Investigating the Deoxyribonuclease Activity of CRM197 with Site-Directed Mutagenesis

The protein cross-reactive material 197 (CRM197) is known to catalyze the hydrolytic cleavage of DNA (DNase activity). A suspected metal-binding site (S109, T111, and E112) and suspected DNA-binding motif (T89, K90, and V91) were predicted within the CRM197 protein X-ray crystal structure (4AE0) using METSITE and DNABindProt, respectively. Between these two predicted sites is a groove (K103, E116, T120, E122, F123, and R126) that may assist in DNase activity. Alanine scanning was performed at these sites to determine which amino acids might be important for DNase activity. These mutations individually or in combination either maintained or increased the overall DNase activity compared to the unmodified CRM197. Mutation at the suspected metal-binding site showed similar fluctuations to the overall DNase activity whether the DNase assays were run with Mg2+ and Ca2+ or Mn2+. However, many of the mutations within the suspected DNA-binding motif saw significant differences depending on which metal was used. Only some of the improvements in DNase activity could be attributed to improved folding of the mutants compared to the unmodified CRM197. This study should provide a basis for further mutagenesis studies to remove the DNase activity of CRM197.