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Quang-Kim Tran,
D. J. Black, and
Anthony Persechini
Division of Molecular Biology & Biochemistry, School of Biological Sciences, University of Missouri, Kansas City, Missouri 64110-2499
Abstract:
Measurements of cellular Ca2+-calmodulin concentrations havesuggested that competition for limiting calmodulin may couplecalmodulin-dependent activities. Here we have directly testedthis hypothesis. We have found that in endothelial cells theamount of calmodulin bound to nitric-oxide synthase and thecatalytic activity of the enzyme both are increased 3-foldupon changes in the phosphorylation status of the enzyme. Quantitativeimmunoblotting indicates that the synthase can bind up to 25%of the total cellular calmodulin. Consistent with this, simultaneous determinations of the free Ca2+ and Ca2+-calmodulin concentrationsin these cells performed using indo-1 and a fluorescent calmodulinbiosensor (Kd = 2 nM) indicate that increased binding of calmodulinto the synthase is associated with substantial reductions inthe Ca2+-calmodulin concentrations produced and an increasein the [Ca2+]50 for formation of the calmodulin-biosensor complex.The physiological significance of these effects is confirmedby a corresponding 40% reduction in calmodulin-dependent plasmamembrane Ca2+ pump activity. An identical reduction in pumpactivity is produced by expression of a high affinity (Kd =0.3 nM) calmodulin biosensor, and treatment to increase calmodulinbinding to the synthase then has no further effect. This suggeststhat the observed reduction in pump activity is due specificallyto reduced calmodulin availability. Increases in synthase activitythus appear to be coupled to decreases in the activities of other calmodulin targets through reductions in the size ofa limiting pool of available calmodulin. This exemplifies whatis likely to be a ubiquitous mechanism for coupling among diversecalmodulin-dependent activities.
Received for publication April 18, 2003.
Revision received May 8, 2003.
* This work was supported by National Institutes of Health GrantDK 53863 (to A. P.). The costs of publication of this articlewere defrayed in part by the payment of page charges. Thisarticle must therefore be hereby marked "advertisement" inaccordance with 18 U.S.C. Section 1734 solely to indicate thisfact.
To whom correspondence should be addressed: Division of Molecular Biology & Biochemistry, School of Biological Sciences, University of Missouri-Kansas City, Rm. 412, Biological Sciences Bldg., 5007 Rockhill Rd., Kansas City, MO 64110-2499. Tel.: 816-235-5972; Fax: 816-235-5595; E-mail: persechinia{at}umkc.edu.
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