TMEM8B

Transmembrane protein 8B is a protein that in humans is encoded by the TMEM8B gene. It encodes for a transmembrane protein that is 338 amino acids long, and is located on human chromosome 9.[5] Aliases associated with this gene include C9orf127, NAG-5, and NGX61.[6]

TMEM8B
Identifiers
AliasesTMEM8B, C9orf127, NAG-5, NGX6, NAG5, transmembrane protein 8B
External IDsOMIM: 616888 MGI: 2441680 HomoloGene: 72894 GeneCards: TMEM8B
Gene location (Human)
Chr.Chromosome 9 (human)[1]
Band9p13.3Start35,814,451 bp[1]
End35,865,518 bp[1]
RNA expression pattern
More reference expression data
Orthologs
SpeciesHumanMouse
Entrez

51754

242409

Ensembl

ENSG00000137103

ENSMUSG00000078716

UniProt

A6NDV4

B1AWJ5

RefSeq (mRNA)

NM_001085508
NM_001355718
NM_001368840
NM_001368841
NM_001368842

RefSeq (protein)

NP_001078977
NP_001342647
NP_001355769
NP_001355770
NP_001355771

Location (UCSC)Chr 9: 35.81 – 35.87 MbChr 4: 43.67 – 43.69 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Gene

Location

Cytogenic location: 9p13.3[7] Located on chromosome 9 in the human genome. It starts at base pair 35,814,451, and ends at 35,865,518, and contains 19 exons. There are 13 transcript variants that are protein encoding, and the longest transcript variant is 790 amino acids long.

Expression

Using information from NCBI's EST Abundance Profile page on TMEM8B, expression levels vary in 32 different human tissues. The highest levels of expression can be found in the brain, ovaries, prostate, placenta, and the pancreas.[8] Expression levels are down regulated in some cancerous tissue, specifically nasopharyngeal and colorectal carcinomas. TMEM8B is expressed in all stages of development, including fetal stages, as low levels of expression are present in the fetal liver, brain, and thymus.[8]

mRNA

Splice Variants

TMEM8B has 13 known mRNA splice variants in humans: Refer to the table below. All 13 variants are protein encoding, and all contain 19 exons.

NameAccession NumberAmino Acid LengthmRNA
Isoform ANP_001036055.1472NM_001042589.2
Isoform BNP_057530.2338NM_016446.3
Isoform X1XP_011516213.1508XM_011517911.2
Isoform X2XP_011516204.1498XM_011517902.2
Isoform X3XP_024303339.1482XM_024447571.1
Isoform X4XP_011516205.1399XM_011517903.2
Isoform X5XP_024303338.1373XM_024447570.1
Isoform X6XP_011516206.1790XM_011517904.3
Isoform X7XP_011516207.1334XM_011517905.1
Isoform X8XP_016870294.1675XM_017014805.1
Isoform X9XP_011516218.1450XM_011517916.2
Isoform X10XP_016870296.1406XM_017014807.1
Isoform X11XP_011516220.1398XM_011517918.3

The figure below from NCBI Gene depicts the chromosomal location of each isoform in comparison to TMEM8B.

Figure 1: Location of each isoform relative to the original TMEM8B.

Protein

Protein Analysis

Protein analysis was completed on Isoform A. TMEM8B isoform A is 472 amino acids long. The molecular weight is 36.8 kDa,[9] and the isoelectric point is 6.773.[10] There are 7 transmembrane domains, resulting in 52% of the protein to be within the plasma membrane.[11] The C-charge> N-charge, and therefore the C-terminal end is on the inside. Transmembrane domains are conserved in most orthologs, including all mammals. Relative to other proteins, TMEM8B has higher than normal levels of K, Lysine, and L, Leucine.[9] There are three repeating leucine-rich regions within conserved domains of TMEM8B, all 4 amino acids long. Leucine rich regions can result in hydrophobic interactions within themselves.[12]

Secondary Structure

Identifying the secondary structure is helpful in further analyzing the function of this protein. Alpha helices are the strongest indicators of transmembrane regions, as the helical structure can satisfy all backbone hydrogen-bonds internally. This is why the secondary structure of this protein is practical, as many of the alpha helices lie in the predicted transmembrane regions. Other key structures identified in this protein include extended strands, which are hypothesized to be important folding regions, and random coils, a class of conformations in the absence of a regular secondary structure.

