<scp>MuGger</scp> Toxins: Exploring the Selective Binding Mechanism of Clostridial Glucosyltransferase Toxin B and Host <scp>GTPases</scp>

(a) Clostridioides difficile ( C. difficile ) bacterium can cause severe diarrhea and its over-colonization in the host's intestinal tract lead to the development of pseudomembranous colitis, generally due to antibiotic usage. The primary exotoxins involved are toxin A (TcdA) and toxin B (TcdB), the latter being more pathogenic. TcdB has glucosyltransferase activity and mediates monoglycosylation by targeting host cell enzymes (mainly Rho and Ras family of GTPases) with differential selectivity. Here, we aim to provide structural and dynamic insights into how TcdB impacts the host's intestinal epithelial cells focusing on the glycosylation mechanism of Rho GTPases, Cdc42, and Rac1, at the molecular level. To this aim, we modeled the unknown TcdB-host protein complex structures, based on the available experimental structures of TcdB, through protein-protein docking. Then, we elaborated on TcdB-Rho GTPase models as TcdB is known to selectively interact with GDP-bound inactive states of Rho GTPases, over the GTP-bound active ones, but the mechanism is unclear. Through a total of 6 μs-long molecular dynamics simulation of TcdB and GTP/GDP-bound Rac1 and Cdc42 complexes, TcdB's selective binding mechanism was revealed for Rac1. TcdB-Rac1 complexes were further analyzed with enhanced sampling techniques such as well-tempered metadynamics simulations and umbrella sampling to reveal selective binding mechanism between TcdB and GDP-bound Rac1. Our results show that TcdB selectively binds to GDP-bound Rac1, over the GTP-bound one, driven by its affinity for the Mg²⁺ ion. A destabilized Mg²⁺ ion incapable of coordinating GDP disrupts Rac1's GTPase function, shedding light on the molecular basis of TcdB's pathogenic effects.