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{{Infobox_gene}}
{{PBB_Controls
'''Bok (Bcl-2 related ovarian killer)''' is a protein-coding gene of the [[Bcl-2 family]] that is found in many invertebrates and vertebrates. It induces [[apoptosis]], a special type of cell death. Currently, the precise function of Bok in this process is unknown.
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<!-- The GNF_Protein_box is automatically maintained by Protein Box Bot.  See Template:PBB_Controls to Stop updates. -->
== Discovery and homology ==
{{GNF_Protein_box
In 1997, the protein Bcl-2-related ovarian killer (Bok) was identified in a yeast [[Two-hybrid screening|two-hybrid experiment]] with a rat ovarian cDNA library in a screen for proteins interacting with Mcl-1, an abundant anti-apoptotic protein.<ref name="Hsu1997">{{cite journal | vauthors = Hsu SY, Kaipia A, McGee E, Lomeli M, Hsueh AJ | title = Bok is a pro-apoptotic Bcl-2 protein with restricted expression in reproductive tissues and heterodimerizes with selective anti-apoptotic Bcl-2 family members | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 94 | issue = 23 | pages = 12401–6 | date = November 1997 | pmid = 9356461 | pmc = 24966 | doi = 10.1073/pnas.94.23.12401 }}</ref> The overexpression of Bok induces [[apoptosis]]. Because of its high sequence similarity to [[Bcl-2 homologous antagonist killer|Bak]] and [[Bcl-2-associated X protein|Bax]],<ref name="Hsu1998">{{cite journal | vauthors = Hsu SY, Hsueh AJ | title = A splicing variant of the Bcl-2 member Bok with a truncated BH3 domain induces apoptosis but does not dimerize with antiapoptotic Bcl-2 proteins in vitro | journal = The Journal of Biological Chemistry | volume = 273 | issue = 46 | pages = 30139–46 | date = November 1998 | pmid = 9804769 | doi = 10.1074/jbc.273.46.30139 }}</ref> Bok is classified as a member of the Bcl-2 protein family.<ref name="Soleymanlou2005">{{cite journal | vauthors = Soleymanlou N, Wu Y, Wang JX, Todros T, Ietta F, Jurisicova A, Post M, Caniggia I | title = A novel Mtd splice isoform is responsible for trophoblast cell death in pre-eclampsia | journal = Cell Death and Differentiation | volume = 12 | issue = 5 | pages = 441–52 | date = May 2005 | pmid = 15775999 | pmc = | doi = 10.1038/sj.cdd.4401593 }}</ref>
| image =
The mouse homologue of Bok is called Matador (Mtd). This name is derived from the Latin term mactator which means butcher or killer.<ref name="Inohara1998">{{cite journal | vauthors = Inohara N, Ekhterae D, Garcia I, Carrio R, Merino J, Merry A, Chen S, Núñez G | title = Mtd, a novel Bcl-2 family member activates apoptosis in the absence of heterodimerization with Bcl-2 and Bcl-XL | journal = The Journal of Biological Chemistry | volume = 273 | issue = 15 | pages = 8705–10 | date = April 1998 | pmid = 9535847 | doi = 10.1074/jbc.273.15.8705 }}</ref> Additionally, homologous proteins were found in ''[[Drosophila melanogaster]]'' (fruit fly) and ''[[Gallus gallus]]'' (chicken).<ref name="Zhang2000">{{cite journal | vauthors = Zhang H, Holzgreve W, De Geyter C | title = Evolutionarily conserved Bok proteins in the Bcl-2 family | journal = FEBS Letters | volume = 480 | issue = 2-3 | pages = 311–3 | date = September 2000 | pmid = 11034351 | doi = 10.1016/s0014-5793(00)01921-9 }}</ref>
| image_source =
| PDB =
| Name = BCL2-related ovarian killer
| HGNCid = 1087
| Symbol = BOK
| AltSymbols =; BCL2L9; BOKL; MGC4631
| OMIM = 605404
| ECnumber =
| Homologene = 9632
| MGIid = 1858494
| GeneAtlas_image1 = PBB_GE_BOK_gnf1h00796_at_tn.png
| GeneAtlas_image2 = PBB_GE_BOK_gnf1h08469_s_at_tn.png
| Function = {{GNF_GO|id=GO:0046983 |text = protein dimerization activity}}
| Component = {{GNF_GO|id=GO:0005575 |text = cellular_component}}
| Process = {{GNF_GO|id=GO:0006917 |text = induction of apoptosis}} {{GNF_GO|id=GO:0042981 |text = regulation of apoptosis}}
| Orthologs = {{GNF_Ortholog_box
    | Hs_EntrezGene = 666
    | Hs_Ensembl = ENSG00000176720
    | Hs_RefseqProtein = NP_115904
    | Hs_RefseqmRNA = NM_032515
    | Hs_GenLoc_db =
    | Hs_GenLoc_chr = 2
    | Hs_GenLoc_start = 242146865
    | Hs_GenLoc_end = 242162222
    | Hs_Uniprot = Q9UMX3
    | Mm_EntrezGene = 51800
    | Mm_Ensembl = ENSMUSG00000026278
    | Mm_RefseqmRNA = NM_016778
    | Mm_RefseqProtein = NP_058058
    | Mm_GenLoc_db =
    | Mm_GenLoc_chr = 1
    | Mm_GenLoc_start = 95516100
    | Mm_GenLoc_end = 95526168
    | Mm_Uniprot = Q3TH93
  }}
}}
'''BCL2-related ovarian killer''', also known as '''BOK''', is a human [[gene]].<ref name="entrez">{{cite web | title = Entrez Gene: BOK BCL2-related ovarian killer| url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=666| accessdate = }}</ref>


