The Na+/Ca2+ exchanger, in concert with the
plasma membrane Ca2+ ATPase (PMCA) and the sarcoplasmic reticulum
Ca2+ ATPase (SERCA), removes Ca2+ ions that enter the cytosol via
L-type Ca2+ channels and/or from internal Ca2+ stores during the
action potential. Driven by the Na+ gradient generated by the Na+/K+
ATPase, the exchanger predominantly expels one Ca2+ ion for the
uptake of 3 Na+ ions , may however, under certain conditions,
also operate in reverse mode .
Structurally, NCX appears to consist of nine transmembrane a-helices
[3,4] and a large, approximately 500 residue-long cytosolic loop.
Recently, we demonstrated the presence of two, rather than the previously
assumed one Ca2+ binding domain in the cytosolic exchanger loop
. We also proposed a model in which the Ca2+ binding sites of
the first Ca2+ binding domain (CBD1) are approximately 90 Å
away from the transport Ca2+ binding site in the ion-conducting
transmembrane domain. In contrast, Ca2+ binding sites of the second
Ca2+ binding domain (CBD2) are close to a predicted third domain
that shows homology with a-catenin and that we therefore designated
as catenin-like domain (CLD).
Ion-transport of the exchanger is regulated by concentration-dependent
interactions of Na+ and Ca2+ ions with the cytosolic exchanger domains.
Binding of Ca2+ to CBD1 with an affinity of 140-400 nM [5-7] activates
ion-transport. Once the local intracellular Ca2+ concentration drops
below the CBD1 Ca2+ affinities, the exchanger enters a process referred
to as Ca2+-dependent or I2 inactivation . Concomitantly, Na+
concentrations rising above 15 mM can lead to Na+-dependent or I1
We determined the solution structures of
residues 371-509 and 501-657 of canine NCX1 that encode the two
Ca2+ sensors, CBD1 and CBD2, in NCX. While CBD1 and CBD2 look very
similar in the Ca2+ bound form, [1H,15N]-HSQC spectra of the domains
in the absence of Ca2+ suggest striking differences in their apo-structures.
Mapping shifted resonances onto the molecular structure of CBD1
shows a loss of structural integrity in the Ca2+-binding sites of
the CBD1 apo-form. Examination of the two domains reveals that only
CBD2, but not CBD1, possesses basic residues in the Ca2+ binding
sites that can form salt-bridges with surrounding acidic residues
and in this way reduce repulsion. The large conformational change
upon Ca2+-binding and a 5-10 times higher affinity for Ca2+ makes
CBD1 the primary Ca2+ sensor in NCX. The markedly different behaviour
of the two domains we visualized in a molecular movie.
Data & Reprints
 Hilge M, Aelen J, Vuister GW. Ca2+ regulation in the Na+/Ca2+
exchanger involves two markedly different Ca2+ sensors. Mol Cell
22, 15-25 (2006).
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Coordinates & Chemical shifts
The coordinates and all experimental data (chemical shifts and constraints)
of the Ca2+ bound CBD1 and CBD2 ensembles have been deposited with
the Protein Data Bank (accession codes 2FWS and 2FWU, respectively).
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