitation sensor: A high-affinity sensor, distinct from Syt1, binds to Ca_res in between APs and raises p by interacting with the fusion machinery.

C) Spike broadening: Inactivation of K⁺ channels during the first AP broadens the waveform of the second AP, prolonging the time-course and total amount of I_Ca.

D) Calcium current facilitation: Ca²⁺ from the first AP binds to NCS-1 or CaM, which interact with the IQ-like motif (IM) on the intracellular C-terminus of P/Q channels to increase I_Ca during the second AP.

E) Buffer saturation: During the first AP there is enough unbound Ca²⁺ buffer to capture incoming ions and decrease the Ca²⁺ signal sensed by Syt1 (Ca_local). During the second AP the buffer becomes saturated and cannot constrain Ca_local.