Exocytosis

A cell frequently wants to secrete larger molecules than can be accommodated through the transport systems dealt with in Topic E3. Exocytosis obtains to the movement of proteins out of the cell across the plasma membrane into the extracellular space. The Proteins  intended  to  be  secreted  from  the  cell  are  translated   on  ribosomes attached to the RER. Membrane-bound vesicles containing these proteins then bud off from the RER, migrate by the cytosol and fuse with the membrane of the Golgi apparatus that is shown in the Figure. On transport by the endoplasmic Golgi and reticulum    apparatus,   several    post- translational modifications to the proteins take place, like as glycosylation.

The Golgi apparatus is the main sorting center of the cell where lipids and proteins are packaged into discrete vesicles and then goaled to the appropriate category of the cell.  For instance, vesicles containing lysosomal proteins will be targeted to the lysosomes. If the vesicles contain no specific targeting signal they will migrate by the cytosol to the plasma membrane the so-called constitutive secretory or default way. Transport  vesicles destined  for the plasma membrane  in the constitutive  secretory  path  leave  the  Golgi  apparatus  in  a  steady  stream.  Membrane proteins   and lipids in these vesicles give new material   for the plasma membrane, although the soluble proteins inside the vesicles are secreted to the extracellular space. Overall cells have this constitutive secretory path.

In certain cells, moreover, an additional secretory path exists, the regulated secretory pathway. This pathway is found mostly in cells which are specialized  for  secreting  products  rapidly  on  demand  in  response  to  a  particular stimulus. For instance, the hormone insulin and digestive enzymes are secreted through the pancreas, although neurotransmitters are secreted through nerve cells. In these cells like substances are initially stored in secretory vesicles that form

                       Figure:  Exocytosis of proteins by the constitutive and regulated secretory pathways.

through clathrin-coated  budding from the Golgi apparatus.  The clathrin coat  then  vesicles  and  the dissociates  remain  in  the  cytosol  until  signaled  to release their contents on fusion with the plasma membrane.