<!-- The PBB_Summary template is automatically maintained by Protein Box Bot.  See Template:PBB_Controls to Stop updates. -->
== Promoter and gene structure ==
{{PBB_Summary
The human BOK [[promoter (genetics)|promoter]] is activated by the overexpression of members of the [[E2F]] hand transcription factor family. Typically, these transcription factors are involved in the promotion of S-phase, so there might be a connection between Bok expression and [[cell cycle]] progression.<ref name="Rodriguez2006">{{cite journal | vauthors = Rodriguez JM, Glozak MA, Ma Y, Cress WD | title = Bok, Bcl-2-related Ovarian Killer, Is Cell Cycle-regulated and Sensitizes to Stress-induced Apoptosis | journal = The Journal of Biological Chemistry | volume = 281 | issue = 32 | pages = 22729–35 | date = August 2006 | pmid = 16772296 | pmc = 2134790 | doi = 10.1074/jbc.M604705200 }}</ref> Due to this regulation of Bok expression by the cell cycle, it was proposed that Bok sensitizes growing cells to stress-induced apoptosis.<ref name="Rodriguez2006" />
| section_title =  
| summary_text = The protein encoded by this gene belongs to the BCL-2 protein family. BCL-2 family members form hetero- or homodimers and act as anti- and pro-apoptotic regulators that are involved in a wide variety of cellular activities. This protein contains all four BCL-2 like domains (BH1, 2, 3 and 4) and is a pro-apoptotic BCL-2 protein identified in the ovary.<ref name="entrez">{{cite web | title = Entrez Gene: BOK BCL2-related ovarian killer| url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=666| accessdate = }}</ref>
}}


