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PR01139

Identifier
CONNEXINB2  [View Relations]  [View Alignment]  
Accession
PR01139
No. of Motifs
4
Creation Date
22-APR-1999
Title
Gap junction beta-2 protein (Cx26) signature
Database References
PRINTS; PR00206 CONNEXIN
PRODOM; PD012575
INTERPRO; IPR002268
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).

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-2 protein (also called connexin26, or Cx26) is a connexin
of 226 amino acid residues that is often found together with Cx32 in
epithelial tissues. In rodents, it seems essential for normal embryonic
development; mice lacking Cx26 die in utero at about embryonic day 11.
Absence of Cx26 impairs transplacental movement of glucose, thus impairing 
embryonic development. In humans, spontaneously-occurring mutations in the
gene encoding Cx26 have been found to be associated with a quite different
disorder, non-syndromic neurosensory autosomal recessive deafness, the most
common form of inherited hearing loss. Consistent with a role in auditory
transduction, Cx26 is the predominant connexin isoform expressed in the
organ of Corti of the cochlea. Here, it may be involved in maintaining the
ionic balance of the endolymph, by providing a pathway for ions necessary
to maintain this potassium-rich fluid.
 
CONNEXINB2 is a 4-element fingerprint that provides a signature for the
gap junction beta-2 protein. The fingerprint was derived from an initial
alignment of 3 sequences: the motifs were drawn from conserved regions
within the C-terminal half of the alignment, focusing on those sections
that characterise the gap junction beta-2 isoform but distinguish it from
others - motif 1 resides within the putative cytoplasmic loop between TM
domains 2 and 3; motif 2 lies within the extracellular domain between TM
domains 3 and 4; motif 3 encodes part of the fourth TM domain; and motif 4
encodes the cytoplasmic C-terminus. Two iterations on SPTR37_9f were
required to reach convergence, at which point a true set comprising 4
sequences was identified. 
Summary Information
4 codes involving  4 elements
0 codes involving 3 elements
0 codes involving 2 elements
Composite Feature Index
44444
30000
20000
1234
True Positives
CXB2_HUMAN    CXB2_MOUSE    CXB2_RAT      CXB2_SHEEP    
Sequence Titles
CXB2_HUMAN  GAP JUNCTION BETA-2 PROTEIN (CONNEXIN 26) (CX26) - HOMO SAPIENS (HUMAN). 
CXB2_MOUSE GAP JUNCTION BETA-2 PROTEIN (CONNEXIN 26) (CX26) - MUS MUSCULUS (MOUSE).
CXB2_RAT GAP JUNCTION BETA-2 PROTEIN (CONNEXIN 26) (CX26) - RATTUS NORVEGICUS (RAT).
CXB2_SHEEP GAP JUNCTION BETA-2 PROTEIN (CONNEXIN 26) (CX26) - OVIS ARIES (SHEEP).
Scan History
SPTR37_9f  2  200  NSINGLE    
Initial Motifs
Motif 1  width=6
Element Seqn Id St Int Rpt
IEEIKT CXB2_HUMAN 118 118 -
IEEIKT CXB2_RAT 118 118 -
IEEIKN CXB2_SHEEP 118 118 -

Motif 2 width=5
Element Seqn Id St Int Rpt
NAWPC CXB2_HUMAN 170 46 -
NAWPC CXB2_RAT 170 46 -
NAWPC CXB2_SHEEP 170 46 -

Motif 3 width=7
Element Seqn Id St Int Rpt
IAVSGIC CXB2_HUMAN 196 21 -
ISVSGIC CXB2_RAT 196 21 -
IAVSGIC CXB2_SHEEP 196 21 -

Motif 4 width=12
Element Seqn Id St Int Rpt
IRYCSGKSKKPV CXB2_HUMAN 215 12 -
IRYCSGKSKRPV CXB2_RAT 215 12 -
IRFCSGKSKKPV CXB2_SHEEP 215 12 -
Final Motifs
Motif 1  width=6
Element Seqn Id St Int Rpt
IEEIKT CXB2_HUMAN 118 118 -
IEEIKT CXB2_RAT 118 118 -
IEEIKT CXB2_MOUSE 118 118 -
IEEIKN CXB2_SHEEP 118 118 -

Motif 2 width=5
Element Seqn Id St Int Rpt
NAWPC CXB2_HUMAN 170 46 -
NAWPC CXB2_RAT 170 46 -
NAWPC CXB2_MOUSE 170 46 -
NAWPC CXB2_SHEEP 170 46 -

Motif 3 width=7
Element Seqn Id St Int Rpt
IAVSGIC CXB2_HUMAN 196 21 -
ISVSGIC CXB2_RAT 196 21 -
ISVSGIC CXB2_MOUSE 196 21 -
IAVSGIC CXB2_SHEEP 196 21 -

Motif 4 width=12
Element Seqn Id St Int Rpt
IRYCSGKSKKPV CXB2_HUMAN 215 12 -
IRYCSGKSKRPV CXB2_RAT 215 12 -
VRYCSGKSKRPV CXB2_MOUSE 215 12 -
IRFCSGKSKKPV CXB2_SHEEP 215 12 -