Proximity ligation identifies the WPB interactome. Newly formed WPBs acquire the Rab3b-APEX2 or Rab27a-APEX2 fusion proteins which, upon addition of biotin, generate an active zone of biotinylation around the organelle, randomly labeling proteins that are associated with the granule or that are in its immediate vicinity. As the WPB makes its way through the cell it encounters (and interacts with) various subcellular organelles and intracellular membranes, a path that is illuminated through the identification of sets of proteins that are found on the endoplasmic reticulum (ER), Golgi, mitochondria, and endosomal compartments such as the late endosomes or multivesicular bodies (LE/MVB), recycling endosomes (RE), and early endosomes (EE). PM, plasma membrane.