==References==
Bok mRNA comprises five exons which code for a 213 amino acid protein,<ref name="Soleymanlou2005" /> called Bok-L. This protein consists of four Bcl-2 homology domains (abbreviated BH1, BH2, BH3, BH4, respectively) and a C-terminal transmembrane region <ref name="Soleymanlou2005" /> (Figure 1). Its BH3 domain contains a stretch with many leucine residues. This is unique among the Bcl-2 family members. The leucine-rich stretch functions as a nuclear export signal.<ref name="Bartholomeusz2006">{{cite journal | vauthors = Bartholomeusz G, Wu Y, Ali Seyed M, Xia W, Kwong KY, Hortobagyi G, Hung MC | title = Nuclear translocation of the pro-apoptotic Bcl-2 family member Bok induces apoptosis | journal = Molecular Carcinogenesis | volume = 45 | issue = 2 | pages = 73–83 | date = February 2006 | pmid = 16302269 | pmc =  | doi = 10.1002/mc.20156 }}</ref> It is recognized by the nuclear exportin Crm1. Mutations in the leucine-rich stretch impair the binding of Crm1 to Bok.<ref name="Bartholomeusz2006" /> Consequently, Bok accumulates in the nucleus and triggers apoptosis.<ref name="Bartholomeusz2006" />
{{reflist|2}}
==Further reading==
{{refbegin | 2}}
{{PBB_Further_reading
| citations =
*{{cite journal  | author=Hsu SY, Kaipia A, McGee E, ''et al.'' |title=Bok is a pro-apoptotic Bcl-2 protein with restricted expression in reproductive tissues and heterodimerizes with selective anti-apoptotic Bcl-2 family members. |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=94 |issue= 23 |pages= 12401-6 |year= 1997 |pmid= 9356461 |doi=  }}
*{{cite journal  | author=Inohara N, Ekhterae D, Garcia I, ''et al.'' |title=Mtd, a novel Bcl-2 family member activates apoptosis in the absence of heterodimerization with Bcl-2 and Bcl-XL. |journal=J. Biol. Chem. |volume=273 |issue= 15 |pages= 8705-10 |year= 1998 |pmid= 9535847 |doi=  }}
*{{cite journal  | author=Leo CP, Hsu SY, Chun SY, ''et al.'' |title=Characterization of the antiapoptotic Bcl-2 family member myeloid cell leukemia-1 (Mcl-1) and the stimulation of its message by gonadotropins in the rat ovary. |journal=Endocrinology |volume=140 |issue= 12 |pages= 5469-77 |year= 1999 |pmid= 10579309 |doi=  }}
*{{cite journal  | author=Zhang H, Huang Q, Ke N, ''et al.'' |title=Drosophila pro-apoptotic Bcl-2/Bax homologue reveals evolutionary conservation of cell death mechanisms. |journal=J. Biol. Chem. |volume=275 |issue= 35 |pages= 27303-6 |year= 2000 |pmid= 10811653 |doi= 10.1074/jbc.M002846200 }}
*{{cite journal  | author=Zhang H, Holzgreve W, De Geyter C |title=Evolutionarily conserved Bok proteins in the Bcl-2 family. |journal=FEBS Lett. |volume=480 |issue= 2-3 |pages= 311-3 |year= 2000 |pmid= 11034351 |doi=  }}
*{{cite journal  | author=Strausberg RL, Feingold EA, Grouse LH, ''et al.'' |title=Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences. |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |issue= 26 |pages= 16899-903 |year= 2003 |pmid= 12477932 |doi= 10.1073/pnas.242603899 }}
*{{cite journal | author=Yakovlev AG, Di Giovanni S, Wang G, ''et al.'' |title=BOK and NOXA are essential mediators of p53-dependent apoptosis. |journal=J. Biol. Chem. |volume=279 |issue= 27 |pages= 28367-74 |year= 2004 |pmid= 15102863 |doi= 10.1074/jbc.M313526200 }}
*{{cite journal  | author=Gerhard DS, Wagner L, Feingold EA, ''et al.'' |title=The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC). |journal=Genome Res. |volume=14 |issue= 10B |pages= 2121-7 |year= 2004 |pmid= 15489334 |doi= 10.1101/gr.2596504 }}
*{{cite journal  | author=Soleymanlou N, Wu Y, Wang JX, ''et al.'' |title=A novel Mtd splice isoform is responsible for trophoblast cell death in pre-eclampsia. |journal=Cell Death Differ. |volume=12 |issue= 5 |pages= 441-52 |year= 2005 |pmid= 15775999 |doi= 10.1038/sj.cdd.4401593 }}
*{{cite journal  | author=Gao S, Fu W, Dürrenberger M, ''et al.'' |title=Membrane translocation and oligomerization of hBok are triggered in response to apoptotic stimuli and Bnip3. |journal=Cell. Mol. Life Sci. |volume=62 |issue= 9 |pages= 1015-24 |year= 2005 |pmid= 15868100 |doi= 10.1007/s00018-005-4543-3 }}
*{{cite journal  | author=Bartholomeusz G, Wu Y, Ali Seyed M, ''et al.'' |title=Nuclear translocation of the pro-apoptotic Bcl-2 family member Bok induces apoptosis. |journal=Mol. Carcinog. |volume=45 |issue= 2 |pages= 73-83 |year= 2006 |pmid= 16302269 |doi= 10.1002/mc.20156 }}
*{{cite journal  | author=Rodriguez JM, Glozak MA, Ma Y, Cress WD |title=Bok, Bcl-2-related Ovarian Killer, Is Cell Cycle-regulated and Sensitizes to Stress-induced Apoptosis. |journal=J. Biol. Chem. |volume=281 |issue= 32 |pages= 22729-35 |year= 2006 |pmid= 16772296 |doi= 10.1074/jbc.M604705200 }}
}}
{{refend}}


{{protein-stub}}
== Splice variants ==
{{WikiDoc Sources}}
Due to [[alternative splicing]], ''Bok'' mRNA gives rise to different Bok proteins: Figure 1 illustrates the different splice variants schematically. Full length Bok is named Bok-L.
 
