1.A.85 The Poliovirus 2B Viroporin (2B Viroporin) Family

Viroporins are involved in destroying the morphology of host cells and disturbing their biological functions to complete the life cycle of the virus. The 2B proteins encoded by enteroviruses, which belong to the family Picornaviridae, can form transmembrane pores by oligomerization, increase the permeability of plasma membranes, disturb the homeostasis of calcium in cells, induce apoptosis, and cause autophagy; these abilities are shared among viroporins. Ao et al. 2014 introduced the structure and biological characteristics of various 2B proteins encoded by enteroviruses.

Poliovirus provides a well-characterized system for understanding how nonenveloped viruses enter and infect cells. It forms a pore in the host cell membrane to facilitate entry of its nucleic acid into the host cell. It is localized to the endoplasmic reticulum (ER) and the Golgi complex, inducing membrane remodeling and blockade of glycoprotein trafficking (Martínez-Gil et al. 2011). Two hydrophobic regions appear to cooperate to insert viroporin 2B into the ER-derived microsomal membrane. These viruses are single stranded RNA (positive strand) viruses. The human poliovirus nonstructural 2B protein is involved in cell membrane permeabilization during late viral infection. It inserts into anionic phospholipid unilamellar vesicles to form pores. Molecules of a size less than 1000 daltons can pass through these channels. Oligomerization is required for activity, and an SDS-resistant tetramer was demonstrated (Agirre et al. 2002). The 2B viroporin is 97 aas long. 2B insertion and pore-opening are mechanistically distinguishable events modulated by the target membrane anionic phospholipids (Agirre et al. 2008). A tetrameric bundle model of 2B has been proposed (Patargias et al. 2009). The bundles show a pore lining motif of three lysines followed by a serine. Different possible orientations of the helices in the membrane may influence the in vivo activity of 2B.

Upon binding its receptor, poliovirus undergoes an irreversible conformational change to the 135S cell entry intermediate (Bubeck et al. 2005). This transition involves shifts of the capsid protein beta barrels, accompanied by the externalization of VP4 and the N terminus of VP1 (Tuthill et al. 2006). Both polypeptides associate with membranes and are postulated to facilitate entry by forming a translocation pore for the viral RNA. Each N terminus of VP1 exits the capsid though an opening in the interface between VP1 and VP3 at the base of the canyon that surrounds the fivefold axis. Comparison with reconstructions of 135S particles in which the first 31 residues of VP1 were proteolytically removed revealed that the externalized N terminus is located near the tips of propeller-like features surrounding the threefold axes.

The 2B viroporin is derived by proteolytic processing from the polyprotein of poliovirus (P03300), a protein of 2209 amino acyl residues. 2B viroporin corresponds to residues 1031-1127 in this polyprotein. The 2B viroporin is homologous to proteins of somewhat divergent sequence in various poliovirus strains as well as in other viruses such as coxsackieviruses, enteroviruses, rhinoviruses, echoviruses, and vesicular disease viruses. These proteins are generally all >40% identical to each other.

The poliovirus 2B protein has been recently implicated in apoptosis. The pore-forming P3 peptide, derived from the 2B amphipathic domain, translocates through the plasma membrane of culture cells and targets mitochondria. Cell permeabilization by P3 versions of different lengths, together with peptide uptake analyses supported an internalization mechanism dependent on the P3 capacity to interact with lipid bilayers and establish permeating pores. Internalized P3 was found associated with mitochondria, but in contrast to the parental 2B protein, the shorter peptide did not affect the morphology or cell distribution of these organelles; nor did it induce apoptosis. P3 constitutes a mitochondriotropic sequence, which is devoid of 2B pro-apoptotic activity (Madan et al. 2010).

Enterovirus polypeptide (EP) (2209aas) is degraded to several proteins/peptides including the VP1, VP2, VP3 and VP4 capsid proteins. VP1 (CAB65072; 302aas; residues 580-881 in EP) and myristoylated VP4 (BAE06026; 69aas; residues 1-69 in EP) may form pores (Bubeck et al., 2005; Tuthill et al., 2006). Viroporin (Poliovirus 2B; YP_001497168, 99aas; residues 1031-1127 in EP; Agirre et al., 2002), known to form pores, is possibly involved in virus particle release. It also derived from the enterovirus polypeptide. The active peptide in the poliovirus 2B viroporin is only 21 residues long (residues 35-55) and forms tetrameric α-helical transmembrane hydrophilic pores (Madan et al., 2007a,b; Nieva et al., 2003). Viroporins are reviewed by Gonzalez and Carrasco (2003). Poliovirus 3B can also form pores (Madan et al., 2007a). The 2B viroporin membranolytic domains, TM1 (residues 35-55) and TM2 (residues 59-82) act together to create cytolytic activity in the endomembranes targeted by 2B (Sánchez-Martínez et al. 2008).

The reaction catalyzed by the 2B viroporin is:

small molecules (in) ⇌ small molecules (out)