Abstract
Multifunctional Ca(2+)-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 Ser(48). 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 [Ca(2+)](i).
Highlights
In skeletal muscle, a regulatory subunit called GM [1, 2] is responsible for the targeting of glycogen and PP1c to the sarcoplasmic reticulum (SR)1 [3,4,5,6]
We further show that GM, glycogen synthase (GS), and PP1c assemble in a structural complex that selectively localizes to nonjunctional SR and that GM is phosphorylated by SR-bound CaMKII and dephosphorylated by PP1c
It is noteworthy that GM and GS co-fractionated throughout the entire preparation
Summary
A regulatory subunit called GM ( known as G, RGl, R3, and PP1R3A) [1, 2] is responsible for the targeting of glycogen and PP1c (protein phosphatase 1 catalytic subunit) to the sarcoplasmic reticulum (SR)1 [3,4,5,6]. A key role for GM phosphorylation in regulation of glycogen synthase (GS) is suggested by a number of observations: (i) cAMP-dependent protein kinase (PKA) phosphorylates GM on Ser and Ser both in vitro [3,4,5] and in vivo in response to adrenaline catecholamines [10, 14]. Both phosphorylation sites are conserved between rabbit and human GM [7,8]. GM, thereby inactivating its phosphatase activity [4, 5]. (ii) GM is phosphorylated by GSK3 at Ser and Ser44 [15]. (iii) GM is possibly phosphorylated by insulin-dependent protein kinase at Ser48 [14, 16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