[[File:Splice variants of Bok.png|thumb|right|350px|'''Figure 1: Splice variants of ''Bok'' mRNA and the resulting proteins'''. ''Bok'' mRNA comprises five exons and two alternative start codons for translation (AUG). Exons are visualized as arrows and boxes in different colors; ovals indicate sequences that code for Bcl-2 homology domains (BH1, BH2, BH3, BH4) or the transmembrane domain (TM). In the Bok-S variant, the BH3 domain is truncated and fused to the shortened BH1 domain. Another known variant, Bok-P, lacks the BH4 domain and contains a truncated BH3 domain.]]
 
The shorter version, Bok-S, lacks exon 3. This results in a fusion of the BH3 domain with the BH1 domain.<ref name="Hsu1998" /><ref name="Hsu2000">{{cite journal | vauthors = Hsu SY, Hsueh AJ | title = Tissue-specific Bcl-2 protein partners in apoptosis: An ovarian paradigm | journal = Physiological Reviews | volume = 80 | issue = 2 | pages = 593–614 | date = April 2000 | pmid = 10747202 }}</ref> The BH3 domain is involved in the interaction of Bok with Mcl-1 and other molecules. It is dispensable for the induction of apoptosis.<ref name="Hsu2000" /> Expression of Bok-S may be an immediate response to stress signals. It has been shown to induce apoptosis regardless of the presence of anti-apoptotic molecules.<ref name="Hsu1998" />
 
Another splice variant termed Bok-P was found in [[placenta]]l tissue from patients suffering from pre-eclampsia. While Bok-S misses exon 3, Bok-P lacks exon 2. This deletion includes the BH4 domain and parts of the BH3 domain. Bok-P may be the cause for trophoblast cell death in [[pre-eclampsia]],<ref name="Soleymanlou2005" /> a dangerous pregnancy complication. In pre-eclampsia, typical alterations occur in the maternal kidney and lead to hypertension and proteins in the urine. To date, the cause of this medical condition as well as an appropriate treatment have not been discovered.
 
== Expression pattern ==
The ''Bok'' gene is activated and produces protein in different tissues. In mice, elevated Bok levels were detected in the [[ovary]], the [[testis]], and the [[uterus]].<ref name="Hsu1997" /> Nevertheless, it also exists in the [[brain]] and at low levels in most other tissues.<ref name="Ke2012">{{cite journal | vauthors = Ke F, Voss A, Kerr JB, O'Reilly LA, Tai L, Echeverry N, Bouillet P, Strasser A, Kaufmann T | title = BCL-2 family member BOK is widely expressed but its loss has only minimal impact in mice | journal = Cell Death and Differentiation | volume = 19 | issue = 6 | pages = 915–25 | date = June 2012 | pmid = 22281706 | pmc = 3354060 | doi = 10.1038/cdd.2011.210 }}</ref> However, the expression pattern of the Bok gene varies among species.
 
In humans, Bok is found in a wide range of tissues. The gene is expressed in the [[Colon (anatomy)|colon]], the [[stomach]], the [[testes]], the [[placenta]], the [[pancreas]], the [[ovary|ovaries]], and the [[uterus]].<ref name="Gao2005">{{cite journal | vauthors = Gao S, Fu W, Dürrenberger M, De Geyter C, Zhang H | title = Membrane translocation and oligomerization of hBok are triggered in response to apoptotic stimuli and Bnip3 | journal = Cellular and Molecular Life Sciences | volume = 62 | issue = 9 | pages = 1015–24 | date = May 2005 | pmid = 15868100 | pmc =  | doi = 10.1007/s00018-005-4543-3 }}</ref> Furthermore,  more Bok is expressed in fetal tissue compared to adult tissue. Thus, Bok may influence [[embryonic development|development]].
 
