Meta-Biol

• Pathways

After passing through the Golgi complex, secretory cargo is packaged into post-Golgi transport intermediates (post-Golgi), which translocate plus-end directed along microtubules to the plasma membrane. There at least two classes of clathrin coated vesicles in cells, one predominantly Golgi-associated, involved in budding from the trans-Golgi network and the other at the plasma membrane. Here the clathrin-coated vesicles emerging from the Golgi apparatus are triggered by the heterotetrameric adaptor protein complex, AP-1 at the trans-Golgi network membrane. The cargo can be transmembrane, membrane associated or golgi luminal proteins. Each step in the vesicle sculpting pathway, gathers cargo and clathrin triskeletons, until a complete vesicular sphere is formed. With the scission of the membrane the vesicle is released and eventually losses its clathrin coat.

The secretory membrane system allows a cell to regulate delivery of newly synthesized proteins, carbohydrates, and lipids to the cell surface, a necessity for growth and homeostasis. The system is made up of distinct organelles, including the endoplasmic reticulum (ER), Golgi complex, plasma membrane, and tubulovesicular transport intermediates. These organelles mediate intracellular membrane transport between themselves and the cell surface. Membrane traffic within this system flows along highly organized directional routes. Secretory cargo is synthesized and assembled in the ER and then transported to the Golgi complex for further processing and maturation. Upon arrival at the trans Golgi network (TGN), the cargo is sorted and packaged into post-Golgi carriers that move through the cytoplasm to fuse with the cell surface. This directional membrane flow is balanced by retrieval pathways that bring membrane and selected proteins back to the compartment of origin.

The transit of proteins and other cargo through the cell requires a cellular transport process in which transported substances are moved in membrane-bounded vesicles. Transported substances are enclosed in the vesicle lumen or located in the vesicle membrane. The transport process begins with the formation of the vesicle itself, often triggered by the interaction of the cargo with the vesicle formation machinery. Vesicular transport pathways can include vesicle formation, coating, budding, uncoating and target membrane fusion depending upon the function of the pathway described. Vesicle-mediated transport occurs from within cell via ER and Golgi transport, as well as functioning in the endocytosis of material taken into the cell via scavenger receptors.