COPII
Encyclopedia
COPII is a type of vesicle
coat protein that transports proteins from the rough endoplasmic reticulum
to the Golgi apparatus
. This is termed anterograde transport. The name "COPII" refers to the specific coat protein complex that initiates the budding process. The coat consists of large protein subcomplexes that are made of four different protein subunits.
These proteins alone are not able to cause the budding of the vesicle or direct the vesicle to the correct target membrane. SNARE, cargo, and other proteins are also needed for these processes to occur. The CopII protein does however cause the binding that forms vesicle coat, and thereby causes the release from the ER. The exact process of how the vesicle is brought to a particular location, or how that location is determined is not yet known.
Sar1
p is a protein that hydrolyzes GTP
and acts like a molecular "switch" that flips between an activated and membrane embedded GTP-bound form, and inactive and soluble GDP-bound form. Inactive GDP-bound Sar1p is attracted to the cytosolic side of the endoplasmic reticulum (ER) membrane by Sec12p. Sec12p is a guanine nucleotide exchange factor, or GEF, and a transmembrane protein residing in the ER membrane. This binding event causes Sar1p to release GDP, and since the concentration of GTP is much higher than GDP in the cytosol, GTP spontaneously binds to Sar1p. Now in a GTP bound state, Sar1p undergoes a conformational change which exposes a hydrophobic tail that can be inserted into the lipid bilayer, binding it to the membrane. Once Sar1p is bound to the membrane the coat protein complexes Sec23p/24p and Sec13p/31p bind to the membrane sequentially. These proteins simultaneously contact Sar1p and cargo proteins destined for the cis-golgi membrane. The Sec23p/24p-Sec13p/31p-Sar1p complexes then coalesce to form a much larger complex. This network deforms the membrane enough to bud a vesicle off.
At some currently unknown point the GTPase Sar1p hydrolyzes its bound GTP into GDP. This process is required before the vesicle can bud off from the membrane.
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
coat protein that transports proteins from the rough endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
to the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. This is termed anterograde transport. The name "COPII" refers to the specific coat protein complex that initiates the budding process. The coat consists of large protein subcomplexes that are made of four different protein subunits.
Coat proteins
There are two protein heterodimers that form the coat complex. These proteins are- Sec23p/Sec24p Heterodimer
- Sec13p/Sec31p Heterotetramer
These proteins alone are not able to cause the budding of the vesicle or direct the vesicle to the correct target membrane. SNARE, cargo, and other proteins are also needed for these processes to occur. The CopII protein does however cause the binding that forms vesicle coat, and thereby causes the release from the ER. The exact process of how the vesicle is brought to a particular location, or how that location is determined is not yet known.
Budding process
The GTPaseGTPase
GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
Sar1
Sar1
SAR1A or Sar1 is a protein involved in membrane trafficking. It is a GTPase found in COPII vesicles. It regulates the assembly and disassembly of COPII coats....
p is a protein that hydrolyzes GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
and acts like a molecular "switch" that flips between an activated and membrane embedded GTP-bound form, and inactive and soluble GDP-bound form. Inactive GDP-bound Sar1p is attracted to the cytosolic side of the endoplasmic reticulum (ER) membrane by Sec12p. Sec12p is a guanine nucleotide exchange factor, or GEF, and a transmembrane protein residing in the ER membrane. This binding event causes Sar1p to release GDP, and since the concentration of GTP is much higher than GDP in the cytosol, GTP spontaneously binds to Sar1p. Now in a GTP bound state, Sar1p undergoes a conformational change which exposes a hydrophobic tail that can be inserted into the lipid bilayer, binding it to the membrane. Once Sar1p is bound to the membrane the coat protein complexes Sec23p/24p and Sec13p/31p bind to the membrane sequentially. These proteins simultaneously contact Sar1p and cargo proteins destined for the cis-golgi membrane. The Sec23p/24p-Sec13p/31p-Sar1p complexes then coalesce to form a much larger complex. This network deforms the membrane enough to bud a vesicle off.
At some currently unknown point the GTPase Sar1p hydrolyzes its bound GTP into GDP. This process is required before the vesicle can bud off from the membrane.