5′-AMP activates the AMP-activated protein kinase cascade, and Ca²⁺/calmodulin activates the calmodulin-dependent protein kinase I cascade, via three independent mechanisms

AMP-activated protein kinase (AMPK) and Ca²⁺/calmodulin (CaM)-dependent protein kinase I (CaMKI) are protein kinases that are regulated both by allosteric activation (AMP and Ca²⁺/CaM, respectively) and by phosphorylation by upstream protein kinases (AMPK kinase (AMPKK) and CaMKI kinase (CaMKIK), respectively). We now report that AMPKK can activate CaMKI and that, conversely, CaMKIK can activate AMPK. CaMKIK is 68-fold more effective at activating CaMKI than AMPK, while AMPKK is 17-fold more effective at activating AMPK than CaMKI. Our results suggest that CaMKIK and AMPKK are distinct enzymes dedicated to their respective kinase targets but with some overlap in their substrate specificities. The availability of alternative substrates for AMPKK and CaMKIK allowed the unequivocal demonstration that AMP and Ca²⁺/ealmodulin promote the activation of AMPK and CaMKI, respectively, via three independent mechanisms: 1) direct activation of AMPK and CaMKI, 2) activation of AMPKK and CaMKIK, and 3) by binding to AMPK and CaMKI, inducing exposure of their phosphorylation sites. Since AMP and Ca²⁺/calmodulin each has a triple effect in its respective system, in vivo, the two systems would be expected to be exquisitely sensitive to changes in concentration of their respective activating ligands.