5'-AMP ACTIVATES THE AMP-ACTIVATED PROTEIN-KINASE CASCADE AND CA2+ CALMODULIN ACTIVATES THE CALMODULIN-DEPENDENT PROTEIN-KINASE-I CASCADE, VIA 3 INDEPENDENT MECHANISMS/

ARIP-activated protein kinase (AMPK) and Ca2+/calmodulin (CaM) depende nt protein kinase I (CaMKI) are protein kinases that are regulated bot h by allosteric activation (AMP and Ca2+/CaM, respectively) and by pho sphorylation by upstream protein kinases (AMPK kinase (AMPKK) and CaMK I kinase (CaMKIK), respectively). We now report that AMPKK can activat e CaMKI and that, conversely, CaMKIK can activate AMPK. CaMKIK is 68-f old more effective at activating CaMKI than AMPK, while AMPKK is 17-fo ld more effective at activating AMPK than CaMKI. Our results suggest t hat CaMKIK and AMPKK are distinct enzymes dedicated to their respectiv e kinase targets but with some overlap in their substrate specificitie s. The availability of alternative substrates for AMPKK and CaMKIK all owed the unequivocal demonstration that AMP and Ca2+/calmodulin promot e the activation of AMPK and CaMKI, respectively, via three independen t mechanisms: 1) direct activation of AMPK and CaMKI, 2) activation of AMPKK and CaMKIK, and 3) by binding to AMPK and CaMKI, inducing expos ure of their phosphorylation sites. Since AMP and Ca2+/calmodulin each has a triple effect in its respective system, in vivo, the two system s would be expected to be exquisitely sensitive to changes in concentr ation of their respective activating ligands.