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{{Short description|Cell structure involved in protein transport}}
{{for|the 1996 United Nations Climate Change Conference in Geneva, Switzerland|COP 2 (climate conference)}}

The '''coat protein complex II''', or '''COPII''', is a group of [[protein]]s that facilitate the formation of [[Vesicle (biology and chemistry)|vesicle]]s to transport proteins from the [[endoplasmic reticulum]] to the [[Golgi apparatus]] or [[endoplasmic-reticulum–Golgi intermediate compartment]]. This process is termed [[Axoplasmic transport#Anterograde transport|anterograde transport]], in contrast to the [[Axoplasmic transport#Retrograde transport|retrograde transport]] associated with the [[COPI]] complex. COPII is assembled in two parts: first an inner layer of Sar1, Sec23, and Sec24 forms; then the inner coat is surrounded by an outer lattice of Sec13 and Sec31.

==Function==
The COPII coat is responsible for the formation of vesicles from the endoplasmic reticulum (ER). These vesicles transport cargo proteins to the Golgi apparatus (in yeast) or the endoplasmic-reticulum-Golgi intermediate compartment (ERGIC, in mammals).<ref name=Peotter2019/>

Coat assembly is initiated when the [[cytosol]]ic [[Ras GTPase]] Sar1 is activated by its [[guanine nucleotide exchange factor]] Sec12.<ref name=Peotter2019/> Activated Sar1-GTP inserts itself into the ER membrane, binding preferentially to areas of membrane curvature. As Sar1-GTP inserts into the membrane, it recruits Sec23 and Sec24 to make up the inner cage.<ref name=Peotter2019/> Once the inner coat is assembled, the outer coat proteins Sec13 and Sec31 are recruited to the budding vesicle.<ref name=Peotter2019/> Hydrolysis of the Sar1 GTP to GDP promotes disassembly of the coat.

Some proteins are found to be responsible for selectively packaging cargos into COPII vesicles. More recent research suggests the Sec23/Sec24-Sar1 complex participates in cargo selection.{{r|Fath 2007}} For example, Erv29p in ''Saccharomyces cerevisiae'' is found to be necessary for packaging glycosylated pro-α-factor.<ref name="pmid11711675">{{cite journal | vauthors = Belden WJ, Barlowe C | title = Role of Erv29p in collecting soluble secretory proteins into ER-derived transport vesicles | journal = Science | volume = 294 | issue = 5546 | pages = 1528–31 | date = November 2001 | pmid = 11711675 | doi = 10.1126/science.1065224 | bibcode = 2001Sci...294.1528B | s2cid = 29870942 }}</ref>

Sec24 proteins recognize various cargo proteins, packaging them into the budding vesicles.

==Structure==
[[File:PDB 2gao EBI.jpg|thumb|Human Sar1A bound to GDP]]
The COPII coat consists of an inner layer – a flexible meshwork of Sar1, Sec23, and Sec24 – and an outer layer made of Sec13 and Sec31.<ref name=Peotter2019>{{cite journal |vauthors=Peotter J, Kasberg W, Pustova I, Audhya A |title=COPII-mediated trafficking at the ER/ERGIC interface |journal=Traffic |volume=20 |issue=7 |pages=491–503 |date=July 2019 |pmid=31059169 |pmc=6640837 |doi=10.1111/tra.12654}}</ref> Sar1 resembles other Ras-family GTPases, with a core of six [[beta strand]]s flanked by three [[alpha helix|alpha helices]], and two flexible "switch domains". Unlike other Ras GTPases, Sar1 inserts into membranes via an N-terminal helix (rather than [[myristoylation]] or [[prenylation]]).<ref name=Peotter2019/>

These coat proteins are necessary but insufficient to direct or dock the vesicle to the correct target membrane.  [[SNARE (protein)|SNARE]], cargo, and other proteins are also needed for these processes to occur.

Pre-budding complex (composed of Sar1-GTP and Sec23/24) recruits the flexible Sec13p/31p complex, characterized by polymerization of the Sec13/31 complex with other Sec13/31 complexes to form a [[cuboctahedron]] with a broader lattice than its [[Clathrin]] vesicle analog. The formation of the cuboctahedron deforms the ER membrane and detaches the COPII vesicle (alongside cargo proteins and v-SNAREs), completing the COPII vesicle budding process.<ref name="Fath 2007">{{cite journal | vauthors = Fath S, Mancias JD, Bi X, Goldberg J | title = Structure and organization of coat proteins in the COPII cage | journal = Cell | volume = 129 | issue = 7 | pages = 1325–36 | date = June 2007 | pmid = 17604721 | doi = 10.1016/j.cell.2007.05.036 | s2cid = 10692166 | doi-access = free }}</ref>

==Regulation==
The signal(s) that triggers Sec12 to initiate COPII assembly remains unclear, though some regulators of coat formation are now known.<ref name=Lodish14>{{cite book|chapter=14 - Vesicular Traffic, Secretion, and Endocytosis |title=Molecular Cell Biology |edition=8 |pages=639–641 |vauthors=Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A, Martin KC |publisher=W. H Freeman |location=New York |date=2016 |isbn=9781464183393}}</ref> The frequency of COPII formation is regulated in part by Sec16A and [[Tango1]] proteins, likely by concentrating Sec12 in a given location, so it can more efficiently activate Sar1.<ref name=Peotter2019/>

