viroporin

thumb|right|The transmembrane helical tetramer of the [[influenza A virus M2 protein, which functions as a proton channel, in complex with the channel-blocking drug amantadine (shown in red). Highly conserved tryptophan and histidine residues known to play key roles in mediating proton transport are shown as sticks. From .]] Viroporins are small and usually hydrophobic multifunctional viral proteins that modify cellular membranes, thereby facilitating virus release from infected cells. Viroporins are capable of assembling into oligomeric ion channels or pores in the host cell's membrane, rendering it more permeable and thus facilitating the exit of virions from the cell. Many viroporins also have additional effects on cellular metabolism and homeostasis mediated by protein-protein interactions with host cell proteins. Viroporins are not necessarily essential for viral replication, but do enhance growth rates. They are found in a variety of viral genomes but are particularly common in RNA viruses. Many viruses that cause human disease express viroporins. These viruses include hepatitis C virus, HIV-1, influenza A virus, poliovirus, respiratory syncytial virus, and SARS-CoV.

== Structure == Viroporins are usually small - under 100 or 120 amino acid residues - and contain at least one region capable of folding into an amphipathic transmembrane helix. Some examples also contain stretches of basic amino acids, or stretches of aromatic amino acids thought to reside in the interfacial region of the membrane. Oligomers of these proteins, most often tetramers, form ion channels or pores of usually weak ion selectivity that permit diffusion of ions across the cell membrane. The molecular architecture of the pore, its degree of selectivity, the extent to which it incorporates lipids from the surrounding membrane, and the presence of portions of the protein that extend beyond the membrane all vary among viroporins and indicate that these proteins have a diverse array of functional roles.