Koni P A, Ellar D J
Department of Biochemistry, University of Cambridge, UK.
Microbiology (Reading). 1994 Aug;140 ( Pt 8):1869-80. doi: 10.1099/13500872-140-8-1869.
The entomocidal delta-endotoxins CytA and CytB produced by Bacillus thuringiensis (Bt) subspecies israelensis and kyushuensis respectively showed a similar level of toxicity to mosquito larvae but were not toxic to the larvae of the lepidopteran Manduca sexta. CytA and CytB are also similar in sequence, predicted secondary structure and alpha-helical content, the only obvious difference being a C-terminal fifteen residue 'tail' on CytB. Investigations of the function, if any, of the CytB C-terminal 'tail' showed that this delta-endotoxin is highly expressed and forms inclusions in an acrylstalliferous Bt mutant without the aid of the 20 kDa 'helper' protein from Bt subspecies israelensis which is essential for CytA inclusion formation. After proteinase K treatment, CytA and CytB were processed to virtually the same points in a sequence alignment and were equally haemolytic in vitro. However, the results suggested that unprocessed CytB differs from unprocessed CytA in that the former is not haemolytic.
苏云金芽孢杆菌以色列亚种和九州亚种分别产生的杀昆虫δ-内毒素CytA和CytB对蚊幼虫显示出相似的毒性水平，但对鳞翅目烟草天蛾幼虫无毒。CytA和CytB在序列、预测的二级结构和α-螺旋含量方面也相似，唯一明显的差异是CytB上有一个C端十五个残基的“尾巴”。对CytB C端“尾巴”的功能（如果有的话）进行的研究表明，这种δ-内毒素在一个无晶体的Bt突变体中高表达并形成包涵体，无需来自苏云金芽孢杆菌以色列亚种的20 kDa“辅助”蛋白，而该蛋白对CytA包涵体形成至关重要。经蛋白酶K处理后，CytA和CytB在序列比对中被加工到几乎相同的位点，并且在体外具有同等的溶血活性。然而，结果表明未加工的CytB与未加工的CytA不同，前者不具有溶血活性。
