Figure 213

Diagram showing two pathways for exocytosis. Newly synthesized proteins are synthesized in the rER. After their initial posttranslational modification, they are delivered in COP-ll-coated vesicles to the Golgi apparatus. After additional modification in the Golgi apparatus, sorting, and packaging, the final secretory product is transported to the plasma membrane in vesicles that form from the frans-Golgi network (TGN). Two distinct pathways are recognized. Blue arrows indicate the constitutive pathway in which proteins leave the cell immediately after their synthesis. In cells using this pathway, little secretory product accumulates, and thus few secretoiy vesicles are present in the cytoplasm. Red arrows indicate the regulated secretoiy pathway in which protein secretion is regulated by hormonal or neural stimuli. In cells using this pathway, such as the pancreatic acinar cells in Figure 2.12, secretoiy proteins are concentrated and transiently stored in secretory vesicles within the cytoplasm. After appropriate stimulation, the secretory vesicles fuse with the plasma membrane and discharge their contents.

cell and with the Golgi apparatus. Coated vesicles formed at the plasma membrane fuse only with early en-dosomes because of their expression of specific surface receptors. The receptor remains a resident component of the early endosomal membrane.

• In the maturation model, early endosomes are formed de novo from endocytotic vesicles originating from the plasma membrane. Therefore, the composition of the early endosomal membrane changes progressively as some components are recycled between the cell surface and the Golgi apparatus. This maturation process leads to formation of late endosomes and then to lysosomes. Specific receptors present on early endosomes, e.g., for coated vesicles, are removed by recycling, degradation, or inactivation as this compartment matures.

Both models actually complement rather than contradict each other in describing, identifying, and studying the pathways of internalized molecules.

Endosomes destined to become lysosomes receive newly synthesized lysosomal enzymes that are targeted via the mannose-6-phosphate receptor

Some endosomes also communicate with the vesicular transport system of the rER. This pathway provides constant delivery of newly synthesized lysosomal enzymes, or hydrolases. Hydrolases are synthesized in the rER and are glycosylated. The protein then folds in a specific way so that a signal patch is formed and exposed on the surface of the molecule. This recognition signal is created when specific amino acids are brought into close proximity by the three-dimensional folding of the protein. The sig-

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