The ATPase Activity of Expressed AAA-ATPase Protein

ATPases are the enzymes that breakdown ATP to ADP and release inorganic phosphate (Pi). Here we provide a detailed protocol to determine the ATPase activity of a recombinant AAA-ATPase protein (GENERAL CONTROL NON-REPRESSIBLE-4 [GCN4]) by spectrophotometric absorption at 360 nm to measure the accumulated inorganic phosphate. In general, the substrate 2-amino-6-mercapto-7-methylpurine riboside (methylthioguanosine, a guanosine analog: MESG) is enzymatically converted in the presence of Pi by purine nucleoside phosphorylase (PNP) to ribose 1-phosphate and 2-amino-6-mercapto-7-methylpurine. The spectrophotometric shift in maximum absorbance at 330 nm for the MESG substrate and subsequent conversion product at 360 nm due to enzymatic conversion was measured. The GCN4-His-tagged recombinant protein was expressed in BL21 cells and purified using Ni-NTA column. This purified protein was then used for the quantitation of Pi in solution or the continuous determination of Pi released due to the ATPase activity of GCN4, an AAA-ATPase protein conserved in many eukaryotes, which in plants regulates stomatal aperture during biotic and abiotic stress in plants.