Identification of candidate La sorting factors by unbiased proximity labeling. (A) Schematic of the proximity labeling strategy to identify La sorting factors. APEX2 or La-APEX2 proximal proteins were labeled with biotin upon addition of biotin phenol and H₂O₂. The cells were then fractionated to obtain a 20,000 × g membrane pellet, and the biotinylated proteins were purified and identified by mass spectrometry. (B) GO analysis for the subcellular localization of proteins that were enriched more than twofold in La-APEX2 compared with APEX2. GO terms for late endosome, extracellular exosome, “extracellular organelle,” and “extracellular space” are colored yellow. (C) Table listing proteins identified in the APEX2 proximity labeling experiment. The ratio of peptides between La-APEX2 and APEX2 for each hit is listed.