The Human Polyoma JC Virus Agnoprotein Acts as a Viroporin

Abstract

Virus infections can result in a range of cellular injuries and commonly this involves both the plasma and intracellular membranes, resulting in enhanced permeability. Viroporins are a group of proteins that interact with plasma membranes modifying permeability and can promote the release of viral particles. While these proteins are not essential for virus replication, their activity certainly promotes virus growth. Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease resulting from lytic infection of oligodendrocytes by the polyomavirus JC virus (JCV). The genome of JCV encodes six major proteins including a small auxiliary protein known as agnoprotein. Studies on other polyomavirus agnoproteins have suggested that the protein may contribute to viral propagation at various stages in the replication cycle, including transcription, translation, processing of late viral proteins, assembly of virions, and viral propagation. Previous studies from our and other laboratories have indicated that JCV agnoprotein plays an important, although as yet incompletely understood role in the propagation of JCV. Here, we demonstrate that agnoprotein possesses properties commonly associated with viroporins. Our findings demonstrate that: (i) A deletion mutant of agnoprotein is defective in virion release and viral propagation; (ii) Agnoprotein localizes to the ER early in infection, but is also found at the plasma membrane late in infection; (iii) Agnoprotein is an integral membrane protein and forms homo-oligomers; (iv) Agnoprotein enhances permeability of cells to the translation inhibitor hygromycin B; (v) Agnoprotein induces the influx of extracellular Ca2+; (vi) The basic residues at amino acid positions 8 and 9 of agnoprotein key are determinants of the viroporin activity. The viroporin-like properties of agnoprotein result in increased membrane permeability and alterations in intracellular Ca2+ homeostasis leading to membrane dysfunction and enhancement of virus release.

The Human Metapneumovirus Small Hydrophobic Protein Has Properties Consistent with Those of a Viroporin and Can Modulate Viral Fusogenic Activity

“…HMPV SH forms higher-order oligomers, and studies using sedimentation equilibrium analysis demonstrate that the SH protein TMD is sufficient for oligomerization. Similar to other reported viroporins (30,31), HMPV SH expression increased cell permeability to hygromycin B. HMPV SH expression also significantly altered intracellular localization of a fluorescent dye, suggesting that SH also impacts permeability of intracellular membranes. In addition, HMPV SH expression, while not altering viral glycoprotein trafficking, negatively impacted fusion mediated by the HMPV F protein and to a lesser extent other paramyxovirus fusion proteins.…”

“…To directly test whether HMPV SH expression can alter membrane permeability, we utilized hygromycin B, an antibiotic which blocks cellular protein translation but which does not efficiently penetrate the plasma membrane when present at low concentrations (30,31,44,45). COS-7 cells transiently expressing HMPV F (as a control), HMPV SH-HA, or HMPV HA-SH were metabolically labeled for 1, 2, or 3 h in the presence or absence of 500 g/ml hygromycin B.…”

Type I and II IFNs Inhibit Merkel Cell Carcinoma via Modulation of the Merkel Cell Polyomavirus T Antigens

“…For several DNA viruses, including polyomaviruses, induction of apoptosis of the infected cell is part of the viral replication cycle ensuring the release of the virions. This host cell apoptosis is initiated by expression of the very late viral protein (33), which, however, has not been elucidated for MCV yet. It should be further noted, that specific miRNAs have been suggested to be involved in MCV virion release (34).…”

The Presumed Polyomavirus Viroporin VP4 of Simian Virus 40 or Human BK Polyomavirus Is Not Required for Viral Progeny Release

“…Recently, several viruses have been proposed to produce viroporins, small hydrophobic proteins that oligomerize in host cell membranes, forming hydrophilic pores affecting several steps in the replication cycle, including progeny release (reviewed in reference 12). Four viroporins have been described for polyomaviruses: JCPyV agnoprotein (13), SV40 VP2 and VP3 (14), and SV40 VP4 (15,16).…”