Surface binding, uptake and fate of cationic ferritin in a steroid producing ovarian cell

Ovarian granulosa cells (GC) were exposed to cationic ferritin (CF) in an effort to determine the binding, intracellular fate of endocytosed negatively charged plasma membrane. Following labeling at zero degrees or after pre-fixation, CF accumulated in patches over the cell surface. Exposure to methylamine (MA) resulted in an even distribution of CF over the GC surface. Endocytosis occurred in non-clathrin coated regions of the GC surface and CF was subsequently observed in a variety of smooth surfaced vesicles. Following a 60 min exposure to CF many of the CF containing vesicles appeared to fuse with each other forming larger vesicles. Numerous examples of small CF containing vesicles surrounding large CF containing vesicles were observed. Also observed at 60 min were CF containing multivesicular and vesicular bodies. Tubular evaginations of the large vesicular structures were often observed; some containing CF. Acid phosphatase activity was observed in multivesicular bodies and the large CF filled vesicles. CF-containing vesicles were also observed in the Golgi region, but CF was never observed in the saccules of this organelle. Our study suggests that endocytosed CF does not pass through the Golgi complex. Many of the internalized vesicles become associated with the lysosomal system. Since GC's secrete progesterone in culture, these observations may indicate that membrane recycling in steroid secreting cells differs from protein secreting cells.