物理系-普通生物学-20195-membrane transport.ppt

* * * * * * * * A Low-density Lipoprotein (LDL) Particle The Receptor-mediated Endocytosis of LDL Vesicular Transport Coated Vesicles Functions 1. Concentrates specific membrane proteins in a specialized membrane patch and helps select the molecules for transport. 2. Deforms the membrane patch and molds the forming vesicles to give a relative uniform size of vesicle. Three Types of Coated Vesicles Clathrin-coated vesicles – mediate transport from the Golgi and from the plasma membrane. COPI- and COPII-coated vesicles – mediate transport from the ER and Golgi. Each type is used for different transport steps in the cell. Clathrin-coated, COPI-coated, and COPII-coated Vesicles Clathrin-coated Pits and Vesicles Clathrin – three large and three small polypeptide chains to form a三联体骨架(triskelion) The Assembly and Disassembly of a Clathrin Coat Adaptin, a second major coat protein in clathrin-coated vesicles, Required both to bind the clathrin coat to the membrane and to trap various transmembrane proteins including cargo receptors Dynamin in Pinching off Clathrin-coated Vesicles from the Membrane Dynamin, A GTPase, regulates the rate with which vesicles pinch off from the membrane. To seal the membrane, dynamin recruits other proteins to the neck of the budding vesicle to bend the membrane. COPI-coated and COPII-coated Vesicles COPII-coated package bud from the ER COPI-coated packages bud form pre-Golgi compartments and Golgi cisternae. The coats of COPI and COPII vesicles consist of large protein complexes. COPI – seven subunits COPII – four subunits Some COPI coat-protein subunits have sequence similarity to adaptins. Different Coats in Vesicle Traffic A Model of COPII-coated Vesicle Formation Monomeric GTPases Control Coat Assembly Coat-recruitment GTPases ensure that membrane traffic to and form an organelles is balanced and coat protein assembled only when needed. ARF proteins – responsible for both COPI coat and clathrin coat assembly