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J. Biol. Chem., Vol. 280, Issue 28, 26448-26456, July 15, 2005

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Ubiquitin-Proteasome-mediated Degradation, Intracellular Localization, and Protein Synthesis of MyoD and Id1 during Muscle Differentiation^*

Liping Sun, Julie S. Trausch-Azar, Aaron Ciechanover, and Alan L. Schwartz¶

From the Edward Mallinckrodt Department of Pediatrics and Department of Molecular Biology and Pharmacology, Washington University School of Medicine and St. Louis Children's Hospital, St. Louis, Missouri 63110 and the Department of Biochemistry and Rappaport Institute for Research in Medical Sciences, Faculty of Medicine, Technion-Israel Institute for Technology, Haifa 31096, Israel

Mammalian skeletal myogenesis results in the differentiation of myoblasts to mature syncytial myotubes, a process regulated by an intricate genetic network of at least three protein families: muscle regulatory factors, E proteins, and Id proteins. MyoD, a key muscle regulatory factor, and its negative regulator Id1 have both been shown to be degraded by the ubiquitin-proteasome system. Using C2C12 cells and confocal fluorescence microscopy, we showed that MyoD and Id1 co-localize within the nucleus in proliferating myoblasts. In mature myotubes, in contrast, they reside in distinctive subcellular compartments, with MyoD within the nucleus and Id1 exclusively in the cytoplasm. Cellular abundance of Id1 was markedly diminished from the very onset of muscle differentiation, whereas MyoD abundance was reduced to a much lesser extent and only at the later stages of differentiation. These reductions in MyoD and Id1 protein levels seem to result from a change in the rate of protein synthesis rather than the rate of degradation. In vivo protein stability studies revealed that the rates of ubiquitin-proteasome-mediated MyoD and Id1 degradation are independent of myogenic differentiation state. Id1 and MyoD were both rapidly degraded, each with a t 1 h in myoblasts and in myotubes. Furthermore, relative protein synthesis rates for MyoD and Id1 were significantly diminished during myoblast to myotube differentiation. These results provide insight as to the interaction between MyoD and Id1 in the process of muscle differentiation and have implications for the involvement of the ubiquitin-proteasome-mediated protein degradation and protein synthesis in muscle differentiation and metabolism under abnormal and pathological conditions.

Received for publication, January 11, 2005 , and in revised form, April 18, 2005.

^* 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.

^¶ To whom correspondence should be addressed: Dept. of Pediatrics, Washington University School of Medicine, 660 S. Euclid Ave., C.B. 8116, St. Louis, MO 63110. Tel.: 314-454-6005; Fax: 314-454-0537; E-mail: Schwartz{at}kids.wustl.edu.

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