Zlotkin E, Gurevitz M, Fowler E, Adams M E
Institute of Life Sciences, Hebrew University, Jerusalem, Israel.
Arch Insect Biochem Physiol. 1993;22(1-2):55-73. doi: 10.1002/arch.940220107.
The present study examines the similarity in the symptoms and binding properties between the depressant and excitatory insect-selective neurotoxins, derived from scorpion venom. A comparison of their primary structures and neuromuscular effects is presented. A new depressant toxin (LqhIT2) was purified from the venom of the scorpion Leiurus quinquestriatus hebraeus. The effects of this toxin on a prepupal housefly neuromuscular preparation mimic its effects on the intact insect, i.e, a brief period of repetitive bursts of regular junction potentials (JPs) is followed by reduced amplitude JPs ending with a block of the neuromuscular transmission. "Loose" patch clamp recordings indicate that the repetitive activity has a presynaptic origin (the motor nerve) and resembles the effect of the excitatory toxin AaIT. The final synaptic block is supposed to be the end result of neuronal membrane depolarization. Such an effect is not caused by an excitatory toxin, which induces long "trains" of repetitive firing. The amino acid sequences of three depressant toxins were determined by automatic Edman degradation indicating a high degree of sequence homology. This conservation differs from those of other groups of scorpion toxins. The opposing pharmacological effects of depressant toxins are discussed in light of the above neuromuscular effects and sequence analysis. A genetic approach in the study of the structure-function relationships of the depressant toxins was initiated by isolating cDNA clones encoding the LqhIT2 and BjIT2 toxins. Their sequence analysis revealed the precursor form of these toxins: A 21 amino acid residue signal peptide followed by a 61 amino acid region of the mature toxin, and three additional amino acids at the carboxy terminus.
本研究考察了源自蝎毒的抑制性和兴奋性昆虫选择性神经毒素在症状和结合特性方面的相似性。文中对它们的一级结构和神经肌肉效应进行了比较。从以色列金蝎（Leiurus quinquestriatus hebraeus）的毒液中纯化出一种新的抑制性毒素（LqhIT2）。这种毒素对家蝇蛹前神经肌肉标本的作用与其对完整昆虫的作用相似，即先是一段短暂的规则接头电位（JPs）重复爆发期，随后JPs幅度降低，最终神经肌肉传递受阻。“松散”膜片钳记录表明，这种重复活动起源于突触前（运动神经），类似于兴奋性毒素AaIT的作用。最终的突触阻滞被认为是神经元膜去极化的最终结果。这种效应不是由兴奋性毒素引起的，兴奋性毒素会诱导长时间的重复放电“串”。通过自动埃德曼降解法测定了三种抑制性毒素的氨基酸序列，结果表明它们具有高度的序列同源性。这种保守性与其他蝎毒素组不同。根据上述神经肌肉效应和序列分析，对抑制性毒素相反的药理作用进行了讨论。通过分离编码LqhIT2和BjIT2毒素的cDNA克隆，开启了研究抑制性毒素结构-功能关系的遗传学方法。它们的序列分析揭示了这些毒素的前体形式：一个21个氨基酸残基的信号肽，后面跟着一个61个氨基酸的成熟毒素区域，以及在羧基末端的另外三个氨基酸。
