Question

With respect to newly synthesized secreted proteins describe how they are sorted and transported through the Golgi?

Answer 1

Most newly made proteins in the ER lumen or membrane are incorporated into small (?50-nm-diameter) transport vesicles. These either fuse with the cis-Golgi or with each other to form the membrane stacks known as the cis-Golgi reticulum (network). From the cis-Golgi certain proteins, mainly ER-localized proteins, are retrieved to the ER via a different set of retrograde transport vesicles. In the process called cisternal migration, or cisternal progression, a newcis-Golgi stack with its cargo of luminal protein physically moves from the cis position (nearest the ER) to the transposition (farthest from the ER), successively becoming first a medial-Golgi cisterna and then a trans-Golgi cisterna. As this happens, membrane and luminal proteins are constantly being retrieved from later to earlier Golgi cisternae by small retrograde transport vesicles. By this process enzymes and other Golgi resident proteins come to be localized either in the cis- or medial- or trans-Golgi cisternae.

Proteins destined to be secreted move by cisternal migration to the trans face of the Golgi and then into a complex network of vesicles termed the trans-Golgi reticulum. From there a secretory protein is sorted into one of two types of vesicles. In all cell types, at least some of the secretory proteins are secreted continuously. Examples of such constitutive (or continuous) secretion include collagen secretion by fibroblasts and secretion of serum proteins by hepatocytes. These proteins are sorted in the trans-Golgi network into transport  vesicles that immediately move to and fuse with the plasma membrane, releasing their contents by exocytosis.

In certain cells, the secretion of a specific set of proteins is not continuous; these proteins are sorted in the trans-Golgi network into secretory  vesicles that are stored inside the cell awaiting a stimulus for exocytosis. Such regulated secretion occurs in pancreatic acinar cells, which secrete precursors of digestive enzymes, and hormone-secreting endocrine cells. The release of each of these stored proteins is initiated by different neural and hormonal stimuli. In most cases of regulated secretion studied so far, a rise in the cytosolic Ca2+ concentration, induced by binding of the hormone to its receptor, triggers fusion of the secretory-vesicle membrane with the plasma  membrane and release of the vesicle contents by exocytosis. nerve cells also store neurotransmitters in similar types of vesicles, which also fuse with the membrane in response to an elevation in cytosolic Ca2+, releasing their contents.