Structured clustering of the glycosphingolipid GM1 is required for membrane curvature induced by cholera toxin

ABSTRACT AB5 bacterial toxins and polyomaviruses induce membrane curvature as a mechanism to facilitate their entry into host cells. How membrane bending is accomplished is not yet fully understood but has been linked to the simultaneous binding of the pentameric B-subunit to multiple copies of their glycosphingolipid receptors. Here, we probe the toxin membrane binding and internalization mechanisms by using a combination of super-resolution and polarized localization microscopy. We show that cholera toxin subunit B (CTxB) can induce membrane curvature only when bound to multiple copies of its glycosphingolipid receptor, GM1, and the ceramide structure of GM1 is likely not a determinant of this activity as assessed in model membranes. A mutant CTxB capable of binding only a single GM1 fails to generate curvature either in model membranes or in cells and clustering the mutant CTxB-single-GM1 complexes by antibody cross-linking does not rescue the membrane curvature phenotype. We conclude both the multiplicity and specific geometry of GM1 binding sites are necessary for the induction of membrane curvature. We expect this to be a general rule of membrane behavior for all AB5-subunit toxins and polyomaviruses that bind glycosphingolipids to invade host cells. SIGNIFICANCE STATEMENTMembrane binding toxins demonstrate both a public health challenge and a bioengineering opportunity due to their efficient internalization into cells. These toxins multivalently bind to naturally occurring lipid receptors at the plasma membrane and initiate endocytosis. This manuscript reports the importance of structured lipid-receptor clustering for the induction of membrane bending. We also observed that the magnitude of membrane curvature to be correlated to the stoichiometry of toxin-bound receptors. The initiation of curvature by these prototypical clathrin-independent cargoes provides mechanistic insight into the early steps in endocytosis.