N端延长对大肠杆菌铁蛋白结构稳定性及其自组装的影响

Effects of N-terminal elongation on the stability and self-assembly of Escherichia coli bacterioferritin

【目的】研究延长的N端肽链对铁蛋白结构稳定性及蛋白自组装的影响,为在N端融合生物分子构建功能性纳米材料奠定基础。【方法】在大肠杆菌铁蛋白N端添加6个组氨酸残基,所得重组蛋白采用体积排阻色谱、非变性凝胶电泳和圆二色谱等手段与野生型铁蛋白的聚合态、二级结构及热力学稳定性进行对比分析。【结果】体积排阻色谱表明野生型铁蛋白在溶液中以24聚体和二聚体混合物形式存在,但突变蛋白仅以二聚体的形式存在;非变性凝胶电泳实验结果与体积排阻色谱结果吻合,即突变蛋白仅显示1条带,且与野生型铁蛋白的2聚体迁移位置基本相同;圆二色谱结果表明突变蛋白的二级结构与野生型类似,呈典型的α-螺旋结构,突变蛋白的Tm值比野生型降低了1.1℃。【结论】在大肠杆菌铁蛋白亚基的N端增加6个组氨酸残基所得突变蛋白的二级结构没有发生明显变化,但却无法形成24聚体壳状结构,在溶液中仅以二聚体的形式存在,且热力学稳定性有所降低。N端的延长可能导致铁蛋白单体构型发生改变,因而无法进一步组装成壳状结构。

[Objective]Investigate the effect of N-terminal elongation on the structural stability and protein self-assembly of the nano-cage bacterioferritin to provide the basis for the construction of functional nanomaterials by incorporating bio- molecules at the N-terminus of bacterioferritin. [ Method ] Escherichia coli bacterioferritin protein was genetically engineered by elongating the N-terminus with six histidine residues. The mutant oligomerization state, secondary structure and thermo-stability were characterized and compared with that of the wild type using size exclusion chromatog- raphy, native gel electrophoresis and circular dichroism spectroscopy.[ Result] The wild type formed a mixture of 24-mer and dimer in solution, while the mutant formed solely dimer. Consistent with the size exclusion chromatography experi- ment, only one band was observed corresponding to the wild-type dimer according to the native gel electrophoresis. Char- acterization of the mutant using circular dichroism demonstrated that it still folded into o^-helical structure, exhibiting similar secondary structure as compared to the wild type. The melting temperature of the mutant was found to be 1.1 ~C lower than that of the wild type. [ Conclusion] Although it still folded into the o^-helical structure, the protein with an e- longated N-terminus completely lost the ability to form the nano-cage structure, and formed dimer solely in solution with reduced thermo-stability compared with that of the parent protein. This study indicates that N-terminal elongation might result in a conformational change of bacterioferritin subunits, preventing the protein from self-assembling into the nano- cage structure.