CPLX3, also known as complexin 3, belongs to the complexin/synaphin family. As a SNARE-binding protein, complexin (CPX), can act either as a facilitator or as an inhibitor of membrane fusion, constituting a controversial dilemma. CPX acts sequentially on assembling SNAREpins, first facilitating zippering by nearly doubling the distance at which v- and t-SNAREs can engage and then clamping them into a half-zippered fusion-incompetent state. Specifically, the central helix of CPX allows SNAREs to form this intermediate energetic state at 9-15 nm but not when the bilayers are closer than 9 nm. Stabilizing the activated-clamped state at separations of less than 9 nm requires the accessory helix of CPX, which prevents membrane-proximal assembly of SNAREpins. CPLX3 binds to the SNARE core complex containing SNAP25, VAMP2 and STX1A.