Phosphorylation control of cardiac acetyl-CoA carboxylase by cAMP-dependent protein kinase and 5 '-AMP activated protein kinase

Acetyl-CoA carboxylase (ACC) is regarded in liver and adipose tissue to be the rate-limiting enzyme for fatty acid biosynthesis; however, in heart tis sue it functions as a regulator of fatty acid oxidation. Because the contro l of fatty acid oxidation is important to the functioning myocardium, the r egulation of ACC is a key issue. Two cardiac isoforms of ACC exist, with mo lecular masses of 265 kDa and 280 kDa (ACC265 and ACC280). In this study, t hese proteins were purified from rat heart and used in subsequent phosphory lation and immunoprecipitation experiments. Our results demonstrate that 5' AMP-activated protein kinase (AMPK) is able to phosphorylate both ACC265 a nd ACC280, resulting in an almost complete loss of ACC activity. Although c AMP-dependent protein kinase phosphorylated only ACC280, a dramatic loss of ACC activity was still observed, suggesting that ACC280 contributes most, if not all, of the total heart ACC activity. ACC280 and ACC265 copurified u nder all experimental conditions, and purification of heart ACC also result ed in the specific copurification of the alpha(2) isoform of the catalytic subunit of AMPK. Although both catalytic subunits of AMPK were expressed in crude heart homogenates, our results suggest that alpha(2), and not alpha( 1), is the dominant isoform of AMPK catalytic subunit regulating ACC in the heart. Immunoprecipitation studies demonstrated that specific antibodies f or both ACC265 and ACC280 were able to coimmunoprecipitate the alternate is oform along with the alpha(2) isoform of AMPK. Taken together, the immunopr ecipitation and the purification studies suggest that the two isoforms of A CC in the heart exist in a heterodimeric structure, and that this structure is tightly associated with the alpha(2) subunit of AMPK.