Cholera toxin as a model AB₅ toxin: functional significance of B-subunit pentamerization in GM1 recognition,...

AB₅ toxins constitute a major family of bacterial exotoxins. Their pathogenicity depends on the coordinated actions of two components: an enzymatically active A subunit and a pentameric B subunit. The B subunit mediates host cell recognition and intracellular trafficking. Among them, cholera toxin represents a representative model for investigating structure-function relationships governing toxin entry and delivery. While the catalytic activity of the A subunit has been extensively characterized, growing evidence indicates that pathogenic outcomes are critically shaped by trafficking strategies encoded within the B subunit pentamer. This review focuses on the cholera toxin B subunit (CTB) as a model system to elucidate how pentameric organization enables multivalent GM1 recognition, receptor clustering, and efficient intracellular transport. We summarize current knowledge on the molecular basis of CTB pentamerization, its thermodynamic and structural advantages, and the role of multivalency in amplifying delivery efficiency and pathogenicity. In addition, we examine how CTB-mediated intracellular trafficking influences uptake pathway selection and intracellular routing, thereby contributing to consistent and robust toxic outcomes. Beyond pathogenic mechanisms, we briefly discuss the potential implications of CTB-mediated trafficking for translational applications. By integrating structural, cellular, and functional perspectives, this review demonstrates how CTB-mediated mobility governs intracellular routing and uptake pathway selection. This framework provides a basis for understanding AB₅ toxin pathogenicity and its broader biological significance.