Palmitoylation-dependent association with Annexin II directs hepatitis E virus ORF3 sorting into vesicles and quasi-enveloped virions

Significance

The hepatitis E virus (HEV)-encoded ORF3 protein is essential for HEV infection. However, the mechanism by which ORF3 regulates the formation of quasi-enveloped virions remains unclear. In this study, we report that HEV ORF3 can be self-secreted into vesicles and identify Annexin II (ANXA2) as crucial for efficient ORF3 self-secretion and quasi-enveloped virion formation. ANXA2 binds to palmitoylated ORF3, and mutation of the palmitoylation sites in ORF3 disrupts its association with ANXA2, leading to dissociation between ORF3 and the cytoskeleton. Furthermore, an HEV mutant harboring mutations at palmitoylation sites within ORF3 was unable to efficiently secrete infectious viral particles, both in vitro and in vivo. Our findings provide insight into the egress mechanism of quasi-enveloped virions.

Abstract

Historically considered to be nonenveloped, hepatitis E virus (HEV), an important zoonotic pathogen, has recently been discovered to egress from infected cells as quasi-enveloped virions. These quasi-enveloped virions circulating in the blood are resistant to neutralizing antibodies, thereby facilitating the stealthy spread of infection. Despite abundant evidence of the essential role of the HEV-encoded ORF3 protein in quasi-enveloped virus formation, the underlying mechanism remains unclear. Here, we demonstrate that the HEV ORF3 protein possesses an inherent capacity for self-secretion and that palmitoylation at two cysteine residues within the ORF3 N-terminal region is essential for its secretion and quasi-enveloped virus formation. We further found that only palmitoylated ORF3 proteins hijacked Annexin II for transport to the cytoskeleton and are then directed into multivesicular bodies through the nSMase-endosomal sorting complexes required for transport-III pathway for secretion. Finally, we show that infection of gerbils with HEV mutants harboring mutations at palmitoylation sites within ORF3 showed no fecal viral shedding but competent replication in the liver. Our study fills a gap in the understanding of the assembly and release of quasi-enveloped virions mediated by ORF3 and offers the potential for designing therapeutic strategies to control HEV infection.