COPII Vesicle

COPII vesicles are transport intermediates from the endoplasmic reticulum. The process is driven by recruitment of the soluble proteins that form the coat structure called COPII from the cytoplasm to the membrane. 

Jones B, Jones EL, Bonney SA, Patel HN, Mensenkamp AR, Eichenbaum-Voline S, Rudling M, Myrdal U, Annesi G, Naik S, Meadows N, Quattrone A, Islam SA, Naoumova RP, Angelin B, Infante R, Levy E, Roy CC, Freemont PS, Scott J, Shoulders CC (2003) Mutations in a Sari GTPase of COPII vesicles are associated with lipid absorption disorders. Nat Genet 34 29-31... 

Lee, M.C., Orci, L., Hamamoto, S., Futal, E., Ravazzola, M., Schekman, R. Sarlp N-terminal helix initiates membrane curvature and completes the fission of a COPII vesicle. Cell 2005,122,605-17. 

COPII vesicles transport proteins from the rough ER to the Golgi. 

Yeast cells, like all eukaryotic cells, express more than 20 different related v-SNARE and t-SNARE proteins. Analyses of yeast sec mutants defective in each of the SNARE genes have identified the specific membrane-fusion event in which each SNARE protein participates. For all fusion events that have been examined, the SNAREs form four-helix bundled complexes, similar to the VAMP/syntaxin/SNAP-25 complexes that mediate fusion of secretory vesicles with the plasma membrane. However, in other fusion events (e.g., fusion of COPII vesicles with the c7s-Golgi network), each participating SNARE protein contributes only one a helix to the bundle (unlike SNAP-25, which contributes two helices) in these cases the SNARE complexes comprise one v-SNARE and three t-SNARE molecules. 

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. 

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. 

Bi, X., et al. 2002. Structure of the Sec23/24-Sarl pre-budding complex of the COPII vesicle coat. Nature 419 271-277. 

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