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Breaking down the defenses

SBKB [doi:10.1038/sbkb.2011.37]
Featured Article - September 2011
Short description: The structure of ISG15 bound to NS1B gives insight into the mechanism by which influenza B virus specifically targets humans.

An NS1B-NTR homodimer bound to two ISG15 molecules (PDB 3SDL). Reprinted with permission from Proc. Natl. Acad. Sci. USA 1 .

One of the primary methods of host defense in the battle against viruses is the interferon-induced ISG15 pathway, which inhibits the replication of several viruses, including influenza A and B viruses. ISG15 is an ubiquitin-like protein comprising two ubiquitin-like domains (UblDs) that are connected by a short linker. The ISG15 pathway closely parallels that of ubiquitin and other ubiquitin-like molecules, including activation by the E1 Ube1L, conjugation by the E2 UbcH8, and ligation by an E3 ligase, principally Herc5. This enzyme system conjugates ISG15 onto host and viral proteins.

To counter this antiviral mechanism, the influenza B virus has evolved the ability to inhibit ISG15 conjugation via its NS1 protein, NS1B. The N-terminal region of NS1B binds the N-terminal UblD of ISG15, although how this interaction leads to inhibition of the host antiviral response is not fully understood. Interestingly, the ability of NS1B to overcome the ISG15 barrier extends only to human and non-human primate ISG15; NS1B cannot bind mouse ISG15 and therefore primarily targets humans. It is known that the ISG15 linker region is important to this specificity, but whether NS1B binds the linker, or whether the linker instead regulates the conformation of ISG15 was not yet known.

To understand how NS1B enables the influenza B virus to overcome the interferon-mediated ISG15 response, Montelione, Guan, Krug, and colleagues (PSI NESG) solved the X-ray crystal structure of human ISG15 in complex with the N-terminal region of NS1B (NS1B-NTR). The complex is a heterotetramer, with the NS1B-NTR homodimer binding one ISG15 monomer on each side. Each ISG15 binding site on the NS1B-NTR dimer surface consists of residues from both monomers. The two ISG15 molecules have similar interactions with NS1B, with a few minor differences, although the biological importance of the asymmetry is not yet clear. The ISG15 linker binds the NS1B-NTR using species-specific residues in the linker, answering the question as to the role of the linker in binding NS1B. Additionally, ISG15 residues 38–51, which have several differences compared to mouse ISG15, make further contacts with NS1B-NTR, providing another level of sequence-specific recognition in this interaction.

Interestingly, the C-terminal UblD of ISG15, which is involved in conjugation, does not interact with the NS1B-NTR, and previous work has shown that NS1B-bound ISG15 can still interact with E1 and E2 conjugating enzymes. Therefore the mechanism by which NS1B inhibits ISG15 conjugation is still a mystery, although the interactions seen in the structure are crucial as mutating two key residues prevents NS1B from inhibiting ISG15 conjugation in cells. The authors propose that NS1B may function by sequestering ISG15 away from cellular interactions, an intriguing hypothesis that will undoubtedly be tested further. In the meantime, the structure presented here provides valuable insights into the mechanism of influenza B virus infection and potential sites for antiviral drug discovery efforts.

Steve Mason

References

  1. R. Guan et al. Structural basis for the sequence-specific recognition of human ISG15 by the NS1 protein of influenza B virus.
    Proc. Natl. Acad. Sci. USA (1 Aug 2011). doi:10.1073/pnas.1107032108

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