| Literature References | 1. PHELAN, P., BACON, J.P., DAVIES, J.A., STEBBINGS, L.A., TODMAN, M.G.,
AVERY, L., BAINES, R.A., BARNES, T.M., FORD, C., HEKIMI, S., LEE, R.,
SHAW, J.E., STARICH, T.A., CURTIN, K.D., SUN, Y. AND WYMAN, R.J.
Innexins: a family of invertebrate gap-junction proteins.
TRENDS GENET. 14 348-349 (1998).
2. DERMIETZEL, R. AND SPRAY, D.C.
Gap junctions in the brain: where, what type, how many and why?
TRENDS NEUROSCIENCE 16 186-192 (1993).
3. GOODENOUGH, D.A., GOLIGER, J.A. AND PAUL, D.L.
Connexins, connexons, and intercellular communication.
ANNU.REV.BIOCHEMISTRY 65 475-502 (1996).
4. KUMAR, N.M. AND GILULA, N.B.
The gap junction communication channel.
CELL 84 381-388 (1996).
5. KUMAR, N.M. AND GILULA, N.B.
Molecular biology and genetics of gap junction channels.
SEMIN.CELL BIOL. 3 3-16 (1992).
6. NICHOLSON, S.M. AND BRUZZONE, R.
Gap junctions: getting the message through.
CURR.BIOL. 7 340-344 (1997).
7. SIMON, A.M. AND GOODENOUGH, D.A.
Diverse functions of vertebrate gap junctions.
TRENDS CELL BIOL. 8 477-483 (1998).
8. SPRAY, D.C. AND DERMIETZEL, R.
X-linked dominant Charcot-Marie-Tooth disease and other potential gap-
junctions diseases of the nervous system.
TRENDS NEUROSCIENCE 18 256-262 (1995).
9. GIMLICH, R.L., KUMAR, N.M. AND GILULA, N.B.
Sequence and developmental expression of mRNA coding for a gap junction
protein in Xenopus.
J.CELL BIOL. 107 1065-1073 (1988).
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| Documentation | The connexins are a family of integral membrane proteins that oligomerise
to form intercellular channels that are clustered at gap junctions. These
channels are specialised sites of cell-cell contact that allow the passage
of ions, intracellular metabolites and messenger molecules (with molecular
weight <1-2 kDa) from the cytoplasm of one cell to its apposing neighbours.
They are found in almost all vertebrate cell types, and somewhat similar
proteins have been cloned from plant species. Invertebrates utilise a
different family of molecules, innexins, that share a similar predicted
secondary structure to the vertebrate connexins, but have no sequence
identity to them [1].
Vertebrate gap junction channels are thought to participate in diverse
biological functions. For instance, in the heart they permit the rapid
cell-cell transfer of action potentials, ensuring coordinated contraction
of the cardiomyocytes. They are also responsible for neurotransmission at
specialised `electrical' synapses. In non-excitable tissues, such as the
liver, they may allow metabolic cooperation between cells. In the brain,
glial cells are extensively-coupled by gap junctions; this allows waves of
intracellular Ca2+ to propagate through nervous tissue, and may contribute
to their ability to spatially-buffer local changes in extracellular K+
concentration [2].
The connexin protein family is encoded by at least 13 genes in rodents, with
many homologues cloned from other species. They show overlapping tissue
expression patterns, most tissues expressing more than one connexin type.
Their conductances, permeability to different molecules, phosphorylation and
voltage-dependence of their gating, have been found to vary. Possible
communication diversity is increased further by the fact that gap junctions
may be formed by the association of different connexin isoforms from
apposing cells. However, in vitro studies have shown that not all possible
combinations of connexins produce active channels [3,4].
Hydropathy analysis predicts that all cloned connexins share a common
transmembrane (TM) topology. Each connexin is thought to contain 4 TM
domains, with two extracellular and three cytoplasmic regions. This model
has been validated for several of the family members by in vitro biochemical
analysis. Both N- and C-termini are thought to face the cytoplasm, and the
third TM domain has an amphipathic character, suggesting that it contributes
to the lining of the formed-channel. Amino acid sequence identity between
the isoforms is ~50-80%, with the TM domains being well conserved. Both
extracellular loops contain characteristically conserved cysteine residues,
which likely form intramolecular disulphide bonds. By contrast, the single
putative intracellular loop (between TM domains 2 and 3) and the cytoplasmic
C-terminus are highly variable among the family members. Six connexins are
thought to associate to form a hemi-channel, or connexon. Two connexons then
interact (likely via the extracellular loops of their connexins) to form the
complete gap junction channel.
Two sets of nomenclature have been used to identify the connexins. The
first, and most commonly used, classifies the connexin molecules according
to molecular weight, such as connexin43 (abbreviated to Cx43), indicating
a connexin of molecular weight close to 43 kDa. However, studies have
revealed cases where clear functional homologues exist across species
that have quite different molecular masses; therefore, an alternative
nomenclature was proposed based on evolutionary considerations, which
divides the family into two major subclasses, alpha and beta, each with a
number of members [5]. Due to their ubiquity and overlapping tissue
distributions, it has proved difficult to elucidate the functions of
individual connexin isoforms. To circumvent this problem, particular
connexin-encoding genes have been subjected to targeted-disruption in mice,
and the phenotype of the resulting animals investigated. Around half the
connexin isoforms have been investigated in this manner [6,7]. Further
insight into the functional roles of connexins has come from the discovery
that a number of human diseases are caused by mutations in connexin genes.
For instance, mutations in Cx32 give rise to a form of inherited
peripheral neuropathy called X-linked dominant Charcot-Marie-Tooth disease
[8]. Similarly, mutations in Cx26 are responsible for both autosomal
recessive and dominant forms of nonsyndromic deafness, a disorder
characterised by hearing loss, with no apparent effects on other organ
systems.
Gap junction beta-1 protein (also called connexin32, or Cx32) is a connexin
of 283 amino acid residues (human isoform) that is widely expressed in many
tissues, including the liver, exocrine pancreas, central nervous system and
epithelium of the gastrointestinal tract. The amphibian isoform from the
African clawed frog, Xenopus laevis, is slightly shorter, containing 264
amino acid residues. In the adult frog, the protein is present in the lungs,
alimentary tract and ovaries [9].
In humans, Cx32 appears to be critical to the functioning of Schwann cells,
which are responsible for the myelination of nerves in the peripheral
nervous system. Mutations in the gene encoding Cx32 give rise to a form of
inherited neuropathy called X-linked Charcot-Marie-Tooth disease, which
affects nervous conduction in both motor and sensory axons. To date, >40
different mutations have been identified, and these are spread throughout
most of the Cx32 molecule. The effects of some of these mutations have been
determined, and several of them lead to a complete loss of gap junction
function. Targeted-gene disruption of Cx32 in mice has confirmed its role
in Schwann cell function; such Cx32-null mice also develop a form of
peripheral neuropathy similar to Charcot-Marie-Tooth disease.
CONNEXINB1 is a 4-element fingerprint that provides a signature for the
gap junction beta-1 protein. The fingerprint was derived from an initial
alignment of 5 sequences: the motifs were drawn from conserved regions
spanning virtually the full alignment length, focusing on those sections
that characterise the gap junction beta-1 isoform but distinguish it from
others - motif 1 resides at the start of the putative cytoplasmic N-terminus;
motif 2 lies within the cytoplasmic loop between TM domains 2 and 3; motif 3
resides within the fourth TM domain; and motif 4 lies within the cytoplasmic
C-terminus. A single iteration on SPTR37_9f was required to reach convergence,
no further sequences being identified beyond the starting set.
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