== Subcellular localization ==
The subcellular localization of Bok protein is controversial. In [[cell growth|proliferating cells]], Bok is found in the [[Cell nucleus|nucleus]].<ref name="Ray2010">{{cite journal | vauthors = Ray JE, Garcia J, Jurisicova A, Caniggia I | title = Mtd/Bok takes a swing: proapoptotic Mtd/Bok regulates trophoblast cell proliferation during human placental development and in preeclampsia | journal = Cell Death and Differentiation | volume = 17 | issue = 5 | pages = 846–59 | date = May 2010 | pmid = 19942931 | pmc =  | doi = 10.1038/cdd.2009.167 }}</ref> Upon induction of apoptosis, it was found to tightly associate with [[mitochondrial membrane]]s.<ref name="Gao2005" /><ref name="Ray2010"/> On the other hand, another group found Bok shuttling between the [[cytoplasm]] and the nucleus. In their experiments, increased nuclear (not mitochondrial) localization correlated with a stronger apoptotic activity.<ref name="Bartholomeusz2006" />
 
== Regulation ==
It was found that the cellular ratio of pro-apoptotic to anti-apoptotic Bcl-2 family members effects late [[apoptosis|apoptotic events]] such as release of [[cytochrome c]] from the mitochondria and the activation of caspases. Higher levels of pro-apoptotic proteins compared to anti-apoptotic proteins seem to cause apoptosis. In a current model, the formation of [[heterodimer]]s between pro-apoptotic and anti-apoptotic proteins prevents induction of apoptosis.<ref name="Hsu2000" />
 
== Interactions ==
The binding of Bok to its interacting partners seems to be mediated by its BH3 domain.<ref name="Soleymanlou2005" /> The splice variant Bok-S lacks this domain and is unable to form heterodimers with other proteins of the Bcl-2 family.
 
In yeast two-hybrid experiments, Bok was found to interact with the anti-apoptotic proteins Mcl-1, BHRF-1, and Bfl-1. However, interactions with other anti-apoptotic proteins such as Bcl-2, Bcl-xL, and Bcl-w were not detectable  (1). Later studies aimed at confirming an interaction between Bok and pro-apoptotic [[Bcl-2 homologous antagonist killer|Bak]] or [[Bcl-2-associated X protein|Bax]] but were not successful.<ref name="Inohara1998" />
 
Accordingly, coexpression of anti-apoptotic proteins such as Mcl-1 suppresses apoptosis induced by Bok overexpression.<ref name="Hsu1997" /> Consistent with the results mentioned above, coexpression of anti-apoptotic Bcl-2 does not prevent Bok-induced apoptosis.<ref name="Hsu1997" />
 
== Knock-out mouse ==
Since its discovery in 1997, several attempts have been made to characterize Bok. Due to the increased expression levels in fetal tissue, scientists anticipated a [[embryonic development|developmental]] role for Bok. Recently, the Bok [[knock-out mouse]] was created. This mouse shows, however, no developmental defects and normal fertility.<ref name="Ke2012" /> This finding indicates that the function of Bok seems to overlap with the function of the related pro-apoptotic proteins [[Bcl-2 homologous antagonist killer|Bak]] and [[Bcl-2-associated X protein|Bax]].
 
Several other roles were proposed for Bok, especially in developing cells.<ref name="Hsu2000" /><ref name="Suominen2001">{{cite journal | vauthors = Suominen JS, Yan W, Toppari J, Kaipia A | title = The expression and regulation of Bcl-2-related ovarian killer (Bok) mRNA in the developing and adult rat testis | journal = European Journal of Endocrinology | volume = 145 | issue = 6 | pages = 771–8 | date = December 2001 | pmid = 11720903 | doi = 10.1530/eje.0.1450771 }}</ref><ref name="Soane2008">{{cite journal | vauthors = Soane L, Siegel ZT, Schuh RA, Fiskum G | title = Postnatal developmental regulation of Bcl-2 family proteins in brain mitochondria | journal = Journal of Neuroscience Research | volume = 86 | issue = 6 | pages = 1267–76 | date = May 2008 | pmid = 18058945 | pmc = 2566804 | doi = 10.1002/jnr.21584 }}</ref><ref name="Jaaskelainen2010">{{cite journal | vauthors = Jääskeläinen M, Nieminen A, Pökkylä RM, Kauppinen M, Liakka A, Heikinheimo M, Vaskivuo TE, Klefström J, Tapanainen JS | title = Regulation of cell death in human fetal and adult ovaries--role of Bok and Bcl-X(L) | journal = Molecular and Cellular Endocrinology | volume = 330 | issue = 1-2 | pages = 17–24 | date = December 2010 | pmid = 20673843 | pmc =  | doi = 10.1016/j.mce.2010.07.020 }}</ref> Since the action of Bok in triggering apoptosis seems to be redundant, it is difficult to assign a specific role to Bok in the presence of Bak and Bax. The study of cells deficient in Bak and Bok or deficient in Bax and Bok, respectively, could help to better characterize the role of Bok in apoptosis. If Bok exerts a critical function, it is likely that this function is limited to certain circumstances, e.g. specific cell types, stress conditions. Thus, these aspects should be assessed in more detail to analyze the physiological and pathological role of Bok.
 
