COPI

COPI is a coatomer, a protein complex that coats vesicles transporting proteins from the "cis" end of the Golgi complex back to the 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 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 (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 (GEFs) and GTPase activating proteins (GAPs). ARF is post-translationally modified at the N-terminus by the addition of the fatty acid myristate.
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 and ARF GAPs. 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.
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 an interconnected assembly. The triads are linked together with contacts of variable valence making up four different types of contacts.