The carboxyterminal processing protease of D1 protein: expression, purification and enzymology of the recombinant and native spinach proteins

The carboxyterminal processing protease of D1 protein (CtpA) is predicted to be an excellent target for a general broad-spectrum herbicide. The gene for spinach CtpA has been expressed in Escherichia coli. The expressed protein that was found mainly in inclusion bodies has been purified and refolded on a nickel-chelate column. Active recombinant CtpA was recovered. Two assays for CtpA activity were developed, a medium-throughput HPLC assay using a fluorescent substrate and a high-throughput assay based on fluorescence polarization capable of application in a high-throughput 96-well plate format. This high-throughput assay was developed to screen chemistry for CtpA inhibitors. Native spinach CtpA was partially purified and the native and recombinant enzymes were compared kinetically for their K(m) and V(max) values using different peptide substrates. Native CtpA partially purified from spinach was shown to have similar kinetic properties to recombinant CtpA. Antibodies developed against the recombinant protein were used to estimate the in planta abundance of the native enzyme in spinach. Since only a small proportion of the recombinant protein is refolded during isolation and it appears that only a small proportion of this enzyme is active, size-exclusion chromatography and light scattering experiments were performed on rCtpA in order to gain insight into its structure and the reasons why most of the protein is not active. The use of rCtpA to screen for herbicidal compounds and the more general question of how good a herbicide target the enzyme is are discussed.