Fig. 9. Transport pathways of lysosomal enzymes in rat thyroid epithelial cells. Schematic drawing summarizing the results on lysosomal enzyme trafficking in FRT or FRTL-5 cells. The occurrence of mature cathepsin B at the surface of thyroid epithelial cells might be explained by extracellular processing of secreted procathepsin B (light green) to the mature enzyme (dark green, left portion, crossed arrows). This report shows that intralysosomal processing was the prerequisite for the retrograde transport of mature cathepsin B from lysosomes to the apical plasma membrane and its subsequent secretion into the extracellular space (dark green, right portion, bold arrow). Extracellularly occurring CB-EGFP was resistant to proteolytic degradation for long time intervals of up to two days, suggesting the stability of secreted lysosomal enzymes, thus explaining their function in the extracellular proteolysis of Tg at the surface of thyroid epithelial cells.