Muscle phosphorylase kinase is not a substrate of AMP-activated protein kinase

AMP-activated protein kinase (AMPK) and cAMP-dependent protein kinase (cAMP K) have been reported to phosphorylate sites on phosphorylase kinase (PhK). Their target residues Ser 1018 and Ser 1020, respectively, are located in the so-called multi-phosphorylation domain in the PhK alpha subunit. In PhK preparations, only one of these serines is phosphorylated, but never both of them. The aim of this study was to determine whether phosphorylation by cAMPK or AMPK would influence subsequent phosphorylation by the other kinas e. Surprisingly, employing four different PhK substrates, it could be demon strated that, in contradiction to previous reports, PhK is not phosphorylat ed by AMPK.