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J. Biol. Chem., Vol. 282, Issue 46, 33494-33506, November 16, 2007

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Calmodulin Binds and Stabilizes the Regulatory Enzyme, CTP:Phosphocholine Cytidylyltransferase^*

Bill. B. Chen and Rama K. Mallampalli^¶1

From the Departments of Internal Medicine and Biochemistry and the ^¶Department of Veterans Affairs Medical Center and the Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa 52242

CTP:phosphocholine cytidylyltransferase (CCT) is a proteolytically sensitive enzyme essential for production of phosphatidylcholine, the major phospholipid of animal cell membranes. The molecular signals that govern CCT protein stability are unknown. An NH₂-terminal PEST sequence within CCT did not serve as a degradation signal for the proteinase, calpain. Calmodulin (CaM) stabilized CCT from calpain proteolysis. Adenoviral gene transfer of CaM in cells protected CCT, whereas CaM small interfering RNA accentuated CCT degradation by calpains. CaM bound CCT as revealed by fluorescence resonance energy transfer and two-hybrid analysis. Mapping and site-directed mutagenesis of CCT uncovered a motif (LQERVDKVK) harboring a vital recognition site, Gln²⁴³, whereby CaM directly binds to the enzyme. Mutagenesis of CCT Gln²⁴³ not only resulted in loss of CaM binding but also led to complete calpain resistance in vitro and in vivo. Thus, calpains and CaM both access CCT using a structurally similar molecular signature that profoundly affects CCT levels. These data suggest that CaM, by antagonizing calpain, serves as a novel binding partner for CCT that stabilizes the enzyme under proinflammatory stress.

Received for publication, August 6, 2007 , and in revised form, September 4, 2007.

^* This work was supported by an American Heart Association predoctoral fellowship award (to B. B. C.), a Merit Review Award from the Department of Veterans Affairs, and National Institutes of Health R01 Grants HL081784, HL068135, HL071040, and HL080229 (to R. K. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ To whom correspondence should be addressed: University of Iowa, Pulmonary and Critical Care Division, C-33K, GH, Dept. of Internal Medicine, Iowa City, IA 52242. Tel.: 319-356-1265; Fax: 319-335-6506; E-mail: ramamallampalli{at}uiowa.edu.

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				B. B. Chen and R. K. Mallampalli Masking of a Nuclear Signal Motif by Monoubiquitination Leads to Mislocalization and Degradation of the Regulatory Enzyme Cytidylyltransferase Mol. Cell. Biol., June 1, 2009; 29(11): 3062 - 3075. [Abstract] [Full Text] [PDF]

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