Molecular mechanisms of vesicle budding and fusion

  • The 3 well-characterized types of transport vesicles – COPI , COPII, and clathrin-coated vesicles – are distinguished by the proteins that form their coats and the transport routes they mediate.
  • All types of coated vesicles are formed by polymerization of cytosolic coat proteins on a parent (donor) membrane to form vesicle buds that eventually pinch off from the membrane to release a complete vesicle. Shortly after vesicle release, the coat is shed, exposing proteins required for fusion with the target membrane.
  • Small GTP-binding proteins (ARF or Sar1) belonging to the GTPase superfamily control polymerization of coat proteins, the initial step in vesicle budding. After vesicles are released from the donor membrane, hydrolysis of GTP bound to ARF or Sar1 triggers disassembly of the vesicle coats.
  • Specific sorting signals in membrane and luminal proteins in donor organelles interact with coat proteins during vesicle budding, thereby recruiting cargo proteins to vesicles.
  • A second set of GTP-binding proteins, the Rab proteins, label specific vesicle type and enable their targeting to the appropriate membrane. Activated Rab.GTP in a vesicle can bind to a specific type of effector protein. One type of effector is a filamentous tethering protein or large protein complex that enables tethering of the vesicle to the target membrane. Another type of effector is a motor protein that enables vesicles to move along cytoskeletal filaments to their correct destination.
  • Each v-SNARE in a vesicular membrane specifically binds to a complex of cognate t-SNARE proteins in the target membrane, inducing fusion of the two membranes. After fusion is completed, the SNARE complex is disassembled in an ATP-dependent reaction mediated by other cytosolic proteins.

Ref. Lodish p.644

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