Figure 6.

Model for Ca²⁺ cooperativity of synaptic transmission. Under resting conditions, wild-type (WT) flies have the full complement of SNARE proteins and SNARE complexes exist in a preformed “loosely” associated state (green). Upon the admittance of Ca²⁺, SNARE complexes make the transition from the loose state to a “tight” state (purple). Cooperativity arises when the formation of a tight SNARE complex increases the probability that neighboring loose complexes will form tight complexes in a Ca²⁺-dependent manner (red). If there is a reduced-level SNARE protein (Mutant) when a complex makes the transition from loose to tight, there are fewer neighboring complexes that can be influenced by it, and thus cooperativity is reduced.