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Journal of Virology, December 1999, p. 9867-9878, Vol. 73, No. 12
Institute for Medical Microbiology and
Hygiene, Medical University of Lübeck, Lübeck, Germany
Received 1 June 1999/Accepted 1 September 1999
The orchestrated liberation of viral proteins by
3Cpro-mediated proteolysis is pivotal for gene expression
by picornaviruses. Proteolytic processing is regulated either by the
amino acid sequence at the cleavage site of the substrate or by
cofactors covalently or noncovalently linked to the viral proteinase.
To determine the role of the amino acid sequence at cleavage sites
3A/3B and 3B/3C that are essential for the liberation of
3Cpro from its precursors and to assess the function of the
stable processing intermediates 3AB and 3ABC, we studied the effect of cleavage site mutations on hepatitis A virus (HAV) polyprotein processing, particle formation, and replication. Using the recombinant vaccinia virus system, we showed that the normally retarded cleavage at
the 3A/3B junction can be improved by altering the amino acid sequence
at the scissile bond such that it matches the preferred HAV 3C cleavage
sites. In contrast to the processing products of the wild-type
polyprotein, 3ABC was no longer detectable in the mutant. VP0 and VP3
were generated less efficiently, implying that processing of the
structural protein precursor P1-2A depends on the presence of stable
3ABC and/or 3AB. In addition, cleavage of 2BC was impaired in
3AB/3ABC-deficient mutants. Formation of HAV particles was not affected
in mutants with blocked 3A/3B and/or 3B/3C cleavage sites. However,
3ABC-deficient mutants produced small numbers of HAV particles, which
could be augmented by coexpressing 3AB or 3ABC. The hydrophobic domain
of 3A that has been proposed to mediate membrane anchorage of the
replication complex was crucial for restoration of defective particle
formation. In vitro transcripts of the various cleavage site mutants
were unable to initiate an infectious cycle, and no progeny viruses
were obtained even after blind passages. Taken together, the data
suggest that accumulation of uncleaved HAV 3AB and/or 3ABC is pivotal
for both viral replication and efficient particle formation.
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Improving Proteolytic Cleavage at the 3A/3B Site of
the Hepatitis A Virus Polyprotein Impairs Processing and Particle
Formation, and the Impairment Can Be Complemented in
trans by 3AB and 3ABC
*
Corresponding author. Mailing address: Institute for
Medical Microbiology and Hygiene, Medical University of Lübeck,
Ratzeburger Allee 160, D-23538 Lübeck, Germany. Phone: 49-451-500 4085. Fax: 49-451-500 3637. E-mail:
koussov{at}molbio.mu-luebeck.de.
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