COPI

COPI is a type of vesicle that transports proteins from the cis end of the Golgi complex to the rough endoplasmic reticulum (ER). This type of transport is termed retrograde transport as proteins are typically moved to the Golgi complex from the ER. The name "COPI" refers to the specific coat protein complex that initiates the budding process on the cis-Golgi membrane. The coat consists of large protein subcomplexes that are made of seven different protein subunits.

Coat Proteins
A protein complex called Coatomer which contains seven different protein subunits is the protein that coats the budding vesicle. Although this protein complex plays a crutial role in vesicle budding and formation, other proteins are needed to initiate the budding process, direct the vesicle to the correct target membrane, and fuse the vesicle with that membrane.

Budding Process
The GTPase ARF (ARF1p or ARF2p in yeast), a protein that hydrolyzes GTP, is attracted to the cytosolic side of the cis-golgi membrane. This is accomplished in one of two ways depending on the type of proteins that will be transported.


 * 1. Lumenal Proteins: Proteins found in the lumen of the golgi complex that need to be transported to the lumen of the ER contain a peptide signal-sequence called KDEL.  This sequence is recognized by a membrane-bound KDEL receptor.  In yeast this is Erd2p and in mammals it is KDELR.  This receptor then binds to a ARF-GEF, a class of guanine nucleotide exchange factors.  This protein in turn binds to the GTPase, ARF.  This interaction causes ARF to exchange its bound GDP for GTP.  Once this exchange is made ARF binds to the cytosolic side of the cis-golgi membrane.


 * 2. Membrane Proteins: Proteins found in the membrane of the golgi-complex that need to be returned to the membrane of the ER sometimes contain a peptide signal-sequence called KKXX.  This part of the protein usually sticks out from the membrane into the cytosol.  Coatomer proteins in the cytosol bind to these sequences.  The beta-subunit of the coatomer complex then recruits an ARF protein from the cytosol.  The ARF, bound to a GDP molecule, is prompted by the coatomer to exchange it's GDP for a GTP molecule.  Once this exchange occurs the ARF protein proceeds to bind to the membrane of the cis-golgi complex.

Once enough of these complexes form on the surface of the cis-golgi membrane, the complexes begin to coalesce into a much larger complex. This large network of proteins built on the membrane causes a portion of the membrane to bud off, carrying the cargo proteins inside.

At some currently unknown point the GTPase ARF hydrolyzes it's bound GTP into GDP. This process is required before the vesicle can bud off from the membrane.