AN ENDOCYTOSED TGN38 CHIMERIC PROTEIN IS DELIVERED TO THE TGN AFTER TRAFFICKING THROUGH THE ENDOCYTIC RECYCLING COMPARTMENT IN CHO CELLS

To examine TGN38 trafficking from the cell surface to the TGN, CHO cel ls were stably transfected with a chimeric transmembrane protein, TacT GN38. We used fluorescent and I-125-labeled anti-Tac IgG and Fab fragm ents to follow TacTGN38's postendocytic trafficking. At steady-state, anti-Tac was mainly in the TGN, but shortly after endocytosis it was p redominantly in early endosomes, 11% of cellular TacTGN38 is on the pl asma membrane. Kinetic analysis of trafficking of antibodies bound to TacTGN38 showed that after short endocytic pulses, 80% of internalized anti-Tac returned to the cell surface (t(1/2) = 9 min), and the remai nder trafficked to the TGN. When longer filling pulses and chases were used to load anti-Tac into the TGN, it returned to the cell surface w ith a t(1/2) Of 46 min. Quantitative confocal microscopy analysis also showed that fluorescent anti-Tac fills the TGN with a 46-min t(1/2) U sing the measured rate constants in a simple kinetic model, we predict that 82% of TacTGN38 is in the TGN, and 7% is in endosomes. TacTGN38 leaves the TGN slowly, which accounts for its steady-state distributio n despite the inefficient targeting from the cell surface to the TGN.