The PSAP Motif within the ORF3 Protein of an Avian Strain of the Hepatitis E Virus Is Not Critical for Viral Infectivity In Vivo but Plays a Role in Virus Release

The ORF3 protein of hepatitis E virus (HEV) is a multifunctional protein important for virus replication. The ORF3 proteins from human, swine, and avian strains of HEY contain a conserved PXXP amino acid motif, resembling either Src homology 3 (SH3) cell signaling interaction motifs or "late domains" involved in host cell interactions aiding in particle release. Using an avian strain of HEY, we determined the roles of the conserved prolines within the PREPSAPP motif in HEY replication and infectivity in Leghorn male hepatoma (LMH) chicken liver cells and in chickens. Each proline was changed to alanine to produce 8 avian HEY mutants containing single mutations (P64, P67, P70, and P71 to A), double mutations (P64167A, P64/70A, and P67/70A), and triple mutations (P64/67/70A). The results showed that avian HEY mutants are replication competent in vitro, and none of the prolines in the PXXPXXPP motif are essential for infectivity in vivo; however, the second and third prolines appear to aid in fecal virus shedding, suggesting that the PSAP motif, but not the PREP motif, is involved in virus release. We also showed that the PSAP motif interacts with the host protein tumor suppressor gene 101 (TSG101) and that altering any proline within the PSAP motif disrupts this interaction. However, we showed that the ORF2 protein expressed in LMH cells is efficiently released from the cells in the absence of ORF3 and that coexpression of ORF2 and ORF3 did not act synergistically in this release, suggesting that another factor(s) such as ORF1 or viral genomic RNA may be necessary for proper particle release.