Viral Targeting of Importin Alpha-Mediated Nuclear Import to Block Innate Immunity

Abstract

Cellular nucleocytoplasmic trafficking is mediated by the importin family of nuclear transport proteins. The well-characterized importin alpha (IMPA) and importin beta (IMPB) nuclear import pathway plays a crucial role in the innate immune response to viral infection by mediating the nuclear import of transcription factors such as IRF3, NFκB, and STAT1. The nuclear transport of these transcription factors ultimately leads to the upregulation of a wide range of antiviral genes, including IFN and IFN-stimulated genes (ISGs). To replicate efficiently in cells, viruses have developed mechanisms to block these signaling pathways. One strategy to evade host innate immune responses involves blocking the nuclear import of host antiviral transcription factors. By binding IMPA proteins, these viral proteins prevent the nuclear transport of key transcription factors and suppress the induction of antiviral gene expression. In this review, we describe examples of proteins encoded by viruses from several different families that utilize such a competitive inhibition strategy to suppress the induction of antiviral gene expression.

Keywords: African swine fever virus; Ebola virus; coronavirus; flavivirus; hantavirus; hepatitis B virus; human immunodeficiency virus-1; immune evasion; importin alpha; innate immunity; interferon; vaccinia virus.

Figure 1

Overview of the Canonical Nuclear Import Mechanism. The nuclear cargo (highlighted in orange) harbors a nuclear localization signal (NLS) (depicted in yellow). The NLS is bound by importin-α (colored blue), and importin-β (shown in green) binds importin-α through the IBB domain. The trimeric assembly translocates through the nuclear pore complex into the nucleus, where upon entry, RanGTP (colored black) binds importin-β, mediating disassembly of the complex and the release of cargo.

Figure 2

Classification of Human Importin Alpha Isoforms The seven human importin alpha isoforms are grouped into three subfamilies (SF-1, SF-2, and SF-3) according to their sequence similarities. Subfamily SF-1 includes IMPA1 and IMPA8, which exhibit the lowest sequence identity among the subfamilies. Subfamily SF-2, with the highest sequence homology, consists of IMPA3 and IMPA4. Subfamily SF-3 comprises IMPA5, IMPA6, and IMPA7. The specific sequence identity percentages, calculated using BLAST, are indicated for each isoform to the right of the figure.

Figure 3

Ebola VP24 inhibits STAT1 signaling by binding SF-3 IMPAs. (A) IMPAs are comprised of 10 armadillo (ARM) repeats of 3 helices (PDB 1BK5). cNLS and ncNLS sites are indicated. (B) VP24 functions as a competitive inhibitor of STAT1-IMPA interaction by blocking phosphorylated STAT1 (PY-STAT1) binding via its ncNLS to IMPA (PDB 1BK5, 4U2X, and 1BF5).

Figure 4

Interactions and Structural Insights into MERS-CoV ORF4b Inhibition of Immune Signaling. Normally, an immune stimulus would trigger the NF-B pathway, leading to the transport of p50/p65 into the nucleus. The complex of IMPA2 with p50/p65, which facilitates this process, has been structurally characterized (PDB 7LET). The MERS-CoV ORF4b protein can disrupt this interaction, thereby preventing the nuclear import of p50/p65 and suppressing the expression of ISGs and IFNs. Structural analysis has provided insights into the MERS-CoV ORF4b mode of action by revealing a mutually exclusive binding mechanism at the interface, as shown in the structural data from PDB 7RFZ [44]. In the visualization, IMPA is rendered in green with a surface representation, while p50 and p65 are depicted in stick form in salmon and light blue, respectively. These models are constructed based on high-resolution X-ray crystallography data.