==Evolution==
In mammals there are two Sar1 genes: ''SAR1A'' and ''SAR1B'' (''SAR1B'' was previously known as ''SARA2''<ref>{{cite web|url=https://www.genecards.org/cgi-bin/carddisp.pl?gene=SAR1B|title=''SAR1B'' Gene - Secretion Associated Ras Related GTPase 1B|website=GeneCards: The Human Gene Database|date=4 October 2023|access-date=7 December 2023}}</ref>). In cultured mammalian cells the two Sar1 genes appear redundant; however, in animals SAR1B is uniquely required for the formation of large (over 1 [[micrometre|micrometer]] across) COPII-coated vesicles.<ref name=Peotter2019/>

Similarly, mammals express two Sec23 genes: ''SEC23A'' and ''SEC23B''. The two Sec23 isoforms have identical function but are expressed in different body tissues. Both Sec23 proteins can interact with any of the four Sec24 proteins: SEC24A, SEC24B, SEC24C, and SEC24D.<ref name=Peotter2019/>

==Role in disease==
Lethal or pathogenic variants of most COPII proteins have been described. Loss of Sar1B in mice results in death soon after birth.<ref name=Lu2020>{{cite journal |vauthors=Lu CL, Kim J |title=Consequences of mutations in the genes of the ER export machinery COPII in vertebrates |journal=Cell Stress Chaperones |volume=25 |issue=2 |pages=199–209 |date=March 2020 |pmid=31970693 |pmc=7058761 |doi=10.1007/s12192-019-01062-3 |url=}}</ref> In humans, inheriting two copies of certain SAR1B variants results in [[Chylomicron retention disease]],<ref name=Peotter2019/> and loss of Sar1B causes a combination of chylomicron retention disease and the neuromuscular disorder [[Marinesco–Sjögren syndrome]].<ref name=Lu2020/>

Loss of Sec23A is lethal to mice ''in utero''.<ref name=Lu2020/> In humans, a Sec23A variant causes [[Cranio-lenticulo-sutural dysplasia]], while Sec23B variants are associated with the bone marrow disease [[congenital dyserythropoietic anemia type II]] and some [[cancer]]s.<ref name=Lu2020/><ref name=Peotter2019/> Mice without Sec23B die soon after birth.<ref name=Lu2020/> [[Halperin-Birk syndrome]] (HLBKS), a rare autosomal recessive neurodevelopmental disorder, is caused by a null mutation in the SEC31A.<ref>{{Cite journal |last1=Halperin |first1=Daniel |last2=Kadir |first2=Rotem |last3=Perez |first3=Yonatan |last4=Drabkin |first4=Max |last5=Yogev |first5=Yuval |last6=Wormser |first6=Ohad |last7=Berman |first7=Erez M |last8=Eremenko |first8=Ekaterina |last9=Rotblat |first9=Barak |last10=Shorer |first10=Zamir |last11=Gradstein |first11=Libe |last12=Shelef |first12=Ilan |last13=Birk |first13=Ruth |last14=Abdu |first14=Uri |last15=Flusser |first15=Hagit |date=2018-11-21 |title=''SEC31A'' mutation affects ER homeostasis, causing a neurological syndrome |url=http://dx.doi.org/10.1136/jmedgenet-2018-105503 |journal=Journal of Medical Genetics |volume=56 |issue=3 |pages=139–148 |doi=10.1136/jmedgenet-2018-105503 |pmid=30464055 |s2cid=53717389 |issn=0022-2593|url-access=subscription }}</ref>

==Conformational changes==
CopII has three specific binding sites that can each be complexed.  The adjacent picture (Sed5) uses the Sec22 t-SNARE complex to bind.  This site is more strongly bound, and therefore is more favored. (Embo)

{{Gallery
|title=Crystal structures of CopII
|footer=
|width=250
|Image:CopIIcomplexbet1.png|Conformation of the CopII protein complexed with the [[SNARE (protein)|snare]] protein [[BET1|Bet1]] ({{PDB|1PCX}}).<ref name="pmid12941276">{{PDB2|1PCX}}; {{PDB2|1PD0}}; {{cite journal | vauthors = Mossessova E, Bickford LC, Goldberg J | title = SNARE selectivity of the COPII coat | journal = Cell | volume = 114 | issue = 4 | pages = 483–95 |date=August 2003 | pmid = 12941276 | doi = 10.1016/S0092-8674(03)00608-1 | s2cid = 11379372 | doi-access = free }}</ref>
|Image:CopIIcomplexsedt5.png|Conformation of the CopII protein that is complexed with the snare protein [[STX5|Sed5]] ({{PDB|1PD0}}).<ref name="pmid12941276"/>
}}

==Research==
Mutations the [[threonine]] at position 39 to asparagine generates a [[dominant negative]] Sar1A bound permanently to GDP; mutating histidine 79 to glycine generates a constitutively active Sar1A, with GTP hydrolysis slowed dramatically.<ref name=Peotter2019/>

==See also==
*[[COPI]] vesicles
*[[Clathrin]] vesicles

==References==
{{Reflist}}

{{Vesicular transport proteins}}

{{DEFAULTSORT:Copii}}
[[Category:Cell biology]]