Recombinant Equine herpesvirus 1 Envelope protein US9 homolog (76)

Introduction to Recombinant Equine Herpesvirus 1 Envelope Protein US9 Homolog (76)

Recombinant Equine Herpesvirus 1 Envelope protein US9 homolog (76) refers to a protein encoded by the ORF76 gene in Equine Herpesvirus type 1 (EHV-1) . EHV-1 is an alphaherpesvirus known to cause rhinopneumonitis, abortion, and neurological issues such as equine herpesvirus myeloencephalopathy (EHM) in horses. It can also induce lethal encephalitis in laboratory animals like mice . The ORF76 gene is a homolog of the herpes simplex virus-1 (HSV-1) US9 gene and encodes a tegument protein, US9, which functions as a herpesvirus kinase . US9 homologs are highly conserved among herpesviruses .

Genetic Characteristics and Conservation

The EHV-1 US9 gene consists of 660 nucleotides, encoding a protein of 219 amino acids with a molecular mass of approximately 22.287 kDa . US9 proteins are enriched with serine and threonine residues and share a conserved domain of highly basic residues followed by a nonpolar amino acid region . Homologs of EHV-1 US9 are found in various herpesviruses, including HSV-1, varicella-zoster virus (VZV), pseudorabies virus (PRV), simian herpesvirus B, bovine herpesvirus 1 (BHV-1), feline herpesvirus 1 (FHV-1), canine herpesvirus, and herpes simplex virus-2 (HSV-2) .

Role in Viral Spread and Neurovirulence

US9 is essential for the anterograde spread of alphaherpesviruses, which involves the movement of the virus from the neuron cell body to the axon terminus . Studies using deletion mutants of ORF76 in EHV-1 have demonstrated that US9 is crucial for the virus's anterograde spread from the olfactory epithelium to the olfactory bulbs . In a mouse infection model, deletion of ORF76 prevented the virus from invading the central nervous system (CNS), underscoring its importance in neuroinvasiveness .

Post-Translational Modifications

Western blot assays have detected multiple bands of EHV-1 US9 ranging from 35 to 42 kDa, which is higher than the expected molecular mass . This suggests that the EHV-1 US9 protein undergoes post-translational modifications, likely phosphorylation . Phosphorylation is believed to alter the charge of the protein, affecting its migration in SDS-PAGE gels and resulting in an apparent higher molecular weight .

Functional Analysis Through Mutant Studies

Deletion mutant studies, such as those using the Ab4pΔORF76 mutant, have been instrumental in elucidating the function of US9 . While the deletion of ORF76 does not affect viral replication or cell-to-cell spread in cultured cells, it significantly impairs the virus's ability to spread within the nervous system . Specifically, the Ab4pΔORF76 mutant was unable to transport to the olfactory bulbs in mice, which prevented CNS infection .

Implications for Pathogenesis

US9's role in anterograde spread and neuroinvasiveness highlights its significance in the pathogenesis of EHV-1-related neurological diseases . The EHV-1 Kentucky A (KyA) strain, which has deletions in genes including US6 (gI), US7 (gE), and US8, is attenuated in mice, further supporting the role of specific genes in EHV-1 virulence .