Constitutive secretion is one of the pathways by which proteins can be secreted, or transferred out of a cell. In constitutive secretion proteins are secreted from a cell continuously, regardless of external factors or signals.
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Different cells secrete different types of proteins. Some cells, such as white blood cells, only secrete only one type of protein and are known as unpolarized cells. Other cells, called polarized cells, secrete several classes of proteins that are each destined for delivery to a different location.
As seen in the Protein Modification animation, some of these proteins can be destined for the endosome. They are targeted to the endosome by a specific mannose-phosphate signal. Other proteins are destined for delivery outside of the cell. These proteins are packaged into secretory vesicles and delivered to the cell membrane.
Some secretory vesicles congregate in the cell awaiting an external signal. Once that signal is received, the vesicles rapidly fuse with the membrane, and a large quantity of cargo proteins are released simultaneously. This is called regulated secretion. During any secretion process, the vesicle fuses with the cell membrane by a process called exocytosis.
Alternatively, secretory vesicles are delivered to the cell membrane after they are formed, and the protein product is immediately released. This is called constitutitive secretion . The constitutive pathway is required to maintain the cell membrane and exists in all eucaryotic cells. This animation focuses on constitutive secretion.
Protein cargo originally from the endoplasmic reticulum migrates through the Golgi apparatus. These proteins leave the Trans Golgi Network in a secretory vesicle. Some of these proteins are not modified as they move through the Golgi apparatus.
Unlike modified proteins, these proteins do not contain any signal that would direct their transport to a specific intracellular location such as the ER, endosome, or a previous Golgi cisternae. These proteins enter the default secretion pathway and are immediately secreted from the cell. Because there is no control over their secretion, this process is called constitutive secretion.
Once the secretory vesicle reaches the cell membrane, it fuses immediately with the surface membrane and releases its cargo protein into the extracellular space. The secretory vesicle itself contributes new lipids and its membrane to the plasma membrane of the cell.
Certain white blood cells constitutively secrete specific Interleukins, which are signaling molecules, for the purpose of intercellular communication and play an important role in the function of the immune system. Cells such as Fibroblasts constitutively secrete proteins like Collagen and Proteoglycans into the extracellular matrix and play an important role in maintaining the structural integrity of connective tissues.
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