Anthrax Toxin Protective Antigen Variants That Selectively Utilize either the CMG2 or TEM8 Receptors for Cellular Uptake and Tumor TargetingThis work was supported by the intramural research programs of the NIAID and the NIDCR, National Institutes of Health. K. H. C., S. L., C. E. L., T. H. B., and S. H. L. are inventors on patents of targeted toxins that have all been assigned to the National Institutes of Health. The content is solely the responsibility of the authors and does not necessarily

• Published in: The Journal of biological chemistry Vol. 291; no. 42; pp. 22021 - 22029
• Main Authors: Chen, Kuang-Hua, Liu, Shihui, Leysath, Clinton E., Miller-Randolph, Sharmina, Zhang, Yi, Fattah, Rasem, Bugge, Thomas H., Leppla, Stephen H.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2016
• Subjects: anthrax toxin; CMG2; mutagenesis in vitro; phage display; protective antigen; receptor; TEM8; tumor therapy; anthrax toxin; receptor; phage display; protective antigen; mutagenesis in vitro; CMG2; TEM8; tumor therapy
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M116.753301

The protective antigen (PA) moiety of anthrax toxin binds to cellular receptors and mediates the translocation of the two enzymatic moieties of the toxin to the cytosol. Two PA receptors are known, with capillary morphogenesis protein 2 (CMG2) being the more important for pathogenesis and tumor endothelial marker 8 (TEM8) playing a minor role. The C-terminal PA domain 4 (PAD4) has extensive interactions with the receptors and is required for binding. Our previous study identified PAD4 variants having enhanced TEM8 binding specificity. To obtain PA variants that selectively bind to CMG2, here we performed phage display selections using magnetic beads having bound CMG2. We found that PA residue isoleucine 656 plays a critical role in PA binding to TEM8 but has a much lesser effect on PA binding to CMG2. We further characterized the role of residue 656 in distinguishing PA binding to CMG2 versus TEM8 by substituting it with the other 19 amino acids. Of the resulting variants, PA I656Q and PA I656V had significantly reduced activity on TEM8-expressing CHO cells but maintained their activity on CMG2-expressing CHO cells. The preference of these PA mutants for CMG2 over TEM8 was further demonstrated using mouse embryonic fibroblast cells and mice deficient in the CMG2 and/or the TEM8 receptors. The structural basis of the alterations in the receptor binding activities of these mutants is also discussed.