Non-coated membrane invaginations are involved in binding and internalization of cholera and tetanus toxins

Abstract

The binding of various biologically significant macromolecules to specific cell surface receptors is followed by their internalization, a process called receptor-mediated endocytosis^1,2. In most cases, it has been shown that receptor-bound ligands cluster in characteristic, bristle-coated indentations of the cell surface known as coated pits^1–3. In addition to coated pits, cultured cells have a population of smaller, non-coated membrane invaginations^4–6, which may have a role in endocytosis^2,7. However, no receptor-bound biologically active ligand has been shown to enter cells via these non-coated invaginations. The studies implicating coated pits in receptor-mediated endocytosis concern essentially ligands that bind to receptors thought to be glycoproteins. We have therefore investigated by electron microscopy the endocytosis of cholera toxin and tetanus toxin, ligands which bind to membrane glycolipids, in particular to either GM₁ monosialoganglioside^8–11 or di- and trisialogangliosides^12–15 respectively. We show here that in cultured liver cells, non-coated membrane microinvaginations are preferentially involved in both the initial binding and subsequent internalization of colloidal gold-labelled cholera and tetanus toxin.