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J. Biol. Chem., Vol. 280, Issue 8, 7147-7155, February 25, 2005

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Glycogen- and PP1c-targeting Subunit G_M Is Phosphorylated at Ser⁴⁸ by Sarcoplasmic Reticulum-bound Ca²⁺-Calmodulin Protein Kinase in Rabbit Fast Twitch Skeletal Muscle^*

Roberta Sacchetto¶, Elisa Bovo, Arianna Donella-Deana||, and Ernesto Damiani**

From the Departments of Experimental Biomedical Sciences and ^||Biochemistry, University of Padova, viale Giuseppe Colombo 3, 35121 Padova, Italy and the Department of Experimental and Diagnostic Medicine, University of Ferrara, via Luigi Borsari, 46, 44100 Ferrara, Italy

Multifunctional Ca²⁺-calmodulin-dependent protein kinase (CaMKII) is a Ser/Thr protein kinase uniformly distributed within the sarcoplasmic reticulum (SR) of skeletal muscle. In fast twitch muscle, no specific substrates of CaMKII have yet been identified in nonjunctional SR. Previous electron microscopy data showed that glycogen particles containing glycogen synthase (GS) associate with SR at the I band level. Furthermore, recent evidence implicates CaMKII in regulation of glucose and glycogen metabolism. Here, we demonstrate that the glycogen- and protein phosphatase 1-targeting subunit, also known as G_M, selectively localizes to the SR membranes of rabbit skeletal muscle and that G_M and GS co-localize at the level of the I band. We further show that G_M, GS, and PP1c assemble in a structural complex that selectively localizes to nonjunctional SR and that G_M is phosphorylated by SR-bound CaMKII and dephosphorylated by PP1c. On the other hand, no evidence for a structural interaction between G_M and CaMKII was obtained. Using His-tagged G_M recombinant fragments and site-directed mutagenesis, we demonstrate that the target of CaMKII is Ser⁴⁸. Taken together, these data suggest that SR-bound CaMKII participates in the regulation of GS activity through changes in the phosphorylation state of G_M. Based on these findings, we propose that SR-bound CaMKII participates in the regulation of glycogen metabolism, under physiological conditions involving repetitive raises elevations of [Ca²⁺]_i.

Received for publication, December 2, 2004

^* This work was supported by Ministero Istruzione, Università, Ricerca Grant PRIN03 (to E. D.) and by FIRB RBAU015R84 (to A. D.-D.). 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.

^¶ Recipient of a Fellowship from the Department of Experimental and Diagnostic Medicine, University of Ferrara.

^** To whom correspondence should be addressed. Tel.: 390-498276038; Fax: 390-498276040; E-mail: damiani{at}civ.bio.unipd.it.

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