Expression of the AMP‐activated protein kinase β1 and β2 subunits in skeletal muscle

Abstract

A heterotrimeric member of the AMP-activated protein kinase (AMPK) isoenzyme family was purified from rat skeletal muscle by immunoaffinity chromatography, consisting of an α2 catalytic and two non-catalytic subunits, β2 and γ1. The AMPK β2 cDNA (271 amino acids (aa), molecular weight (MW)=30 307, pI 6.3) was cloned from skeletal muscle and found to share an overall identity of 70% with β1 (270 aa, MW=30 475, pI 6.0). In the liver AMPK β1 subunit, Ser-182 is constitutively phosphorylated whereas in skeletal muscle β2 isoform, we find that Ser-182 is only partially phosphorylated. In addition, the autophosphorylation sites Ser-24, Ser-25 found in the β1 are replaced by Ala-Glu in the β2 isoform. β2 contains seven more Ser and one less Thr residues than β1, raising the possibility of differential post-translational regulation. Immunoblot analysis further revealed that soleus muscle (slow twitch) contains exclusively β1 associated with α2, whereas extensor digitorum longus muscle α2 (EDL, fast twitch) associates with β2 as well as β1. Sequence analysis revealed that glycogen synthase, a known AMPK substrate, co-immunoprecipitated with the AMPK α2β2γ1 complex.