Ca2؉ /calmodulin-dependent protein kinase kinase  (CaMKK) is a known activating kinase for AMP-activated protein kinase (AMPK). In vitro, CaMKK phosphorylates Thr 172 in the AMPK␣ subunit more efficiently than CaMKK␣, with a lower K m (ϳ2 M) for AMPK, whereas the CaMKI␣ phosphorylation efficiencies by both CaMKKs are indistinguishable. Here we found that subdomain VIII of CaMKK is involved in the discrimination of AMPK as a native substrate by measuring the activities of various CaMKK␣/CaMKK chimera mutants. Sitedirected mutagenesis analysis revealed that Leu 358 in CaMKK/ Ile 322 in CaMKK␣ confer, at least in part, a distinct recognition of AMPK but not of CaMKI␣.
Ca2ϩ /calmodulin-dependent protein kinase kinases (CaMKKs) 2 were originally identified as members of the calmodulin (CaM) kinase (CaMK) family that phosphorylate and activate two multifunctional CaMKs, including CaMKI and CaMKIV, constituting Ca 2ϩ -dependent kinase cascades named CaMK cascades (1-3). CaMKK in mammals is derived from two genes (CaMKK␣ and CaMKK), with ϳ70% of amino acid sequence homology in the catalytic domains (4 -6). The CaMKK/CaMKIV cascade has been shown to be involved in the regulation of gene expression through the phosphorylation of transcription factors such as cAMP-response element-binding protein (7,8) and serum response factor (9). In contrast, the CaMKK/CaMKI cascade plays an important role in neuronal development, including axon outgrowth (10), leptin-induced spine formation (11), and activity-dependent synaptogenesis (12). Except for the downstream CaMKs, AMP-activated protein kinase (AMPK) has been identified as a novel target kinase for CaMKK (13-15), and the CaMKK/AMPK pathway has been demonstrated to be functional in various Ca 2ϩ -dependent AMPK-mediated signal transduction processes, including regulation of appetite and glucose homeostasis (16), stimulation of mitochondrial fatty acid oxidation by thyroid hormone T3 (17), and regulation of autophagy by amino acid starvation (18). According to studies using either RNA interference or pharmacological inhibition of CaMKK in HeLa cells, which do not express LKB1, an alternative AMPK kinase, CaMKK has been shown to be responsible for Ca 2ϩ -dependent activation of AMPK in vivo, whereas both CaMKK isoforms are capable of phosphorylating the AMPK␣ subunit at Thr 172 in vitro (13-15). This was confirmed by the fact that STO-609 (19), a CaMKK inhibitor, suppressed ionomycin-induced AMPK phosphorylation in A549 cells (a human lung adenocarcinoma epithelial cell line) expressing STO-609-resistant CaMKK␣ but not . These results indicate that the CaMKK isoform, rather than CaMKK␣, preferably recognizes AMPK, suggesting that the recognition mechanism of AMPK by CaMKK isoforms as a substrate may differ from that of CaMKI. A previous report showed that CaMKK, but not CaMKK␣, forms a stable complex with AMPK, which could explain why CaMKK is an AMPK kinase and CaMKK␣ is not (21). However, Fogarty et al. (22) reported that CaMKK activates AMPK without forming a stabl...