By sequence comparison, the majority of 1,4-α-glucan-branching enzymes (GBEs) consist of an N-terminal carbohydrate-binding domain, a TIM-barrel catalytic domain, and a C-terminal all-beta domain. Among these structures, the GBE from Geobacillus thermoglucosidans STB02 uniquely has a highly charged 26-amino-acid C-terminal extension, whose functional roles are the least understood. In this research, the functional significance of the C-terminal domain in GBE from G. thermoglucosidans STB02 and its extension were assessed using a C-terminal deletion analysis. Mutants lacking of more than 7 residues of the C-terminal all-beta domain could not be detected in lysates of their Escherichia coli expression strains, suggesting that an intact all-beta domain is required for structural stability. In contrast, truncation of the C-terminal extension resulted in greater stability and solubility than the wild type, as well as a lower sensitivity to the presence of added metal ions. Comparison of this mutant with the wild type suggests that the interaction of metal ions with the C-terminal extension influences performance of this enzyme.

中文翻译：

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嗜热葡糖葡聚糖杆菌STB02的嗜热1,4-α-葡聚糖支化酶中C末端延伸的重要性。

通过序列比较，大多数1，4-α-葡聚糖支化酶（GBE）由N末端碳水化合物结合结构域，TIM桶催化结构域和C末端全β结构域组成。在这些结构中，来自热葡糖葡聚糖杆菌STB02的GBE独特地具有高度带电荷的26个氨基酸的C末端延伸，其功能作用最不清楚。在这项研究中，使用C末端缺失分析评估了G. thermoglucosidans STB02在GBE中C末端结构域的功能重要性及其延伸。在其大肠杆菌表达菌株的裂解物中无法检测到缺少7个以上C末端全β结构域残基的突变体，这表明完整的全β结构域是结构稳定性所必需的。相反，C-末端延伸的截短导致比野生型更大的稳定性和溶解性，以及对添加的金属离子的存在更低的敏感性。该突变体与野生型的比较表明，金属离子与C端延伸的相互作用会影响该酶的性能。