| Literature References | 1. PAYNE, J.A. AND FORBUSH, B., III.
Molecular characterization of the epithelial Na-K-Cl cotransporter
isoforms.
CURR.OPIN.CELL BIOL. 7 493-503 (1995).
2. SIMON, D.B. AND LIFTON, R.P.
Mutations in Na(K)Cl transporters in Gitelman's and Bartter's syndromes.
CURR.OPIN.CELL BIOL. 10 450-454 (1998).
3. XU, J.-C., LYTLE, C., ZHU, T.T., PAYNE, J.A., BENZ, E., JR. AND
FORBUSH, B., III.
Molecular cloning and functional expression of the bumetanide-sensitive
Na-K-Cl cotransporter.
PROC.NATL.ACAD.SCI.U.S.A. 91 2201-2205 (1994).
4. PAYNE, J.A., XU, J.-C., HAAS, M., LYTLE, C.Y., WARD, D. AND
FORBUSH, B., III.
Primary structure, functional expression, and chromosomal localization of
the bumetanide-sensitive Na-K-Cl cotransporter in human colon.
J.BIOL.CHEM. 270 17977-17985 (1995).
5. DELPIRE, E., RAUCHMAN, M.I., BEIER, D.R., HEBERT, S.C. AND GULLANS, S.R.
Molecular cloning and chromosome localization of a putative basolateral
Na+-K+-2Cl- cotransporter from mouse inner medullary collecting duct
J.BIOL.CHEM. 269 25677-25683 (1994).
|
| Documentation | The Na-K-Cl co-transporters are a family of integral membrane proteins that
are ubiquitously expressed in animal tissues, serving a variety of
functions. In cells of Cl- absorptive and Cl- secretory epithelia, Na-K-Cl
co-transport serves as the major Cl- entry pathway, and functions in
concert with other membrane ion channels and pumps to carry out net trans-
epithelial movement of salt. This vectorial transport of Cl- across
epithelia is involved in the reabsorption of salt in the vertebrate kidney
(which is crucial for urinary concentration), and in the secretion of salt
in such tissues as the mammalian intestine and trachea. In addition,
Na-K-Cl co-transport is known to play a role in cell volume regulation in
most mammalian cell types. The proteins mediate the coupled, electroneutral
transport of sodium, potassium and chloride ions across the plasma membrane
of cells (with a stoichiometry of 1:1:2, respectively). Co-transport of all
three ions is obligatory, since absence of one is sufficient to prevent ion
movement. Their transport activity does not alter the cell's membrane
potential, thus the driving force for the transport is determined solely
by the chemical gradients of the three transported ions; hence, under
normal physiological conditions, the direction will be inward.
Recent molecular studies have identified two distinct isoforms: one from
Cl- secretory epithelia, NKCC1; and another, NKCC2, found specifically in
the diluting segment of the vertebrate kidney, a Cl- absorptive epithelium
[1]. They show lowish amino acid sequence identity (~58%); nevertheless,
they have rather similar hydropathy profiles, with hydrophilic N- and C-
termini, flanking a central hydrophobic domain. Their N-termini show
considerable variation, unlike the central domain (containing the 12
putative transmembrane (TM) domains) and their C-termini, which are well
conserved (~70%). Both isoforms are known to be glycosylated and, consistent
with this, consensus sites for N-linked glycosylation are located within the
large hydrophilic loop between presumed TM domains 7 and 8. Sequence
comparisons with other cloned ion co-transporters reveals that Na-K-Cl
co-transporters belong to a superfamily of electroneutral cation-chloride
co-transporters, which includes the K-Cl co-transporter (PR01081) and the
thiazide-sensitive Na-Cl co-transporter. All share a similar predicted
membrane topology of 12 TM regions in a central hydrophobic domain,
together with hydrophilic N- and C-termini that are likely cytoplasmic.
Mutations in the gene encoding the renal-specific isoform of the Na-K-Cl
co-transporter (NKCC2) give rise to Bartter's Syndrome Type 1, an inherited
kidney disease characterised by hypokalaemia, metabolic alkalosis, salt-
wasting and hypotension [2].
NKCC1 (also called BSC2) was first cloned from the shark rectal gland, a
model secretory epithelium [3]. Subsequently, mammalian homologues were
cloned from mouse and human tissues. NKCC1 has a wide distribution and is
a basolateral secretory isoform, likely involved in salt secretion in a
diverse range of tissues. Its broad distribution also suggests that it
may be involved in cell volume regulation and ionic homeostasis [4,5].
The human isoform consists of 1212 amino acid residues and shares ~90
identity with the mouse homologue. It shows lower identity to other members
of the cation-chloride co-transporter superfamily, being ~40% identical to
the thiazide-sensitive Na-Cl co-transporter.
NAKCLTRSPRT1 is a 4-element fingerprint that provides a signature for the
Na-K-Cl co-transporter type 1 isoform. The fingerprint was derived from
an initial alignment of 3 sequences: the motifs were drawn from conserved
regions spanning virtually the full alignment length, focusing on those
sections that characterise the Na-K-Cl co-transporter 1 isoform but
distinguish it from others - motif 1 resides within the putative cytoplasmic
N-terminus; motif 2 encodes part of the first putative extracellular domain;
and motifs 3 and 4 reside near the end of the large putative cytoplasmic
C-terminus. Two iterations on SPTR37_9f were required to reach convergence,
at which point a true set comprising 4 sequences was identified.
|