Intracellular Trafficking; The key event in exocytosis

The key event in exocytosis is translocation of newly synthesized protein into the cisternal space of the rough ER (signal hypothesis). Proteins and lipids reach the Golgi apparatus by vesicular transport. Using carbohydratesorting signals, proteins are sorted from the trans-face of the Golgi apparatus to secretory vesicles, the cell membrane, and lysosomes.
The key event in exocytosis is translocation of newly synthesized protein into the cisternal space of the rough ER (signal hypothesis). Proteins and lipids reach the Golgi apparatus by vesicular transport. Using carbohydratesorting signals, proteins are sorted from the trans-face of the Golgi apparatus to secretory vesicles, the cell membrane, and lysosomes.


Lysosomal enzymes are sorted by using a mannose-6-phosphate signal recognized by a receptor on the lysosomal membrane. Absence of mannose 6-phosphate results in default to the secretory pathway and release of enzymes by exocytosis. Nuclear and mitochondrial-sorting signals (positively charged amino acid sequences) are recognized by those organelles.

Endocytosis involves transport from the cell membrane to lysosomes using endosome intermediates. The process originates with a clathrincoated pit that invaginates to form a coated vesicle that fuses with an endosome. This internalization can be receptor-mediated (e.g., uptake of cholesterol). Endosomes subsequently fuse with lysosomes. Internalized receptor/ligand complexes may be conserved, degraded, or recycled.

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