Initiation of ER membrane penetration by murine polyomavirus.

Abstract

The nonenveloped murine polyomavirus (Py) enters the host cell and traffics to the lumen of the endoplasmic reticulum (ER). In order for the viral genome to gain access to the host nucleus, the virus must be transported out of the ER, a process that was previously uncharacterized. Using an unbiased biochemical approach, an ER host factor has been identified that alters the local architecture of the viral capsid. The ER chaperone ERp29 induces a conformational change in the C terminal tail of the major coat protein VP1, yielding an activated viral particle that is hydrophobic and able to bind membranes. ERp29 is also shown to be required in Py infection. The activities of ERp29 depend on its homodimerization via its N-terminal domain, as a mutant of ERp29 that cannot dimerize is unable to facilitate the VP1 conformational change or stimulate Py infection. Furthermore, a novel perforation assay is used to demonstrate that the ERp29-activated Py disrupts the ER membrane, a step that presumably precedes penetration of the membrane by the virus. The membrane-disrupting activity of activated Py is attributed to the minor coat protein VP2, which becomes exposed on the activated viral particle. VP2 can bind, integrate into, and induce disruption of the ER membrane, while the other minor coat protein, VP3, can bind and integrate into the ER membrane, but does not perforate the membrane. VP2-mediated perforation of the ER membrane and penetration of the ER membrane by Py requires the myristic acid moiety on the N-terminus of VP2. Collectively, these results illustrate the initial steps in penetration of the ER membrane by Py, in which the viral particle undergoes local remodeling to expose a component required for membrane disruption that mediates binding to and perforation of the ER membrane by the activated Py particle.