Roles of Non-Structural Protein 4A in Flavivirus Infection

Abstract

Infections with viruses in the genus Flavivirus are a worldwide public health problem. These enveloped, positive sense single stranded RNA viruses use a small complement of only 10 encoded proteins and the RNA genome itself to remodel host cells to achieve conditions favoring viral replication. A consequence of the limited viral armamentarium is that each protein exerts multiple cellular effects, in addition to any direct role in viral replication. The viruses encode four non-structural (NS) small transmembrane proteins (NS2A, NS2B, NS4A and NS4B) which collectively remain rather poorly characterized. NS4A is a 16kDa membrane associated protein and recent studies have shown that this protein plays multiple roles, including in membrane remodeling, antagonism of the host cell interferon response, and in the induction of autophagy, in addition to playing a role in viral replication. Perhaps most importantly, NS4A has been implicated as playing a critical role in fetal developmental defects seen as a consequence of Zika virus infection during pregnancy. This review provides a comprehensive overview of the multiple roles of this small but pivotal protein in mediating the pathobiology of flaviviral infections.

Keywords: autophagy; congenital Zika syndrome; flavivirus; interferon response; transmembrane protein; unfolded protein response.

Figure 1

A membrane topology model of flavivirus NS4As. NS4A consists of an N-terminal cytosolic region and four predicted transmembrane segments: pTMS1, 2, 3 and 4 with the latter referred to as the 2k fragment. The red and yellow triangles mark the site of viral NS2B-NS3 protease and host signalase cleavage sites, respectively. Thick purple and dashed green arrows indicate specific amino acid (aa) residues in DENV and WNV NS4A, respectively, contributing to viral replication. AH1 is the experimentally determined amphipathic helix 1 of DENV NS4A. The position of critical amino acids is indicated.

Figure 2

Roles of NS4As in flavivirus replication. NS4As induce membrane remodeling similar to that induced by flavivirus infection. NS4A and the unprocessed intermediates containing NS4A are essential components of viral replication complexes (VRCs), that interacts with host factors or other flavivirus NS proteins to promote efficient viral replication.

Figure 3

NS4As modulate the interferon response. NS4As manipulate RIG-I-like receptor signaling, DDX42 helicase, JAK/STAT signaling and the unfolded protein response (UPR) to counteract the interferon-α/β (IFN-α/β) response.

Figure 4

NS4As modulate developmental processes. NS4As modulate autophagy to facilitate persistent infection and manipulate a number of signaling pathways, i.e., Akt-mTOR, ANKLE2/VRK1, JAK/STAT and Notch signaling resulting in developmental defects.