Development of the procedures for high-yield expression and rapid purification of active recombinant Csk-homologous kinase (CHK): Comparison of the catalytic activities of CHK and CSK

Csk-homologous kinase (CHK) is an important endogenous inhibitor constraining the oncogenic actions of Src-family kinases (SFKs) in cells. It suppresses SFK activity by specifically phosphorylating the conserved regulatory tyrosine near the C-terminus of SFKs. In addition to phosphorylation, CHK employs a novel non-catalytic inhibitory mechanism to suppress SFK activity. This mechanism involves direct binding of CHK to the active forms of SFKs to form stable protein complexes. Since aberrant activation of SFKs contributes to cancer formation and progression, small-molecule inhibitors mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics. Elucidation of the catalytic and regulatory properties and the structural basis of the CHK non-catalytic inhibitory mechanism would facilitate the development of these small-molecule inhibitors. To this end, we developed procedures for higher level expression in insect cells of active recombinant CHK with a hexa-histidine tag attached to its C-terminus (referred to as CHK-His6) and its rapid purification by a two-step method. Analyses by size-exclusion column chromatography and analytical ultracentrifugation revealed that the purified CHK-His6 exists as a monomeric species in solution. Biochemical analyses demonstrated that CHK-His6 exhibits efficiencies comparable to those of CSK in phosphorylating artificial protein and peptide substrates as well as an intact SFK protein. Our results indicate that the recombinant CHK-His6 can be used for future studies to decipher the three-dimensional structure, and regulatory and catalytic properties of CHK.