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COPI is a
coatomer The coatomer is a protein complex that coats membrane-bound transport vesicles. Two types of coatomers are known: *COPI (retrograde transport from trans-Golgi network to cis-Golgi network and endoplasmic reticulum) *COPII (anterograde transport ...
, a protein complex that coats
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...
s transporting proteins from the ''cis'' end of the
Golgi complex The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles insi ...
back to the rough
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
(ER), where they were originally synthesized, and between Golgi compartments. This type of transport is ''retrograde transport'', in contrast to the ''anterograde transport'' associated with the
COPII The Coat Protein Complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This ...
protein. 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, namely α, β, β', γ, δ, ε and ζ.


Coat proteins

Coat protein, or COPI, is an
ADP ribosylation factor ADP ribosylation factors (ARFs) are members of the ARF family of GTP-binding proteins of the Ras superfamily. ARF family proteins are ubiquitous in eukaryotic cells, and six highly conserved members of the family have been identified in mammal ...
(ARF)-dependent protein involved in membrane traffic. COPI was first identified in retrograde traffic from the ''cis''-Golgi to the rough endoplasmic reticulum (ER) and is the most extensively studied of ARF-dependent adaptors. COPI consists of seven subunits which compose the heteroheptameric protein complex. The primary function of adaptors is the selection of cargo proteins for their incorporation into nascent carriers. Cargo containing the sorting motifs KKXX and KXKXX interact with COPI to form carriers which are transported from the cis-Golgi to the ER. Current views suggest that ARFs are also involved in the selection of cargo for incorporation into carriers.


Budding process

ADP ribosylation factor (ARF) is a GTPase involved in membrane traffic. There are 6 mammalian ARFs which are regulated by over 30
guanine nucleotide exchange factors Guanine nucleotide exchange factors (GEFs) are proteins or protein domains that activate monomeric GTPases by stimulating the release of guanosine diphosphate (GDP) to allow binding of guanosine triphosphate (GTP). A variety of unrelated structu ...
(GEFs) and
GTPase activating proteins GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are a ...
(GAPs). ARF is post-translationally modified at the N-terminus by the addition of the fatty acid
myristate Myristic acid (IUPAC name: tetradecanoic acid) is a common saturated fatty acid with the molecular formula CH3(CH2)12COOH. Its salts and esters are commonly referred to as myristates or tetradecanoates. It is named after the binomial name for nut ...
. ARF cycles between GTP and GDP-bound conformations. In the GTP-bound form, ARF conformation changes such that the myristate and hydrophobic N-terminal become more exposed and associate with the membrane. The interconversion between GTP and GDP bound states is mediated by ARF
GEFs Generalized epilepsy with febrile seizures plus (GEFS+) is a syndromic autosomal dominant disorder where affected individuals can exhibit numerous epilepsy phenotypes. GEFS+ can persist beyond early childhood (i.e., 6 years of age). GEFS+ is ...
and ARF
GAPs Gaps is a member of the Montana group of Patience games, where the goal is to arrange all the cards in suit from Deuce (a Two card) to King. Other solitaire games in this family include Spaces, Addiction, Vacancies, Clown Solitaire, Paganini, ...
. At the membrane, ARF-GTP is hydrolyzed to ARF-GDP by ARF GAPs. Once in the GDP-bound conformation, ARF converts to a less hydrophobic conformation and dissociates from the membrane. Soluble ARF-GDP is converted back to ARF-GTP by GEFs. # Luminal proteins: Proteins found in the lumen of the Golgi complex that need to be transported to the lumen of the ER contain the
signal peptide A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16-30 amino acids long) present at the N-ter ...
KDEL. This sequence is recognized by a membrane-bound KDEL receptor. In yeast, this is
ERD2P ERD may refer to: * Érd, a city in Hungary * Berdyansk Airport, in Ukraine * Economic Relations Division (Bangladesh), of the Bangladeshi Ministry of Finance * Elastic recoil detection * Emergency repair disk * Emergency Reserve Decoration, a Bri ...
and in mammals it is
KDELR KDEL (Lys-Asp-Glu-Leu) endoplasmic reticulum protein retention receptors (KDELR) are the members of a group of receptor proteins: * KDELR1 * KDELR2 * KDELR3 ER lumen protein retaining receptor 3 is a protein that in humans is encoded by the ''KDELR ...
. This receptor then binds to an ARF-GEF, a class of guanine nucleotide exchange factors. This protein in turn binds to the ARF. This interaction causes ARF to exchange its bound
GDP Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is often ...
for GTP. Once this exchange is made ARF binds to the cytosolic side of the cis-Golgi membrane and inserts the myristoylated N-terminal amphipathic alpha-helix into the membrane. # Membrane proteins: Transmembrane proteins which reside in the ER contain sorting signals in their cytosolic tails which direct the protein to exit the Golgi and return to the ER. These sorting signals, or motifs, typically contain the amino acid sequence KKXX or KXKXX, which interact with COPI subunits α-COP and β'-COP. The order in which adaptor proteins associate with cargo, or adaptor proteins associate with ARFs is unclear, however, in order to form a mature transport carrier coat protein, adaptor, cargo, and ARF must all associate. Membrane deformation and carrier budding occurs following the collection of interactions described above. The carrier then buds off of the donor membrane, in the case of COPI this membrane is the cis-Golgi, and the carrier moves to the ER where it fuses with the acceptor membrane and its content is expelled.


Structure

On the surface of a vesicle COPI molecules form symmetric trimers ("triads"). The curved triad structure positions the Arf1 molecules and cargo binding sites proximal to the membrane. The β′- and α-COP subunits form an arch over the γζβδ-COP subcomplex, orienting their N-terminal domains such that the K(X)KXX cargo-motif binding sites are optimally positioned against the membrane. Thus β′- and α-COP do not form a cage or lattice as in COPII and clathrin coats as previously suggested; instead, they are linked to one another via the γζβδ-COP subcomplexes, forming a
interconnected assembly
The triads are linked together with contacts of variable valence making up four different types of contacts.


See also

*
COPII The Coat Protein Complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This ...
vesicles *
Clathrin Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. Whe ...
vesicles * Glyceraldehyde 3-phosphate dehydrogenase#ER to Golgi transport *
Exomer Exomer is a heterotetrameric protein complex similar to COPI and other adaptins. It was first described in the yeast ''Saccharomyces cerevisiae''. Exomer is a cargo adaptor important in transporting molecules from the Golgi apparatus toward the c ...


References

{{Vesicular transport proteins Cell biology