Exportation from the EREdit
How are Proteins exported from the ER?
There are areas of the smooth ER that bud transport vesicles to carry proteins from the ER to the Golgi complex and these areas are called Transitional ER. Only proteins that have been properly folded can leave the ER in transport vesicles. New research has shown that only correctly folded glycoprotein’s destined for insertion into membrane or secretion from the cell travel to the Golgi Complex while the rest of the ER products, oligomannosides and glycopeptides, are transported into the cytosol (Cacan and Verbert 2000).
Purpose of the Protein Coat
Once budded, the individual vesicles merge to form larger vesicles and interconnected tubules, both of which are termed vesicular-tubular carriers (VTCs). It is thought that VTCs then transverse the ER Golgi intermediate compartment (ERGIC) by travelling along microtubule tracks (Karp 2010, p. 464). Each vesicle that buds from the ER is enclosed within a protein coat consisting of two layers of soluble proteins, which assemble on the cytosolic surface around the bud. The outer layer of the protein coat forms a framework around the vesicle, while the inner layer interacts with the contents of the vesicle to select the materials that will be exported from the ER. The protein coat serves two main purposes:
1. It acts as a mechanical device to assist the ER membrane in making a circular transport vesicle.
2. It selects the substances to be exported from the ER including secretary, lysosomal and membrane proteins but also equipment required to help guide and insert the vesicle into its destined membrane. Enzymes assembled in the ER may also be exported by these protein coated vesicles to be used later in the biosynthetic pathway.
COPII Protein Coat assembly and exportation of the finished protein
The protein coating on vesicles transporting ER products to the Golgi complex is called COPII. COPII is assembled due to the regulatory control of a G protein called Sar1. Initially Sar1 is associated with a GDP molecule and is inactive but by exchanging the GDP molecule for a GTP molecule, the Sar1 G protein activates. Activation is usually due to the binding of a Sar1 guanine-nucleotide-exchange factor (GEF) to the Sar1 G protein (Alberts et al 2008, p.759) and this triggers a conformational change, in which Sar1 inserts its N-terminal α helix into the membrane of the ER, causing the membrane to start to bend (Karp 2010, p. 471). Polypeptides, such as Sec23 and 24, are then recruited by the Sar1 G protein to bind it and create a banana shaped dimer. Sec 23 and 24 create the inner layer of the COPII protein coat. This helps to further bend the ER membrane so that it can eventually bud off to form a round vesicle. Sec24 also has a role in selecting the materials that will be exported by interacting with the ‘ER export signals’ on part of the cytosolic tails of specific integral membrane proteins. Other polypeptides, Sec13 and 31, bind to continue the construction of the COPII protein coat by forming a lattice like cage structure around the inner layer. In this framework, each corner is composed of 4 Sec13-31 polypeptides and these combine to form the outer layer of the protein coat. Once fully assembled the vesicle buds off the ER membrane and travels to the Golgi complex (Karp 2010, p.471).
Disassembly of COPII Protein Coat
When the COPII reaches the Golgi complex it must first disassemble before it can fuse to the membrane and empty its contents. Disassembly is caused by hydrolysis of the GTP molecule on the Sar1 G protein to a GDP molecule. The Sar1 then dissociates form the vesicle membrane and then the rest of the COPII subunit proteins can be released back into the cytosol (Karp 2010, p.472).