Vessicle Sorting and Trafficing

Molecules are directed to specific target vessicles by modification or aggregation of these molecules with specific transmembrane proteins in the target vessicles.

• Carbohydrates attached to lysososomal bound proteins are phosphorylated (Fig. 17.39).
• Phophorylated proteins are attached to membrane bound proteins which have lysosomal targeting alpha domain (Fig. 17.40).
  
  
• Regulated vessicles have  specific secretory proteins that aggregates and targets vessicles to be regulated (HyperCell movie).
  
  
• Localization of transmembrane vessicles to apical or basal membranes is controlled by the presence of the transmembrane proteins Rab and V-SNARE (Fig. 17.43)

Pro-proteins are cleaved into active proteins late in the secretion process (Fig. 17.42).

At least three types of coated vessicles transport proteins from organelle to organelle (Fig. 17.50).

• Clathrin                      golgi to lysosomes and plasma membrane and endocytosis (Fig. 17.53).
• Coatamer I (COPI)      golgi to ER   (Fig. 17.58).
• Coatamer II (COPII)   ER to golgi

Depolymerization (by cytosolic Hsc 70) of the clathrin and coatamer exposes transmembrane proteins (V-SNARE) that direct the vessicle to its destination by binding to the specific T-SNARE protein on the target organelle.  The fusion protein SNAP25 causes the vessicle to fuse with the target membrane (Fig. 17.59).

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