Lec17_05_23_04 - Chapter 17 Vesicular traffic, secretion,...

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Chapter 17 Vesicular traffic, secretion, and endocytosis. The focus in on the mechanisms that allow soluble and membrane proteins synthesized on the rough ER to move to their final destination via the secretory pathway. The main mode of transport is by transport vesicles that collect cargo proteins in buds arising from the membrane of one compartment and then deliver these cargo proteins to the next compartment by fusing with the membrane of that compartment. As the transport vesicles bud from one membrane to the next the same face of the membrane remains oriented toward the cytosol. Thus proteins inserted into membrane or lumen of ER protein can be carried along secretory pathway from one organelle to another without exposure to other membranes. Fig 17-1 outlines major routes for protein trafficking in the secretory pathway. Newly synthesized proteins are incorportated into the ER lumen or membrane and packaged into antergrade transport vesicles which fuse with each other to form flattened membrane bound compartments known as cis-Golgi cisterna. During cisternal progression, enymes and other Golgi resident proteins are constantly being retrieved from later to earlier Golgi cisternae by retrograde transport vesicles thereby remaining localized to the cis, medial, or trans-Golgi cisternae. Proteins in the secretory pathway that are destined for compartment other than the ER or Golgi eventually reach a complex network of membranes and vesicles termed the trans-Golgi network. From this major branch point in secretory pathway a protein can be loaded into one of three different kinds of vesicles. The first type of vesicle immediately moves to and fuses with the PM releasing its contents by exocytosis. All cell types release some proteins in this manner including collagen by fibroblasts, serum protein by hepatocytes, and antibodies by activated B lymphocytes. The second type of vesicle to bud from the trans-Golgi network known as secretory vesicles are stored inside the cell until a signal for exocytosis causes release at the PM. Proteins released in this manner are peptide hormones such as insulin from various endocrine cells and precursors of digestive enymes from mammary glands and some neurotransmitters from neurons. Third type of vesicle that buds from the trans-Golgi network is directed to the lysosome. This organelle is responsible for intracellular degradation of macromolecules. Secretory proteins destined for lysosomes are transported by vesicles from the trans-Golgi network to a compartment called the late endosome and then via direct fusion to the lysosomal membrane. Soluble proteins delivered by this pathway are digestive enzymes including proteases, glycosidases and phosphatases and membrane pumps (i.e. H + V-pumps) that pump hydrogens into lumen of endosome and lysosome. The endosome also functions in the endocytic pathway in which vesicles bud from the PM bringing proteins and bound ligands into the cell.
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This note was uploaded on 09/18/2008 for the course BIM 202 taught by Professor Simon during the Spring '06 term at UC Davis.

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Lec17_05_23_04 - Chapter 17 Vesicular traffic, secretion,...

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