== References ==
{{reflist|33em}}
 
== External links ==
* {{UCSC gene info|BOK}}
 
[[Category:Genes]]
[[Category:Protein families]]

Latest revision as of 03:43, 18 February 2017

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Identifiers
Aliases
External IDsGeneCards: [1]
Orthologs
SpeciesHumanMouse
Entrez
Ensembl
UniProt
RefSeq (mRNA)

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n/a

RefSeq (protein)

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Location (UCSC)n/an/a
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Bok (Bcl-2 related ovarian killer) is a protein-coding gene of the Bcl-2 family that is found in many invertebrates and vertebrates. It induces apoptosis, a special type of cell death. Currently, the precise function of Bok in this process is unknown.

Discovery and homology

In 1997, the protein Bcl-2-related ovarian killer (Bok) was identified in a yeast two-hybrid experiment with a rat ovarian cDNA library in a screen for proteins interacting with Mcl-1, an abundant anti-apoptotic protein.[1] The overexpression of Bok induces apoptosis. Because of its high sequence similarity to Bak and Bax,[2] Bok is classified as a member of the Bcl-2 protein family.[3] The mouse homologue of Bok is called Matador (Mtd). This name is derived from the Latin term mactator which means butcher or killer.[4] Additionally, homologous proteins were found in Drosophila melanogaster (fruit fly) and Gallus gallus (chicken).[5]

Promoter and gene structure

The human BOK promoter is activated by the overexpression of members of the E2F hand transcription factor family. Typically, these transcription factors are involved in the promotion of S-phase, so there might be a connection between Bok expression and cell cycle progression.[6] Due to this regulation of Bok expression by the cell cycle, it was proposed that Bok sensitizes growing cells to stress-induced apoptosis.[6]

Bok mRNA comprises five exons which code for a 213 amino acid protein,[3] called Bok-L. This protein consists of four Bcl-2 homology domains (abbreviated BH1, BH2, BH3, BH4, respectively) and a C-terminal transmembrane region [3] (Figure 1). Its BH3 domain contains a stretch with many leucine residues. This is unique among the Bcl-2 family members. The leucine-rich stretch functions as a nuclear export signal.[7] It is recognized by the nuclear exportin Crm1. Mutations in the leucine-rich stretch impair the binding of Crm1 to Bok.[7] Consequently, Bok accumulates in the nucleus and triggers apoptosis.[7]

Splice variants

Due to alternative splicing, Bok mRNA gives rise to different Bok proteins: Figure 1 illustrates the different splice variants schematically. Full length Bok is named Bok-L.

File:Splice variants of Bok.png
Figure 1: Splice variants of Bok mRNA and the resulting proteins. Bok mRNA comprises five exons and two alternative start codons for translation (AUG). Exons are visualized as arrows and boxes in different colors; ovals indicate sequences that code for Bcl-2 homology domains (BH1, BH2, BH3, BH4) or the transmembrane domain (TM). In the Bok-S variant, the BH3 domain is truncated and fused to the shortened BH1 domain. Another known variant, Bok-P, lacks the BH4 domain and contains a truncated BH3 domain.

The shorter version, Bok-S, lacks exon 3. This results in a fusion of the BH3 domain with the BH1 domain.[2][8] The BH3 domain is involved in the interaction of Bok with Mcl-1 and other molecules. It is dispensable for the induction of apoptosis.[8] Expression of Bok-S may be an immediate response to stress signals. It has been shown to induce apoptosis regardless of the presence of anti-apoptotic molecules.[2]

Another splice variant termed Bok-P was found in placental tissue from patients suffering from pre-eclampsia. While Bok-S misses exon 3, Bok-P lacks exon 2. This deletion includes the BH4 domain and parts of the BH3 domain. Bok-P may be the cause for trophoblast cell death in pre-eclampsia,[3] a dangerous pregnancy complication. In pre-eclampsia, typical alterations occur in the maternal kidney and lead to hypertension and proteins in the urine. To date, the cause of this medical condition as well as an appropriate treatment have not been discovered.

