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J. Biol. Chem., Vol. 280, Issue 20, 19527-19534, May 20, 2005

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Phosphorylation of Rat Liver Mitochondrial Glycerol-3-phosphate Acyltransferase by Casein Kinase 2^*

Thomas M. Onorato, Sanjoy Chakraborty, and Dipak Haldar¶

From the Department of Biological Sciences, St. John's University, Queens, New York 11439

We have previously shown rat liver mitochondrial glycerol-3-phosphate acyltransferase (mtGAT), which catalyzes the first step in de novo glycerolipid biosynthesis, is stimulated by casein kinase 2 (CK2) and that a phosphorylated protein of 85 kDa is present in CK2-treated mitochondria. In this paper, we have identified the ³²P-labeled 85-kDa protein as mtGAT. We have also investigated whether the phosphorylation of mtGAT is because of CK2. Mitochondria were treated with CK2 and [-³²P]GTP as the phosphate donor. Autoradiography, Western blot, and immunoprecipitation results showed mtGAT was phosphorylated by CK2. Next, we incubated mitochondria with CK2 and either ATP or GTP, in the presence of heparin, a known inhibitor of CK2. Heparin inhibited CK2-induced stimulation of mtGAT activity; this inhibition resulted in decreased ³²P-labeling of mtGAT. Additionally, mitochondria were treated with CK2 and [-³²P]ATP in the presence of staurosporine (a serine/threonine protein kinase inhibitor), genistein (a tyrosine kinase inhibitor), and 5,6-dichloro-1--D-ribofuranosylbenzimidazole (DRB, a CK2 inhibitor). Only DRB, the CK2 inhibitor, greatly reduced the amount of ³²P-incorporation into mtGAT by CK2. Finally, isolated mitochondrial outer membrane was incubated with cytosol in the presence of [-³²P]GTP; ³²P-labeled mtGAT was detected. Collectively, these data suggest that CK2 phosphorylates mtGAT. The impact of our results in the regulation of mtGAT and other anabolic processes is discussed.

Received for publication, September 10, 2004 , and in revised form, March 11, 2005.

^* This research was supported in part by Grant GM-57643 from the National Institutes of Health. 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.

Supported by Department of Education's Graduate Assistance in Areas of National Need (GAANN) Grant P200A010130. This work formed a portion of a doctoral thesis submitted to the faculty of Biological Sciences, St. John's University. Present address: The Population Council, Center for Biomedical Research, New York, New York 10021.

Present address: Dept. of Physical and Biological Sciences, New York City College of Technology/CUNY, Brooklyn, New York 11201.

^¶ To whom correspondence should be addressed: St. John's University, 8000 Utopia Pkwy., Queens, NY 11439. Tel.: 718-990-1697; Fax: 718-990-5958; E-mail: haldard{at}stjohns.edu.

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