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Fig. S4 AB Complex detection method resolves cytoplasmic protein pools. Proteins that participate in the secretory pathway leave the ER via budding vesicles labeled by Sec23, a component of the COPII machinery (A). Trappc3 mediates fusion of Sec23 vesicles that form the ER-Golgi intermediate compartment (ERGIC) (B). After fusion, Sec23 and other COPII components return to the ER. Vesicles budded from the ERGIC then fuse with the membranes of the Golgi apparatus, labeled by GM130 (C). GM130 and Sec23 do not co-localize despite both having broad cytoplasmic labeling (C). This indicates that colocalization signals detected with the AB complex method are specific and not due to cross-reactivity or generalized labeling of the cell body. Some cell bodies are highlighted in white. Scale bar: 5µm.