COPI Vesicle

Coatomer is the major coat component of COPI vesicles. The Coatomer complex consists of seven different subunits (a -C, COP). 

COPI vesicles mediate anterograde transport from the intermediate compartment to the Golgi, transport within the Golgi apparatus and retrograde transport back from the Golgi to the ER by the recruitment of soluble... 

A pre-requisite for clathrin-coat assembly is the recruitment to the membrane of an adaptor complex. Similar to what has been observed for the recruitment of coatomer to Golgi membranes, adaptor binding is dependent on the presence of ARF-GTP. However, in contrast to COPI vesicle formation, ARF-GTP is suggested to act in a process before budding and not as a stoichiometric coat component. Other differences between COP-coated and clathrin-coated vesicles concern their uncoating mechanism. Disassembly of clathrin-coated vesicles is believed to depend on the chaperoneHSC 70 and on auxilin. 

COPI vesicle budding is triggered by the hydrolysis of GTP to GDP by membrane-associated ARFl. Once in the membranes, ARF-GTP recruits pre-assembled coatomers, resulting in membrane deformation. The cytoplasmic tail of an abundant transmembrane protein, known as p24,... 

J., Satch, A., Pelletier, L., and Warren, G., Golgin tethers define subpopulations of COPI vesicles. Science 307,1095-1098,2005 Short, B Haas, A., and Barr, F.A., Golgins and GTPases, giving identity and structure to the Golgi apparatus, Biochim. Biophys. Acta 1744, 383-395, 2005 Satoh, A., Beard, M., and Warren, G., Preparation and characterization of recombinant golgin tethers. Methods Enzymol. 404, 279-296, 2005. 

COPI vesicles mainly transport proteins In the retrograde direction between Golgi cisternae and from the cis Golgi back to the rough ER. 

COPII vesicles were first recognized when cell-free extracts of yeast rough ER membranes were incubated with cytosol, ATP, and a nonhydrolyzable analog of GTP. The vesicles that formed from the ER membranes had a distinct coat, similar to that on COPI vesicles but composed of different proteins, designated COPII proteins. Yeast cells with mutations in the genes for COPII proteins are class B sec mutants and accumulate proteins In the rough ER (see Figure 17-5). Analysis of such mutants has revealed several proteins required for formation of COPII vesicles. 

COPI Vesicles Mediate Retrograde Transport within the Golgi and from the Golgi to the ER... 

COPI vesicles were first discovered when isolated Golgi fractions were incubated in a solution containing ATP, cytosol, and a nonhydrolyzable analog of GTP (see Figure 17-10). Subsequent analysis of these vesicles showed that the coat is formed from large cytosolic complexes, called coatomers, composed of seven polypeptide subunits. Yeast cells containing temperature-sensitive mutations in COPI proteins accumulate proteins in the... 

As discussed in Chapter 16, the ER contains several soluble proteins dedicated to the folding and modification of newly synthesized secretory proteins. These Include the chaperone BiP and the enzyme protein disulfide Isomerase, which are necessary for the ER to carry out Its functions. Although such ER-resident luminal proteins are not specifically selected by COPII vesicles, their sheer abundance causes them to be continuously loaded passively into vesicles destined for the cis-Golgi. The transport of these soluble proteins back to the ER, mediated by COPI vesicles, prevents their eventual depletion... 

COPII vesicles transport proteins from the rough ER to the cis-Golgi COPI vesicles transport proteins in the reverse direction (see Figure 17-14). 

Membrane proteins needed to form COPII vesicles can be retrieved from the ds-Golgl by COPI vesicles. One of the sorting signals that directs membrane proteins into COPI vesicles is a KKXX sequence, which binds to subunits of the COPI coat. 

COPI vesicles also carry Golgi-resident proteins from later to earlier compartments In the Golgi stack. 

Jayaram, S., Senti, K., Tiklova, K., Tsahouras, V., Hemphala, J. Samakovlis, C. (2008). COPI vesicle transport is a common requirement for tube expansion in Drosophila. PLoS... 

TETHERING ASSAYS FOR COPI VESICLES MEDIATED BY GOLGINS... 

A method is described that allows the attachment of COPI vesicles and Golgi membranes to glass slides that can then be analyzed using electron microscopy (EM) and immuno-EM methods. Subpopulations of COPI vesicles can be bound selectively using recombinant golgins. Alternatively, COPI vesicles can be attached to prebound Golgi membranes. Marking these vesicles selectively with biotin allows their site of attachment to be identified. 

Determining the flow patterns for COPI vesicles has been hampered by the lack of available methods to subfractionate them biochemically, so that... 

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