Soluble Diphtheria Toxin Variant, CRM 197 was Obtained in Escherichia coli at High Productivity Using SUMO Fusion and an Adjusted Expression Strategy

CRM197, a non-toxic diphtheria toxin variant, is widely used as a polysaccharide carrier in a variety of conjugate vaccines and also exhibits antitumor activity. CRM197 commercial production is limited due to the low yield of Corynebacterium diphtheriae C7 (197) tox-. Developing an efficient method for recombinant CRM197 production reduces production costs and is critical for expanding the application coverage of related medical products and basic research. Escherichia coli is a frequently used host for heterologous protein synthesis. However, the primary limitation of this system is the inclusion body formation and the low yield of active protein recovery.As a result, we attempted to produce CRM197 in the soluble form in E. coli using a small ubiquitin-related modifier (SUMO) tag fusion and an expression strategy optimized for protein production.CRM197 was expressed intracellularly in E. coli BL21 (DE3) with its N-terminus fused to a SUMO tag preceded by a histidine tag (HSCRM197). To improve the solubility of HSCRM197 in E. coli, a response surface method (RSM) experimental design was used based on three factors: expression temperature, inducer concentration, and sorbitol inclusion in the culture medium. Metal affinity chromatography was used to purify HSCRM197, and the SUMO tag was removed using the SUMO protease's catalytic domain. After adsorbing the SUMO tag on a Ni-NTA column, CRM197 was obtained. DNA degradation activity was determined for both HSCRM197 and CRM197.When HSCRM197 was expressed in E. coli under common expression conditions (37ºC, 1000 μM inducer), 15.4% of the protein was found in the cellular soluble fraction. However, when the RSM-derived expression conditions were used (30ºC, 510 μM inducer, and 200 mM sorbitol), the obtained HSCRM197 was almost completely soluble (96.5% solubility), and the system productivity was 32.67 μg ml-1 h-1. HSCRM197 and CRM197 both exhibited nuclease activity. However, the activity of CRM197 was greater than that of HSCRM197.These findings established the utility of the method developed in this study to produce CRM197 for medical applications.