ABSTRACT

Many cells secrete proteins constitutively. That is, there is a relatively constant flow of proteins through the endoplasmic reticulum (ER) and Golgi complex and then to the plasma membrane where carrier vesicles fuse with the plasma membrane and release their contents outside the cell (Fig. 3.1 ) . 1 ,2 Such cells include fibro­ blasts, where the secreted proteins are largely components of the extracellular matrix, and hepatocytes, where the secreted proteins circulate in the serum. Other cells exhibit regu­ lated secretion in which the secretory proteins are packaged after reaching the trans-Golgi network (TGN) into secretory vesicles that remain in the cytoplasm until a secretory sig­ nal is received (Fig. 3 .2 ) . 1 -3 These cells include a wide variety of endocrine and exocrine cells as well as neurons. Many secreted proteins are made as larger precursor molecules. They are cleaved to the mature protein within the secretory vesicle.