Lipid-mediated endocytosis

Abstract

Receptor-mediated endocytosis is used by a number of viruses and toxins to gain entry into cells. Some have evolved to use specific lipids in the plasma membrane as their receptors. They include bacterial toxins such as Shiga and Cholera toxin and viruses such as mouse polyoma virus and simian virus 40. Through multivalent binding to glycosphingolipids, they induce lipid clustering and changes in membrane properties. Internalization occurs by unusual endocytic mechanisms involving lipid rafts, induction of membrane curvature, trans-bilayer coupling, and activation of signaling pathways. Once delivered to early endosomes, they follow diverse intracellular routes to the lumen of the ER, from which they penetrate into the cytosol. The role of the lipid receptors is central in these well-studied processes.

Figure 1.

Organization of binding sites in ganglioside ligands. (A) SV40 coat protein VP1 pentamer cocrystallized with GM1 pentasaccharide, three of five binding sites are occupied in the structure. PDB-file: 3bwr. (B) Pentameric cholera toxin β-subunit cocrystallized with GM1 pentasaccharide, PDB file: 1ct1. (C) Pentameric Escherichia coli heat-labile enterotoxin β-subunit cocrystallized with nitrophenyl-galactoside, PDB file: 1pzi. (D) Pentameric Shiga toxin β-subunit cocrystallized with GB3 trisaccharide, six of up to 15 reported binding sites are occupied. PDB file: 1cqf. (E) Pentameric Escherichia coli Shigalike toxin β-subunit cocrystallized with a GB3 analog, PDB file: 1bos. (F) Bivalent antibody (Mus musculus IgG2a), PDB file: 1igt. The structures are shown to scale. In some crystals, not all binding sites are occupied by ligands. (Scale bar, 5 nm.)

Figure 2.

First steps in SV40 endocytosis. (A) Binding to GM1 and lateral motion in the plane of the plasma membrane, (B) formation of a membrane microdomain “lipid raft” and actin-dependent immobilization, (C) invagination of the plasma membrane, (D) recruitment of dynamin-independent scission machinery (actin?) and tyrosine kinase-dependent internalization, and (E) intracellular transport. (Insets) (Left) GM1 mobility in membranes. Individual GM1 molecules undergo fast diffusion in cholesterol-free membranes (A′), but are transiently confined in nanoscopic domains in the presence of cholesterol (A″). When SV40 binds to GM1, diffusion speed is greatly reduced (A′″) and the SV40-GM1-complex likely recruits cholesterol to form a membrane microdomain or “lipid raft.” (Right) Individual SV40 virions bound to cells seem to imprint their shape onto the plasma membrane and are found in small, tight-fitting invaginations. Image of SV40 courtesy of Jürgen Kartenbeck. Structures used for this figure: SV40: 1sva; SV40 pentamer: (Campanero-Rhodes et al. 2007); actin:1j6z, 3b63.

Figure 3.

Trafficking in the endocytic pathway. The main organelles are EEs, LEs, REs, and lysosomes. EEs are complex organelles with tubular and vacuolar domains. The vacuolar domains dissociate with their contents and undergo microtubule-mediated, dynein-dependent movement to the perinuclear region. They mature to LEs, which can fuse with each other and eventually with lysosomes generating endolysosomes. At the level of EEs, LEs, and REs, the endocytic pathway is connected to the TGN and the Golgi complex by vesicle transport and possibly with the ER at the level of endolysosomes. The main trafficking pathways are emphasized for mono- or paucivalent gangliosides (blue), AB5 toxins (red), and polyomaviruses (green).