Quenchbody (Q-body) is a fluorescent biosensor in which a fluorescent dye is tagged near the antigen binding site of an antibody. The fluorescence of the dye is quenched by the tryptophan residues present in the variable region of the antibody, and is recovered when the antigen binds. Q-bodies have been prepared using recombinant DNA technology by introducing one or more tag sequence(s) at either the N-terminal of the Fab or the single chain variable region fragment of the antibody, and labeling the tag with a fluorescent dye. However, preparation of recombinant antibody fragments is time-consuming and the performance of the Q-body is unpredictable. Here we report an antibody-binding quenching probe made from protein M from Mycoplasma genitalium that can transform the IgG antibody into an immunosensor. By using bacterially expressed and purified protein M and labeling the C-terminal cysteine-containing tag, we prepared a TAMRA-labeled PM Q-probe. When the Q-probe was incubated with Fab or IgG recognizing the bone Gla protein, the fluorescence of the probe was quenched and subsequently recovered by the adding of antigens in a dose-dependent manner. We also succeeded in detecting several small biomarkers with nanomolar sensitivity, including thyroxine extracted from human serum. The clone found to be suitable for the detection of cortisol was confirmed to work as a recombinant Q-body as well, which also worked in 50% human serum. The results suggest that the Q-probe can quickly convert an IgG to a biosensor, which will be useful in rapid diagnosis of small biomarkers.

猝灭体（Q-body）是一种荧光生物传感器，其中在抗体的抗原结合位点附近标记了荧光染料。染料的荧光被抗体可变区中的色氨酸残基淬灭，并在抗原结合时被回收。使用重组DNA技术制备Q抗体，方法是在Fab的N端或抗体的单链可变区片段上引入一个或多个标签序列，并用荧光染料标记标签。然而，重组抗体片段的制备是费时的，并且Q-抗体的性能是不可预测的。在这里，我们报告了由生殖支原体的蛋白M制成的抗体结合淬灭探针可以将IgG抗体转化为免疫传感器。通过使用细菌表达和纯化的蛋白M并标记包含C端半胱氨酸的标签，我们制备了TAMRA标记的PM Q探针。当Q探针与识别骨Gla蛋白的Fab或IgG孵育时，探针的荧光被淬灭，随后以剂量依赖性方式通过添加抗原而被回收。我们还成功地检测了几种具有纳摩尔灵敏度的小生物标记，包括从人血清中提取的甲状腺素。确认发现适合检测皮质醇的克隆也可作为重组Q体，也可在50％的人血清中起作用。结果表明，Q-探针可以快速将IgG转化为生物传感器，这将有助于快速诊断小型生物标志物。