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Lysosome Associated Membrane Protein 1 (Lamp1) Traffics Directly from the TGN to Early Endosomes

The precise trafficking routes followed by newly synthesized lysosomal membrane proteins after exit from the Golgi are unclear. To study these events we created a novel chimera (YAL) having a lumenal domain comprising two tyrosine sulfation motifs fused to avidin, and the transmembrane and cytoplasmic domains of lysosome associated membrane protein 1 (Lamp1). The newly synthesized protein rapidly transited from the trans‐ Golgi Network (TGN) to lysosomes (t1/2∼ 30 min after a lag of 15–20 min). However, labeled chimera was captured by biotinylated probes endocytosed for only 5 min, indicating that the initial site of entry into the endocytic pathway was early endosomes. Capture required export of YAL from the TGN, and endocytosis of the biotinylated reagent, and was essentially quantitative within 2 h of chase, suggesting that all molecules were following an identical route. There was no evidence of YAL trafficking via the cell surface. Fusion of TGN‐derived vesicles with 5 min endosomes could be recapitulated in vitro, but neither late endosomes nor lysosomes could serve as acceptor compartments. This suggests that contrary to previous conclusions, most if not all newly synthesized Lamp1 traffics from the TGN to early endosomes prior to delivery to late endosomes and lysosomes.