Inhibition of VP2-mediated entry: a potential antiviral strategy to treat or prevent calicivirus disease

Summary

Felines to the rescue once more: apart from being the best pets they are also helping us understand the biology of caliciviruses and how to prevent infection in both humans and of course felines.

Figure 1. Cryo-em structure of feline calicivirus (FCV) 273 bound to both its proteinaceous protein receptor fJAM-A (feline junctional adhesion molecule A) coloured in red and peptide VP2_87-106 coloured in purple. The colouring scheme represents distance of each density from the centre of the structure. This preprint figure is made available under a CC-BY-NC-ND 4.0 International license.

Background

Caliciviridae are positive-sense RNA viruses that lack an envelope, the outermost layer of many types of viruses. Caliciviridae can infect humans and cause acute gastroenteritis. Propagation of human infecting caliciviruses in lab environments has proven challenging and as a result animal models are used to understand the biology of caliciviruses. In this preprint, the authors study the feline calicivirus (FCV) that can cause respiratory disease in felines.

FCVs are ideal to study since they give high titres in vitro, but also because the proteinaceous receptor of FCV, the feline junctional adhesion molecule A (fJAM-A), has already been identified. Caliciviruses package their genetic material in a capsid, made by the major capsid protein VP1 that is comprised by three domains including the protruding domain (P) that can form spikes. Caliciviruses apart from VP1 also utilise the minor capsid protein VP2 that is essential for infection and interacts with the P-domain.

Upon receptor engagement the P-domain spikes form clefts in which VP2 binds. VP2 was also identified to be essential for genomic delivery to the cytosol from previous work, thus necessary for viral infection and propagation.

Key findings

Peptide mimetic VP2_87-106  lowers infectiousness of FCV isolates

The authors formulated a peptide based on twenty C-terminal VP2 residues from the FCV strain F9, termed VP2_87-106 to prevent viral entry by disrupting the VP2-VP1 interface. The VP2_87-106  proved successful against FCV clinical isolates and reference strains, with the infectivity of the viral particles reduced in a dose-dependent manner of VP2_87-106 . The authors identified that pre-incubation of the peptide with the virus resulted in a higher decrease in infectivity, suggesting that VP2_87-106  can access the VP1 binding without the need for receptor interaction. To test the specificity of VP2_87-106  the sequence was scrambled and tested against FCV strains without any reduction in infectiousness. The peptide’s potency was determined by luciferase assay showing similar IC50 values for both FCV and VS FCV isolates.

Binding site of VP2_87-106 identified by cryo-EM

The authors then solved the structures of VP2_87-106  with both the respiratory clinical isolate FCV 273 and the clinical isolate NSW-1, in the presence or absence of the ectodomain of fJAM-A. In all their structures a clear density could be identified for VP2_87-106  at the VP2 binding cleft in VP1 across the entirety of the virion. The binding of VP2_87-106 could occur in the absence of fJAM-A which suggests that receptor engagement isn’t required for essential structural re-arrangements that allow for VP2_87-106  binding. These findings further support previous observations that VP2_87-106 could prevent viral infection more effectively following pre-incubation with viral particles.

Essential residues of VP2_87-106  for VP1 binding

The authors – using their solved cryo-EM maps – mapped the points of contacts VP2_87-106 makes with the VP1 protein (specifically the P-domain). Each one of the residues essential for these interactions was then mutated to alanine. Their results showed reduced activity for only eight of the twenty VP2_87-106 residues suggesting that only these residues are responsible for the main anti-viral activity of the peptide.

 Importance of this work

The preprint elegantly presents a possible solution to tackle viral infections bringing together expertise from peptide chemistry, biochemistry and structural biology. It showcases the importance of working across fields to solve modern problems with the science presented in a clear and elegant manner. Findings of this research bring to light the capabilities of anti-viral peptides and opens the possibility of using the same methodology and techniques to combat viral infections across other viral families (all while displaying some beautiful cryo-EM structures and models).

Tags: cryo-em, peptides, structural biology, viral infection, virology, virus

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