Gap junction beta-2 protein (GJB2), also known as
connexin
Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, form gap junctions in invertebrates. Each ...
26 (Cx26) — is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
that in humans is encoded by the ''GJB2''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
Clinical significance
Defects in this gene lead to the most common form of congenital
deafness
Deafness has varying definitions in cultural and medical contexts. In medical contexts, the meaning of deafness is hearing loss that precludes a person from understanding spoken language, an audiological condition. In this context it is written ...
in developed countries, called DFNB1 (also known as connexin 26 deafness or ''GJB2''-related deafness). One fairly common mutation is the deletion of one
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...
from a string of six, resulting in a
frameshift
Ribosomal frameshifting, also known as translational frameshifting or translational recoding, is a biological phenomenon that occurs during translation that results in the production of multiple, unique proteins from a single mRNA. The process can ...
and termination of the protein at
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
number 13. Having two copies of this mutation results in deafness.
Connexin 26 also plays a role in
tumor suppression
A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or red ...
through mediation of the cell cycle. The abnormal expression of Cx26, correlated with several types of human
cancers
Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
, may serve as a prognostic factor for cancers such as colorectal cancer, breast cancer, and bladder cancer. Furthermore, Cx26 over-expression is suggested to promote cancer development by facilitating cell migration and invasion and by stimulating the self-perpetuation ability of
cancer stem cells
Cancer stem cells (CSCs) are cancer cells (found within tumors or hematological cancers) that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found in a particular cancer sample ...
.
Function
Gap junction
Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...
s were first characterized by
electron microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
as regionally specialized structures on
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
s of contacting adherent cells. These structures were shown to consist of cell-to-cell channels. Proteins, called
connexin
Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, form gap junctions in invertebrates. Each ...
s, purified from fractions of enriched gap junctions from different tissues differ. The connexins are designated by their molecular mass. Another system of nomenclature divides gap junction proteins into two categories, alpha and beta, according to sequence similarities at the nucleotide and amino acid levels. For example, CX43 (
GJA1
Gap junction alpha-1 protein (GJA1), also known as connexin 43 (Cx43), is a protein that in humans is encoded by the ''GJA1'' gene on chromosome 6. As a connexin, GJA1 is a component of gap junctions, which allow for gap junction intercellular ...
) is designated alpha-1 gap junction protein, whereas
GJB1
Gap junction beta-1 protein (GJB1), also known as connexin 32 (Cx32) is a transmembrane protein that in humans is encoded by the ''GJB1'' gene. Gap junction beta-1 protein is a member of the gap junction connexin family of proteins that regulates ...
(CX32), and GJB2 (CX26; this protein) are called beta-1 and beta-2 gap junction proteins, respectively. This nomenclature emphasizes that GJB1 and GJB2 are more homologous to each other than either of them is to gap junction protein, alpha GJA1.
See also
*
Connexin
Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, form gap junctions in invertebrates. Each ...
*
Gap junction
Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...
*
Vohwinkel syndrome
Palmoplantar keratodermas are a heterogeneous group of disorders characterized by abnormal thickening of the stratum corneum of the palms and soles.
Autosomal recessive, dominant, X-linked, and acquired forms have all been described.
Types
Cli ...
*
Bart–Pumphrey syndrome
Bart–Pumphrey syndrome (also known as "Palmoplantar keratoderma with knuckle pads and leukonychia and deafness") is a cutaneous condition characterized by hyperkeratoses (knuckle pads) over the metacarpophalangeal, proximal and distal interphala ...
References
Further reading
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Connexins