Viral weapons of membrane destruction: variable modes of membrane penetration by non-enveloped viruses

Abstract

Significant progress has recently been obtained in our understanding of cellular entry by nonenveloped viruses (NEVs). A key step in the entry process involves the disruption or remodeling of the limiting cell membrane allowing the virus to gain access to the cellular replication machinery. Biochemical, genetic and structural data from diverse virus groups have shed light on the process of membrane penetration thereby revealing both the conservation and divergence of the mechanisms and principles governing this process. In general, membrane breach is achieved via the highly regulated spatiotemporal exposure of a virally encoded membrane lytic factor, resulting in the transfer of the viral genome or nucleocapsid into the cytosol.

Figure 1. Proposed mechanisms of membrane disruption by NEV lytic factors

Disruption of the limiting membrane is divided into three major categories: transient lipid rearrangement, pore formation, and gross disruption. Parvoviruses deploy a phospholipase A2 domain, which mediates membrane passage via temporary modification of the endosomal bilayer. Reovirus µ1N is released from the virus, and forms size-selective pores in the endosomal membrane. Whether pore formation is linked to osmotic lysis of the endocytic vesicle is still unclear, but could explain how the large virus core translocates to the cytoplasm. AdV protein VI inserts into the lumenal leaflet of the endosomal membrane, inducing positive membrane curvature and total fragmentation of the endosome. This model is based on VI-mediated lysis of unilamellar vesicles [37, 38]. Viruses are scaled to depict the relative size of each particle. Structural models were obtained from the Virus Particle Explorer database (VIPERdb, http://viperdb.scripps.edu/index.php) [55].