Improving the Solubility of Maize Uroporphyrinogen III Methyltransferase as a Red Fluorescent Indicator by Site-directed Mutagenesis

S-adenosylmethionine-dependent uroporphyrinogen III methyltransferase (SUMT) is a novel red fluorescent indicator. However, the production of SUMT in Escherichia coli is restricted by its relatively low solubility, and little is known about the red fluorescent materials that are associated with SUMT. Two individual SUMT mutations, L166A and L88R/L89G double mutants, were produced by site-directed mutagenesis. Both mutants were overexpressed in E. coli and purified by Ni-NTA chromatography. The reddish mixtures isolated from the purified L88R/L89G double mutant were analyzed by UV-visible spectra scanning and mass spectrometry (MS). The L88R/L89G double mutant has enzymatic activity in vivo, whereas L166A mutant loses the activity. Trimethylpyrrocorphin is identified as the main constituent in the isolated pigment. The purified L88R/L89G mutant increases protein solubility, which is potentially applied as the fluorescent indicator denoting the solubility of protein fusion partner.