The Structure, Function, and Mechanisms of Action of Enterovirus Non-structural Protein 2C

Abstract

Enteroviruses are a group of RNA viruses belonging to the family Picornaviridae. They include human enterovirus groups A, B, C, and D as well as non-human enteroviruses. Enterovirus infections can lead to hand, foot, and mouth disease and herpangina, whose clinical manifestations are often mild, although some strains can result in severe neurological complications such as encephalitis, myocarditis, meningitis, and poliomyelitis. To date, research on enterovirus non-structural proteins has mainly focused on the 2A and 3C proteases and 3D polymerase. However, another non-structural protein, 2C, is the most highly conserved protein, and plays a vital role in the enterovirus life cycle. There are relatively few studies on this protein. Previous studies have demonstrated that enterovirus 2C is involved in virus uncoating, host cell membrane rearrangements, RNA replication, encapsidation, morphogenesis, ATPase, helicase, and chaperoning activities. Despite ongoing research, little is known about the pathogenesis of enterovirus 2C proteins in viral replication or in the host innate immune system. In this review, we discuss and summarize the current understanding of the structure, function, and mechanism of the enterovirus 2C proteins, focusing on the key mutations and motifs involved in viral infection, replication, and immune regulation. We also focus on recent progress in research into the role of 2C proteins in regulating the pattern recognition receptors and type I interferon signaling pathway to facilitate viral replication. Given these functions and mechanisms, the potential application of the 2C proteins as a target for anti-viral drug development is also discussed. Future studies will focus on the determination of more crystal structures of enterovirus 2C proteins, which might provide more potential targets for anti-viral drug development against enterovirus infections.

Keywords: 2C protein; anti-viral drug; enterovirus; function; host immune response; structure; type I IFNs.

Figure 1

A schematic representation of enterovirus A71 (EV-A71) genome and proteolytic processing of the polyprotein. The polyprotein was cleaved into four viral proteins, VP1–VP4, and seven non-structural proteins including 2A–2C and 3A–3D.

Figure 2

Functional motifs and sequence alignment of enterovirus 2C protein. (A) Functional motifs in 2C^ATPase protein are shown in detail, including membrane binding-, RNA binding-, zinc binding-, oligomerization-, and amphipathic-motifs. The locations of the known mutations corresponding to morphogenesis, encapsidation, uncoating, capsid interacting, ATPase activity, homo-oligomerization, and temperature sensitivity are shown in different symbols. The precise positions of R finger and zinc-binding sites are indicated according to the crystal structural of EV-A71 2C protein. Amino acid positions in each motif are numbered and illustrated, and the exact species of each mutation is shown behind the amino acid position. (B) Sequence alignment of enterovirus 2C protein. The amino acid positions mentioned in (A) are highlighted on top of the sequences in different colors, with PV 2C in red, EV-A71 2C in green, and positions shared by both in blue. Dashes stand for amino acid residues identical to those of PV 2C protein.

Figure 3

An overview of the functions of enterovirus 2C proteins, which interact with several host factors and regulate viral replication and infection.

Figure 4

Interactions between picornavirus 2C protein and the NF-κB and retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) pathways. Enterovirus 2C proteins are mainly involved in downregulation of pro-inflammatory cytokines by targeting the NF-κB pathway, while foot-and-mouth disease virus (FMDV) 2C regulates the corresponding pathway by suppressing the expression of NOD-2. Two picornavirus 2C proteins, including encephalomyocarditis virus (EMCV) and seneca valley virus (SVV), are shown to target the MDA5 and RIG-I of the RLR pathway which may cause downstream mediators to counteract antiviral innate immunity.