The Toxin Complex (Tc) superfamily consists of toxin translocases that contribute to the targeting, delivery, and cytotoxicity of certain pathogenic Gram-negative bacteria. Membrane receptor targeting is driven by the A-subunit (TcA), which comprises IgG-like receptor binding domains (RBDs) at the surface. To better understand XptA2, an insect specific TcA secreted by the symbiont X. nematophilus from the intestine of entomopathogenic nematodes, we determined structures by X-ray crystallography and cryo-EM. Contrary to a previous report, XptA2 is pentameric. RBD-B exhibits an indentation from crystal packing that indicates loose association with the shell and a hotspot for possible receptor binding or a trigger for conformational dynamics. A two-fragment XptA2 lacking an intact linker achieved the folded pre-pore state like wild type (wt), revealing no requirement of the linker for protein folding. The linker is disordered in all structures, and we propose it plays a role in dynamics downstream of the initial pre-pore state.

ABC toxin complexes are a class of protein toxin translocases comprised of a multimeric assembly of protein subunits. Each subunit displays a unique composition, contributing to the formation of a syringe-like nano-machine with natural cargo carrying, targeting, and translocation capabilities. Many of these toxins are insecticidal, drawing increasing interest in agriculture for use as biological pesticides. The A subunit (TcA) is the largest subunit of the complex and contains domains associated with membrane permeation and targeting. The B and C subunits, TcB and TcC, respectively, package into a cocoon-like structure that contains a toxic peptide and are coupled to TcA to form a continuous channel upon final assembly. In this review, we outline the current understanding and gaps in the knowledge pertaining to ABC toxins, highlighting seven published structures of TcAs and how these structures have led to a better understanding of the mechanism of host tropism and toxin translocation. We also highlight similarities and differences between homologues that contribute to variations in host specificity and conformational change. Lastly, we review the biotechnological potential of ABC toxins as both pesticides and cargo-carrying shuttles that enable the transport of peptides into cells.

ABC toxin complexes (Tcs) are tripartite complexes that come together to form nano-syringe-like translocation systems. ABC Tcs are often compared with Bacillus thuringiensis (Bt) toxins, and as such, they have been highly studied as a potential novel pesticide to combat growing insect resistance. Moreover, it is possible to substitute the cytotoxic hypervariable region with alternative peptides, which promise potential use as a novel peptide delivery system. These toxins possess the unique ability to form active chimeric holotoxins across species and display the capability to translocate a variety of payloads across membrane bilayers. Additionally, mutagenesis on the linker region and the receptor binding domains (RBDs) show that mutations do not inherently cause a loss of functionality for translocation. For these reasons, Tcs have emerged as an ideal candidate for targeted protein engineering. However, elucidation of the specific function of each RBD in relation to target receptor recognition currently limits the use of a rational design approach with any ABC Tc. Additionally, there is a distinct lack of targeting and biodistribution data for many Tcs among mammals and mammalian cell lines. Here, we outline two separate strategies for modifying the targeting capabilities of the A subunit (TcA) from Xenorhabdus nematophilus, Xn-XptA2. We identify novel structural differences that make Xn-XptA2 different than other characterized TcAs and display the modular capabilities of substituting RBDs from alternative TcAs into the Xn-XptA2 scaffold. Finally, we show the first, to our knowledge, biodistribution data of any TcA in mice.

: Cystic fibrosis (CF) is likely underdiagnosed in Caribbean populations due to non‐representative cystic fibrosis transmembrane conductance regulator (CFTR) variant screening panels, limited newborn screening programs, and structural healthcare barriers. Data from 2022 indicate substantial populations with European ancestry in Puerto Rico (1.4 M, 42.7%) and the Dominican Republic (1.4 M, 57.9%), yet the true burden of CF in the broader Caribbean remains largely undocumented. Working Hypothesis: Current diagnostic frameworks, largely based on European‐derived CFTR variant distributions, fail to capture the true burden of CF in Caribbean populations, leading to underestimated prevalence and delayed or missed diagnoses. Objectives: To synthesize registry, clinical, and published data to identify barriers to accurately assessing CF prevalence in Caribbean populations. Study Design and Methods: This narrative literature review integrates CF registries, published data on CFTR variant distribution, population ancestry data, and clinical observations from CF centers in Puerto Rico and the Dominican Republic. Clinical insights were derived from pediatric patients evaluated at the Pediatric Rare Lung and Asthma Institute in Puerto Rico and the CF Clinic at Robert Reid Cabral Children's Hospital in the Dominican Republic. Results: CFTR variant patterns differ from those in the United States, with higher frequencies of rare variants such as p.Ala559Thr. Standard screening panels may miss these variants, contributing to underdiagnosis. Limited newborn screening, misdiagnosis, and restricted access to CFTR modulator therapies further exacerbate disparities. Conclusions: Structural, diagnostic, and genetic factors hinder accurate CF prevalence estimates in the Caribbean, highlighting the need for region‐specific research, improved screening, and expanded access to therapies.

Lipid homeostasis is essential for maintaining cellular function and metabolic balance in all organisms, and its dysregulation can contribute to hypercholesterolemia, atherosclerosis, and cardiovascular disease. Adverse health outcomes are associated with elevated levels of low-density lipoprotein cholesterol (LDL-C), retention of which causes macrophages to transition into foam cells, leading to widespread inflammation, cardiovascular disease, and atherosclerosis. Statins (HMG-CoA reductase inhibitors) are considered the gold standard for LDL and very-low-density lipoprotein (VLDL) reduction, yet they exhibit side effects like myalgia and insulin resistance. Apolipoprotein E (apo E)-based lipid-binding synthetic mimetic peptides have been shown to clear cholesterol from the blood through canonical hepatic pathways such as HSPG receptors and are currently being studied to reduce plasma cholesterol. This review examines the development, biological properties, and therapeutic potential of the apo E mimetic peptide AEM-28. AEM-28 reduces atherogenic lipoproteins from plasma efficiently in multiple preclinical models as well as Phase I clinical trials in humans, where general tolerance of the treatment was observed. AEM-28 utilizes a unique mechanism of action that involves binding to atherogenic lipoproteins with multiple receptor-binding domains, facilitating enhanced clearance via the highly abundant heparan sulfate proteoglycans on hepatocytes. These attributes support the potential of AEM-28 as a complementary therapeutic or even an alternative for patients requiring LDL apheresis and those with hypercholesterolemia, atherosclerosis, or cardiovascular disease.