Expression pattern

The Bok gene is activated and produces protein in different tissues. In mice, elevated Bok levels were detected in the ovary, the testis, and the uterus.[1] Nevertheless, it also exists in the brain and at low levels in most other tissues.[9] However, the expression pattern of the Bok gene varies among species.

In humans, Bok is found in a wide range of tissues. The gene is expressed in the colon, the stomach, the testes, the placenta, the pancreas, the ovaries, and the uterus.[10] Furthermore, more Bok is expressed in fetal tissue compared to adult tissue. Thus, Bok may influence development.

Subcellular localization

The subcellular localization of Bok protein is controversial. In proliferating cells, Bok is found in the nucleus.[11] Upon induction of apoptosis, it was found to tightly associate with mitochondrial membranes.[10][11] On the other hand, another group found Bok shuttling between the cytoplasm and the nucleus. In their experiments, increased nuclear (not mitochondrial) localization correlated with a stronger apoptotic activity.[7]

Regulation

It was found that the cellular ratio of pro-apoptotic to anti-apoptotic Bcl-2 family members effects late apoptotic events such as release of cytochrome c from the mitochondria and the activation of caspases. Higher levels of pro-apoptotic proteins compared to anti-apoptotic proteins seem to cause apoptosis. In a current model, the formation of heterodimers between pro-apoptotic and anti-apoptotic proteins prevents induction of apoptosis.[8]

Interactions

The binding of Bok to its interacting partners seems to be mediated by its BH3 domain.[3] The splice variant Bok-S lacks this domain and is unable to form heterodimers with other proteins of the Bcl-2 family.

In yeast two-hybrid experiments, Bok was found to interact with the anti-apoptotic proteins Mcl-1, BHRF-1, and Bfl-1. However, interactions with other anti-apoptotic proteins such as Bcl-2, Bcl-xL, and Bcl-w were not detectable (1). Later studies aimed at confirming an interaction between Bok and pro-apoptotic Bak or Bax but were not successful.[4]

Accordingly, coexpression of anti-apoptotic proteins such as Mcl-1 suppresses apoptosis induced by Bok overexpression.[1] Consistent with the results mentioned above, coexpression of anti-apoptotic Bcl-2 does not prevent Bok-induced apoptosis.[1]

Knock-out mouse

Since its discovery in 1997, several attempts have been made to characterize Bok. Due to the increased expression levels in fetal tissue, scientists anticipated a developmental role for Bok. Recently, the Bok knock-out mouse was created. This mouse shows, however, no developmental defects and normal fertility.[9] This finding indicates that the function of Bok seems to overlap with the function of the related pro-apoptotic proteins Bak and Bax.

Several other roles were proposed for Bok, especially in developing cells.[8][12][13][14] Since the action of Bok in triggering apoptosis seems to be redundant, it is difficult to assign a specific role to Bok in the presence of Bak and Bax. The study of cells deficient in Bak and Bok or deficient in Bax and Bok, respectively, could help to better characterize the role of Bok in apoptosis. If Bok exerts a critical function, it is likely that this function is limited to certain circumstances, e.g. specific cell types, stress conditions. Thus, these aspects should be assessed in more detail to analyze the physiological and pathological role of Bok.

References

  1. 1.0 1.1 1.2 1.3 Hsu SY, Kaipia A, McGee E, Lomeli M, Hsueh AJ (November 1997). "Bok is a pro-apoptotic Bcl-2 protein with restricted expression in reproductive tissues and heterodimerizes with selective anti-apoptotic Bcl-2 family members". Proceedings of the National Academy of Sciences of the United States of America. 94 (23): 12401–6. doi:10.1073/pnas.94.23.12401. PMC 24966. PMID 9356461.
  2. 2.0 2.1 2.2 Hsu SY, Hsueh AJ (November 1998). "A splicing variant of the Bcl-2 member Bok with a truncated BH3 domain induces apoptosis but does not dimerize with antiapoptotic Bcl-2 proteins in vitro". The Journal of Biological Chemistry. 273 (46): 30139–46. doi:10.1074/jbc.273.46.30139. PMID 9804769.
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