COPII proteins

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. 

Espenshade, P. J., Li, W. P., and Yabe, D. Sterols block binding of COPII proteins to SCAP, thereby controlling SCAP sorting in ER. Proc Natl Acad Sci U S A 99 (2002) 11694-11699. 

We thank Crystal Chan and Robert Lesch for COPII proteins and Matthew Welsh and David G. Drubin for sharing their equipment. We thank Chris Fromme for improving the manuscript and Bruno Antonny for advice on kinetic analysis of real-time assays. We thank... 

The buffer used for the preparation of hposomes should be isoosmotic with the assay buffers in which proteins and hposomes are finally mixed. It is preferable to omit divalent salt (MgC ) in the liposome buffer to prevent long-term fusion. Therefore the liposomes used for experiments with COPI proteins are prepared in 50 mM Hepes-KOH, pH 7.2,120 mM Kacetate, whereas hposomes used for experiments with COPII proteins are prepared in 20 mM Hepes-KOH, pH 7.0, 160 mM Kacetate. 

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. 

Proteins associated with the PDZ ligand of the 5-HT4e receptor C-terminus are completely different from those that bind to the 5-HT4a receptors. They include the neuronal isoform of nitric oxide synthase (nNOS) and CIPP, two PDZ-domain-containing proteins, and Sec 23, which lacks any obvious PDZ domain (9). Sec 23 is a member of a protein complex designated as COPII, which is involved in the budding of vesicles from the endoplasmic reticulum (ER) and ER-to-Golgi transport of proteins. 

Icosahedral capsid viruses and clathrins are examples of coat proteins of which there are many. Another example that has been extensively studied is coat protein II, or COPII, which is composed of an inner cage and outer coat [5], The inner cage is a cuboctahedron approximately 60 nm across. It has square and triangular faces which can only be constructed if four protein strands emanate from the structure s hub, rather than the three seen in clathrins. It also transpires that the proteins interact with each other at the vertices without any of the extensive interdigitation seen in clathrin cages. 

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

A EXPERIMENTAL FIGURE 17-8 Vesicle buds can be visualized during in vitro budding reactions. When purified COPII coat components are incubated with isolated ER vesicles or artificial phospholipid vesicles (liposomes), polymerization of the coat proteins on the vesicle surface induces emergence of highly curved buds. In this electron micrograph of an in vitro budding reaction, note the distinct membrane coat, visible as a dark protein layer, present on the vesicle buds. [From K. Matsuoka etal., 1988, Ce//93(2) 263.[... 

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. 

---