Expression and processing of the Hepatitis E virus ORF1 nonstructural polyprotein.

Area of Science:

• Virology
• Molecular Biology
• Protein Biochemistry

Background:

• Hepatitis E virus (HEV) ORF1 encodes a large polyprotein with putative enzyme domains.
• Previous studies using bacterial and mammalian systems showed no HEV ORF1 polyprotein processing.
• This led to the hypothesis that HEV ORF1 does not undergo functional domain processing.

Purpose of the Study:

• To investigate the expression and processing of HEV ORF1 polyprotein.
• To utilize the baculovirus expression system for high-level expression and post-translational modification analysis.

Main Methods:

• HEV ORF1 expression in a baculovirus vector system.
• Detection of processed fragments using N- and C-terminal tags.
• Inhibition studies using a cysteine protease inhibitor (E-64d).
• Mass spectrometry (MALDI-TOF) for fragment identification.
• Western blot analysis using antibodies against specific domains.

Main Results:

• Baculovirus expression led to time-dependent processing of the ~192 kDa tagged HEV ORF1 polyprotein into smaller fragments.
• Processing was significantly inhibited by the cysteine protease inhibitor E-64d.
• MALDI-TOF analysis identified a 35 kDa fragment as the methyltransferase (MeT) domain.
• Antibody detection confirmed processing patterns consistent with domain cleavage.

Conclusions:

• The HEV ORF1 polyprotein undergoes processing into smaller proteins correlating with functional domains when expressed in a baculovirus system.
• This processing is predominantly mediated by a cysteine protease.
• Further studies are needed to rule out the involvement of other proteases.