Dynamic binding of PH-GFP and PI-4,5-P₂ and postfusion labeling of the secretory granule membrane. PH-GFP reversibly binds to PI-4,5-P₂. The stable labeling of the fused granule membrane by PH-GFP probably reflects diffusion of PI-4,5-P₂, bound or unbound to PH-GFP, with simultaneous association and dissociation of PH-GFP and PI-4,5-P₂. The left and right side of the fusion pore depict diffusion of PH-GFP/PI-4,5-P₂ or unbound PI-4,5-P₂, respectively. De novo synthesis of PI-4,5-P₂ on the fused granule membrane (see text) would also enhance PH-GFP binding.