Related Experiment Video
Updated: May 16, 2026

A Purification and In Vitro Activity Assay for a (p)ppGpp Synthetase from Clostridium difficile
Published on: November 3, 2018
In vitro substrate phosphorylation by Ca²⁺/calmodulin-dependent protein kinase kinase using guanosine-5'-triphosphate
Saki Yurimoto1, Tomohito Fujimoto, Masaki Magari
1Department of Signal Transduction Sciences, Faculty of Medicine, Kagawa University, 1750-1 Miki-cho, Kita-gun, Kagawa 761-0793, Japan.
Background:
Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) phosphorylates and activates particular downstream protein kinases - including CaMKI, CaMKIV, and AMPK- to stimulate multiple Ca2+-signal transduction pathways. To identify previously unidentified CaMKK substrates, we used various nucleotides as phosphate donors to develop and characterize an in vitro phosphorylation assay for CaMKK.
Results:
Here, we found that the recombinant CaMKK isoforms were capable of utilizing Mg-GTP as a phosphate donor to phosphorylate the Thr residue in the activation-loop of CaMKIα (Thr177) and of AMPK (Thr172) in vitro. Kinetic analysis indicated that the Km values of CaMKK isoforms for GTP (400-500 μM) were significantly higher than those for ATP (~15 μM), and a 2- to 4-fold decrease in Vmax was observed with GTP. We also confirmed that an ATP competitive CaMKK inhibitor, STO-609, also competes with GTP to inhibit the activities of CaMKK isoforms. In addition, to detect enhanced CaMKI phosphorylation in brain extracts with Mg-GTP and recombinant CaMKKs, we found potential CaMKK substrates of ~45 kDa and ~35 kDa whose Ca2+/CaM-induced phosphorylation was inhibited by STO-609.
Conclusions:
These results indicated that screens that use STO-609 as a CaMKK inhibitor and Mg-GTP as a CaMKK-dependent phosphate donor might be useful to identify previously unidentified downstream target substrates of CaMKK.
Related Concept Videos
Calmodulin-dependent Signaling
The Ca2+-CaM complex does not have enzymatic activity by itself. Instead, the complex binds downstream target proteins, including membrane proteins or enzymes,...
Protein Kinases and Phosphatases
Protein kinases
Many proteins in the cell are regulated by phosphorylation, the addition of a phosphate group. A family of enzymes called kinases...
cAMP-dependent Protein Kinase Pathways
Phosphorylation
During phosphorylation, protein kinases transfer the terminal phosphate group of ATP to specific amino acid side chains of substrate proteins. Serine, threonine, and tyrosine are the most commonly...
GTPases and their Regulation
Large G-proteins, also known...
IP3/DAG Signaling Pathway

