SNAP receptor activity of platelet membrane extract. Detergent solubilized membrane extract from human platelets (HPE) was tested for SNARE activity using the assay of Wilson et al.10 In the complete reaction, radiolabeled α-SNAP was incubated for 2 hours at 4°C with NSFmyc (12 μg), HPE (40 μg), and beads with a covalently coupled anti-myc antibody. The buffer contained 0.5 mmol/L ATP, 2 mmol/L EDTA, and 1% Triton X-100. After incubation, the beads were captured on a glass fiber filter and the [³⁵S] α-SNAP associated with the anti-myc antibody beads was determined by scintillation counting. To determine the specificity of the interaction, unlabeled α-(63 μg, + α-SNAP ) and γ- (5 μg, + γ-SNAP ) SNAP was added to the initial incubation. To determine the effect of ATP hydrolysis, 6 mmol/L MgCl₂ (+MgCl₂ ) was included. SNARE activity is defined as that which induced the association of [³⁵S] α-SNAP with NSF. The data presented is representative of three separate experiments.