Fig. 1. Localization and possible function of cathepsin B in rat thyroid epithelial cells. A confocal fluorescence micrograph of a segment of a rat thyroid follicle (A), and a conventional fluorescence micrograph of formaldehyde-fixed (B) and Triton-X-100-permeabilized (C) FRT cells after immunolabeling with rabbit anti-rat cathepsin B antibodies. Cathepsin B was recognized within vesicles resembling endosomes or lysosomes (arrows) or in association with the plasma membrane (arrowheads) of rat thyroid epithelial cells in situ (A) and in vitro (B,C). In vitro degradation of Tg with plasma-membrane-associated proteases of FRT cells without (blue curve) or after inhibition of cysteine proteases by E64 (red curve) and identification of liberated thyroid hormones (eluting positions marked by green arrows) by reversed phase chromatography (D). Note that preincubation with E64 completely abolished liberation of T₃ and T₄ by proteases associated with plasma membrane preparations (D, cf. red with blue curve), indicating the contribution of cell-surface-associated cysteine proteases in Tg processing for thyroid hormone liberation. N, nuclei; stars, Golgi cisternae. Bars, 10 µm (A), 50 µm (B,C).