COPII

From Wikipedia, the free encyclopedia

Image:Sec23 Sec24 Heterodimer.png
Heterodimer of Sec23 and Sec24
Sec23A
Identifiers
Symbol	SEC23A
Entrez	856311
HUGO	10701
OMIM	610511
PDB	1M2V
RefSeq	NM_006364
UniProt	Q15436
Other data
Locus	Chr. XVI [1]

Sec24A
Identifiers
Symbol	SEC24A
Entrez	10802
HUGO	10703
OMIM	607183
PDB	1M2V
RefSeq	XM_001132082
UniProt	O95486
Other data
Locus	Chr. 5 [2]

COPII is a type of vesicle 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.

1 Coat Proteins
2 Budding Process
3 See also
4 Sources

[edit] Coat Proteins

There are two protein heterodimers that form the coat complex. These proteins are

Sec23p/Sec24p Heterodimer
Sec13p/Sec31p Heterotetramer

It should be noted that these proteins alone are not able to cause the budding of the vesicle or direct the vesicle to the correct target membrane. Other proteins are needed for these processes.

[edit] Budding Process

The GTPase Sar1p 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 it's bound GTP into GDP. This process is required before the vesicle can bud off from the membrane.

[edit] See also

COPI vesicles
Clathrin vesicles

[edit] Sources

Lodish, Harvey, et al. 2003. Molecular Cell Biology 5th Edition, pages 708-710. W. H. Freeman
Springer, S., et al. 1999. A primer on vesicle budding. Cell 97:145–148.
Alberts, B., et al. 2002. Molecular Biology of The Cell 4th edition. Garland Science