Because entry into this pathway does not require a particular signal, it is also called the default pathway ( Figure 13-55). Most other proteins are transported directly to the cell surface by the nonselective constitutive secretory pathway. We have already noted that proteins destined for lysosomes are tagged for packaging into specific departing vesicles (by mannose-6-phosphate for lysosomal hydrolases), and analogous signals are thought to direct secretory proteins into secretory vesicles. In a cell capable of regulated secretion, at least three classes of proteins must be separated before they leave the trans Golgi network-those destined for lysosomes (via late endosomes), those destined for secretory vesicles, and those destined for immediate delivery to the cell surface. Many Proteins and Lipids Seem to Be Carried Automatically from the Golgi Apparatus to the Cell Surface
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