Cloning and characterization of mouse 5'-AMP-activated protein kinase gamma3 subunit

Naturally occurring mutations in the regulatory gamma-subunit of 5'-AMP-activated protein kinase (AMPK) can result in pronounced pathological changes that may stem from increases in muscle glycogen levels, making it critical to understand the role(s) of the gamma-subunit in AMPK function. In this study we cloned the mouse AMPKgamma3 subunit and revealed that there are two transcription start sites, which result in a long form, gamma3L (AF525500) and a short form, gamma3S (AF525501). AMPKgamma3L is the predominant form in mouse and is specifically expressed in mouse skeletal muscle at the protein level. In skeletal muscle, AMPKgamma3 shows higher levels of expression in fast-twitch white glycolytic muscle (type IIb) compared with fast-twitch red oxidative glycolytic muscle (type IIa), whereas gamma3 is undetectable in soleus muscle, a slow-twitch oxidative muscle with predominantly type I fibers. AMPKgamma3 can coimmunoprecipititate with both alpha and beta AMPK subunits. Overexpression of gamma3S and gamma3L in mouse tibialis anterior muscle in vivo has no effect on alpha1 and alpha2 subunit expression and does not alter AMPKalpha2 catalytic activity. However, gamma3S and gamma3L overexpression significantly increases AMPKalpha1 phosphorylation and activity by approximately 50%. The increase in AMPKalpha1 activity is not associated with alterations in glycogen accumulation or glycogen synthase expression. In conclusion, the gamma3 subunit of AMPK is highly expressed in fast-twitch glycolytic skeletal muscle, and wild-type gamma3 functions in the regulation of alpha1 catalytic activity, but it is not associated with changes in muscle glycogen concentrations.