Figure 2: Predicted secondary structure of TMEM8B, Isoform A. Predicted transmembrane domains are highlighted in green

Tertiary Structure

I-TASSER[13] predicted the 3D tertiary structure of TMEM8B, with strategic folding of the alpha helices and beta sheets. Although there are no high scoring hydrophobic segments of TMEM8B, that would usually be hidden within the interior of the 3D structure, the high amounts of Leuceine (L) amino acids in this protein creates hydrophobic interactions with itself, and these areas are predicted to be buried on the inside of the structure.[12] Refer to the figure below to see a predicted tertiary structure.

Figure 3: Predicted tertiary structure of TMEM8B.

TMEM8B highly resembles a tertiary structure that is similar to the Reelin protein, predicted by a 42% coverage and 14.79% identity[14] The Reelin protein has no transmembrane domains, and is mostly found in the cerebral cortex and the hippocampus, where it plays important roles in the control of neuronal migration and formation of cellular layers during brain development.

Figure 4: Multiple sequence alignment of the Reelin protein and TMEM8B, color coordinating with the tertiary structure.

Homologogy

Orthologs

The orthologs of TMEM8B were sequenced in BLAST[15] and 20 various orthologs were picked. The orthologs are all multicellular organisms, and vary through mammals, rodents, birds, fish, amphibians, echinoderms, chordates, insects, and cnidarians. Refer to the table below. Time tree was a program that was used to find the evolutionary branching shown in MYA,[16] and conserved domains of the genome were found and analyzed using ClustalW.[17]

Genus SpeciesCommon NameDivergence from Humans (MYA)Accession NumberAmino Acid LengthSequence IdentitySequence Similarity
Homo sapiensHumans--EAW58325.1338----
Carlito syrichtaPhilippine tarsier67.1XP_008061336.227396%97%
Trichechus manatus latirostrisFlorida manatee105XP_004372337.127396%97%
Neomonachus schauinslandiHawaiian monk seal96XP_021546789.128096%96%
Pelecanus CrispusDalmatian pelican312XP_009481450.121975%86%
Salmo salarAtlantic salmon435XP_013999021.149468%86%
Struthio camelus australisSouthern ostrich312XP_009675834.128370%81%
Cariama cristataRed-legged seriema312XP_009701221.128068%80%
Egretta garzettaLittle egret312XP_009645653.128268%79%
Sinocyclocheilus grahamGolden line fish435XP_016091386.129562%76%
Charadrius vociferusKildeer312XP_009889203.142063%75%
Chrysochloris asiaticaCape golden mole105XP_006863153.139293%75%
Branchiostoma belcheriBelcher's Lancelet684XP_019646192.120937%54%
Xenopus laevisAfrican clawed frog352XP_018123357.148065%50%
Diachasma alloeumCommon house spider797XP_015126938.125229%47%
Megachile rotundataAlfalfa leafcutting bee797XP_003700975.224229%46%
Strongylocentrotus purpuratusPurple sea urchin684XP_011666469.124023%38%
Cryptotermes brevisTermite794XP_023705434.136131%29%
Exaiptasia pallidaSea anemone824XP_020898578.136129%28%
Ciona intestinalisVase tunicate676XP_009857467.138433%18%

Paralogs

One human paralog was found when this protein was sequenced in BLAST. It is 416 amino acids long, with 40% sequence identity, and 45% sequence similarity. Accision number for this protein is: NP_067082.2.

Divergence of TMEM8B

In an evolutionary comparison of TMEM8B, one species from each group (ex. Mammals, birds, fish) was plotted to avoid overabundance of information on one graph. Also plotted the comparison of the quickly diverging cytochrome C, and slowly diverging fibrinogen. TMEM8B shows divergence somewhere in-between these two proteins.

Figure 5: Evolutionary timeline for ten species uncluding humans.

