Protein kinases are signaling molecules that regulate vital cellular and biological processes by phosphorylating cellular proteins. Kinases are linked to variety of diseases such as cancer, immune deficiencies and degenerative diseases.
This thesis work aimed to identify direct substrates for protein kinases in the CMGC family, which consists of the cyclin-depended kinases (CDK), mitogen activated protein kinases (MAPK), glycogen synthase kinase-3 (GSK3) and CDC-like kinases (CLK). CMGC kinases have been identified as cancer hubs in interactome studies, but large-scale identification of direct substrates has been difficult due to the lack of efficient methods.
Here, we present a heavy-labeled 18O-ATP-based kinase assay combined with LC-MS/MS analysis for direct substrate identification. In the assay, HEK and HeLa cell lysates are treated with a pan-kinase inhibitor FSBA which irreversibly blocks endogenous kinases. After the removal of FSBA, cell lysates are incubated with the kinase of interest and a heavy-labeled ATP, which contains 18O isotope at the γ-phosphate position. Resulting phosphopeptides are enriched with Ti4+- IMAC before the LC-MS/MS analysis, which distinguishes the desired phosphorylation events based on a mass shift caused by the heavy 18O.
With this pipeline of methods, we managed to quantify and identify direct substrates for 26 members of CMGC kinase family. A total of 1345 substrates and 3841 interacting kinase-substrate pairs were identified in cytosolic cell lysates, from which 165 were annotated in the PhosphoSitePlus® database. To identify substrates for kinases with nuclear localization, ten kinases were tested with nuclear HEK cell lysate. We identified 194 kinase-substrate pairs, 141 of which were unique to the nuclear fraction and 27 annotated in the PhosphoSitePlus® database. Finally, kinases with outstandingly high amounts of novel substrates were subjected to gene ontology analysis. We were able to link the gene ontology classifications of novel substrates to the biological processes regulated by the kinase of interest. These results indicate that heavy-labeled 18O-ATP-based kinase assay linked LC-MS/MS is a useful tool for large-scale direct kinase substrate identification.
