Interaction of STXBP1 with SNARE proteins in endothelial cells. (A) Lysates of endothelial cells lentivirally transduced with mEGFP-STXBP1 or mEGFP were incubated with magnetic beads covalently coupled with sheep anti-GFP IgG or an equivalent amount of naive sheep IgG. Lysates (input) and coimmunoprecipitates (IP) were separated by SDS-PAGE and probed with rabbit anti-Slp4-a, anti-syntaxin-3, or mouse anti-GFP followed by incubation with IR dye–labeled donkey anti-rabbit or anti-mouse IgG. Slp4 a and syntaxin-3 bound mEGFP-STXBP1 but not mEGFP. (B,C) Endothelial lysates were incubated with magnetic beads covalently coupled with mouse anti-STXBP1 IgG or an equivalent amount of naive mouse IgG (B) or with rabbit anti-syntaxin-3 IgG or an equivalent amount of naive rabbit IgG (C). Lysates (input) and co- IP were separated by SDS-PAGE and probed with rabbit anti-STXBP1, anti-syntaxin-2, anti-syntaxin-3, or mouse anti-syntaxin-4 followed by incubation with IR dye–labeled donkey anti-rabbit or anti-mouse IgG. Syntaxin-2 and syntaxin-3 but not syntaxin-4 coimmunoprecipitated with STXBP1 (B). Immunoprecipitation of syntaxin-3 coprecipitated STXBP1 but not syntaxin-2 (C).