Nguyen, J. D. M. | bioRxiv Science

bioRxiv2023

Mycolactone A vs. B: Does localization or association explain isomer-specific toxicity?

Mycolactone is an exotoxin produced by Mycobacterium ulcerans that causes the neglected tropical skin disease Buruli ulcer. This toxin inhibits the Sec61 translocon in the endoplasmic reticulum (ER), preventing the host cell from producing many secretory and transmembrane proteins, resulting in cytotoxic and immunomodulatory effects. Interestingly, only one of the two dominant isoforms of mycolactone is cytotoxic. Here, we investigate the origin of this specificity by performing extensive molecular dynamics (MD) simulations with enhanced free energy sampling to query the association trends of the two isoforms with both the Sec61 translocon and the ER membrane, which serves as a toxin reservoir prior to association. Our results suggest that mycolactone B (the cytotoxic isoform) has a stronger association with the ER membrane than mycolactone A due to more favorable interactions with membrane lipids and water molecules. This could increase the reservoir of toxin proximal to the Sec61 translocon. Isomer B also interacts more closely with the lumenal and lateral gates of the translocon, the dynamics of which are essential for protein translocation. These interactions induce a more closed conformation, which has been suggested to block signal peptide insertion and subsequent protein translocation. Collectively, these findings suggest that isomer Bs unique cytotoxicity is a consequence of both increased localization to the ER membrane and channel-locking association with the Sec61 translocon, facets that could be targeted in the development of Buruli Ulcer diagnostics and Sec61-targeted therapeutics.

biophysics↗

bioRxiv2022

Can Membrane Composition Traffic Toxins? Mycolactone and Preferential Membrane Interactions

Mycolactone is a cytotoxic and immunosuppressive macrolide produced by Mycobacterium ulcerans and the sole causative agent of the neglected tropical skin disease Buruli ulcer. The toxin acts by invading host cells and interacting with intracellular targets to disrupt multiple fundamental cellular processes. Mycolactones amphiphilic nature enables strong interactions with lipophilic environments, including cellular membranes; however, the specificity of these interactions and the role of membranes in the toxins pathogenicity remain unknown. It is likely that preferential interactions with lipophilic carriers play a key role in the toxins distribution in the host, which, if understood, could provide insights to aid in the development of needed diagnostics for Buruli ulcer disease. In this work, molecular dynamics simulations were combined with enhanced free energy sampling to characterize mycolactones association with and permeation through models of the mammalian endoplasmic reticulum (ER) and plasma membranes (PM). We find that increased order in the PM not only leads to a different permeation mechanism compared to that in the ER membrane, but also an energetic driving force for ER localization. Increased hydration, membrane deformation, and preferential interactions with unsaturated lipid tails stabilize the toxin in the ER membrane, while disruption of lipid packing is a destabilizing force in the PM. STATEMENT OF SIGNIFICANCEMycolactone is sole the causative agent of Buruli ulcer, a neglected tropical disease involving large necrotic lesions that can cause permanent disfigurement if left untreated. Due to its amphiphilic nature, the toxin hides from traditional diagnostic detection and the host immune system by associating with lipophilic carriers, including cellular membranes. Our work uses extensive all-atom simulations to query if the toxin has preferential interactions with different types of membranes. We find a clear preference for more disordered membranes, like the endoplasmic reticulums, via interactions with unsaturated lipid tails and membrane deformation. The revealed insights can be used to predict host cell distribution between different types of lipophilic carriers and to aid in the design of Buruli ulcer diagnostics.

biophysics↗