The mechanisms by which receptors guide intracellular virus transport are poorly characterized. The murine polyomavirus (Py) binds to the lipid receptor ganglioside GD1a and traffics to the endoplasmic reticulum (ER) where it enters the cytosol and then the nucleus to initiate infection. How Py reaches the ER is unclear. We show that Py is transported initially to the endolysosome where the low pH imparts a conformational change that enhances its subsequent ER-to-cytosol membrane penetration. GD1a stimulates not viral binding or entry, but rather sorting of Py from late endosomes and/or lysosomes to the ER, suggesting that GD1a binding is responsible for ER targeting. Consistent with this, an artificial particle coated with a GD1a antibody is transported to the ER. Our results provide a rationale for transport of Py through the endolysosome, demonstrate a novel endolysosome-to-ER transport pathway that is regulated by a lipid, and implicate ganglioside binding as a general ER targeting mechanism.

No abstract

Polyomaviruses (Pys) are nonenveloped DNA tumor viruses that include the murine polyomavirus (mPy), simian virus 40 (SV40), and the human BK, JC, KI, WU, and Merkel Cell viruses. To cause infection, Pys must enter host cells and navigate through various intracellular compartments, where they undergo sequential conformational changes enabling them to uncoat and deliver the DNA genome into the nucleus. The ensuing transcription and replication of the genome leads to lytic infection or cell transformation. In recent years, a more coherent understanding of how Pys are transported from the plasma membrane to the nucleus is starting to emerge. This review will focus on the decisive steps of Py entry, including engagement of the host cell receptor, targeting to the endoplasmic reticulum (ER), penetration across the ER membrane, nuclear entry, and genome release. Strikingly, a number of these steps resemble the intoxication pathway of the AB(5) bacterial toxins. Thus, as Pys and bacterial toxins hijack similar cellular machineries during infection, a general principle appears to guide their entry into host cells.

How receptors control virus infection is poorly understood. Polyomavirus (Py) binds to the sialic acid-galactose moiety on receptors to gain entry into host cells and cause infection. We previously demonstrated that the sialic acid-galactose-containing glycolipids called gangliosides GD1a and GT1b promote Py infection, in part, by sorting the virus from the endolysosomes to the endoplasmic reticulum (ER), a critical infection route. Whether these glycolipids act as Py entry receptors, however, is not clear. Additionally, as the majority of glycoproteins also harbor terminal sialic acid-galactose residues, their roles in Py infection are also not well established. Using a gangliosidedeficient cell line, we show that GD1a is the functional entry receptor for Py. GD1a binds to Py on the plasma membrane, and the receptor-virus complex is internalized and transported to the late endosomes and then the ER to initiate infection. In contrast, our findings indicate that glycoproteins act as decoy receptors, restricting the ER transport and infection of Py. Thus, glycolipids and glycoproteins, two major constituents of the plasma membrane, execute opposing functions in regulating infection by a defined virus.