Abstract
The cardiac Na/Ca exchanger's (NCX1) role in calcium homeostasis during
myocardial contractility makes it a possible target of signaling
factors regulating inotropy. Caveolae, structured invaginations of
the plasmalemma, are known to concentrate a wide variety of signaling
factors. The predominant coat proteins of caveolae, caveolins, dock
to and regulate the activity of these signaling factors and other
proteins through interaction with their scaffolding domain. In this
study we investigated the interaction of NCX1 with caveolin proteins.
Western blots of bovine cardiac sarcolemmal vesicles revealed the
presence of caveolin-1, -2, and -3. Immunoprecipitation of detergent-solubilized
vesicle proteins with either NCX1 or caveolin-3 antibodies indicated
that NCX1 coprecipitates with caveolin-3, but not with caveolin-1
and -2. Functional disruption of caveolae, by beta-cyclodextrin treatment
of vesicles, diminished coprecipitation of caveolin-3 and NCX1 activity.
NCX1 has five potential caveolin-binding motifs, two of which are
in the transporter's exchange inhibitory peptide (XIP) domain. The
presence of 50 mM XIP peptide enhanced coprecipitation of caveolin-3
with NCX1 independent of calcium concentration. We conclude that
NCX1 associates specifically with caveolin-3. Partitioning of NCX1
in caveolae has implications for temporal and spatial regulation
of excitation-contraction and -relaxation coupling in cardiac myocytes.
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