Clinical significance

TMEM8B shows lower expression rates in nasopharyngeal carcinomas, and expression is also down regulated in colorectal cancers. This gene also plays a negative role in an Epidermal Growth Factor Receptor (EGFR) pathway.[5] It can delay cell cycle G0-G1 progression, and thus inhibit cell proliferation in nasopharyngeal carcinoma cells.[5]

Mutations with this gene can be pathogenic, and cause chronic pain disorders, specifically erythromelalgia symptoms.[5][18][19] Erythromelalgia is a rare condition that affects the extremities (hands and feet), and is characterized by intense, burning pain, severe redness, and increased skin temperature.[20] Medications are available to reduce symptoms, however, there is no cure for this rare condition.[20]

Interacting Proteins

Two interacting proteins were found: EGF protein, and ATXN1L protein.

EGF plays a role in cell adhesion in nasopharyngeal carcinomas (TMEM8B also plays a role in these carcinomas). This protein is expressed on the cell surface as a glycoprotein, and ectopic induction of EGF can impair NPC cell migration and improve cell adhesion and gap junctional intercellular communication.[21]

ATXN1L protein has a correlation with neurodegenerative disorders. Neurodegenerative disorders are characterized by a loss of balance due to the cerebellar Purkinje degeneration. Ataxia-causing proteins share interacting partners, a subset of which has been found to modify neurodegeneration in animal models. Interactome provides a tool for understanding pathogenic mechanisms common for neurodegenerative disorders.[22]

References
  1. GRCh38: Ensembl release 89: ENSG00000137103 - Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000078716 - Ensembl, May 2017
  3. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. Zhang XM, Wang XY, Sheng SR, Wang JR, Li J (August 2003). "Expression of tumor related genes NGX6, NAG-7, BRD7 in gastric and colorectal cancer". World Journal of Gastroenterology. 9 (8): 1729–33. doi:10.3748/wjg.v9.i8.1729. PMC 4611532. PMID 12918109.
  6. NCBI, Nucleotide
  7. "NCBI Protein". NCBI. Retrieved 24 April 2018.
  8. "Synthetic construct Homo sapiens clone ccsbBroadEn_08344 TMEM8B gene, - Nucleotide - NCBI". www.ncbi.nlm.nih.gov. Retrieved 3 May 2018.
  9. "SAPS < Sequence Statistics < EMBL-EBI". SAPS. Retrieved 23 April 2018.
  10. Kozlowski, Lukasz P. "IPC - ISOELECTRIC POINT CALCULATION OF PROTEINS AND PEPTIDES". isoelectric.org.
  11. "TMHMM Server, v. 2.0". www.cbs.dtu.dk.
  12. "Protein Structure: Primary, Secondary, Tertiary, Quatemary Structures". www.particlesciences.com. Retrieved 3 May 2018.
  13. "I-TASSER results". zhanglab.ccmb.med.umich.edu. Retrieved 1 May 2018.
  14. {{cite web|title=SWISS-MODEL |
  15. BLAST protein sequence, c9orf127
  16. Time Tree http://www.timetree.org/resources
  17. Clustal W, Multiple Sequence Alignment
  18. Andersson B, Wentland MA, Ricafrente JY, Liu W, Gibbs RA (April 1996). "A "double adaptor" method for improved shotgun library construction". Analytical Biochemistry. 236 (1): 107–13. doi:10.1006/abio.1996.0138. PMID 8619474.
  19. "C9orf127 chromosome 9 open reading frame 127". Entrez Gene.
  20. "Erythromelalgia - NORD (National Organization for Rare Disorders)". NORD (National Organization for Rare Disorders). Retrieved 2 May 2018.
  21. Ma, J. (16 September 2004). "Role of a novel EGF-like domain-containing gene NGX6 in cell adhesion modulation in nasopharyngeal carcinoma cells". Carcinogenesis. 26 (2): 281–291. doi:10.1093/carcin/bgh312. PMID 15498789.
  22. Lim, Janghoo; Hao, Tong; Shaw, Chad; Patel, Akash J.; Szabó, Gábor; Rual, Jean-François; Fisk, C. Joseph; Li, Ning; Smolyar, Alex; Hill, David E.; Barabási, Albert-László; Vidal, Marc; Zoghbi, Huda Y. (19 May 2006). "A protein-protein interaction network for human inherited ataxias and disorders of Purkinje cell degeneration". Cell. 125 (4): 801–814. doi:10.1016/j.cell.2006.03.032. PMID 16713569. S2CID 13709685.

Further reading
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