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J Virol, May 1998, p. 4095-4103, Vol. 72, No. 5
Department of Microbiology, University of
Alabama at Birmingham, Birmingham, Alabama 35294
Received 3 November 1997/Accepted 10 February 1998
Virus assembly represents one of the last steps in the retrovirus
life cycle. During this process, Gag polyproteins assemble at specific sites within the cell to form viral capsids and induce membrane extrusion (viral budding) either as assembly progresses (type
C virus) or following formation of a complete capsid (type B and type D
viruses). Finally, the membrane must undergo a fusion event to pinch
off the particle in order to release a complete enveloped virion.
Structural elements within the MA region of the Gag polyprotein
define the route taken to the plasma membrane and direct the process of
virus budding. Results presented here suggest that a distinct region of
Gag is necessary for virus release. The pp24 and pp16 proteins of the
type D retrovirus Mason-Pfizer monkey virus (M-PMV) are
phosphoproteins that are encoded in the gag gene of the
virus. The pp16 protein is a C-terminally located cleavage product of
pp24 and contains a proline-rich motif (PPPY) that is conserved among
the Gag proteins of a wide variety of retroviruses. By performing a
functional analysis of this coding region with deletion mutants, we
have shown that the pp16 protein is dispensable for capsid
assembly but essential for virion release. Moreover, additional
experiments indicated that the virus release function of pp16
was abolished by the deletion of only the PPPY motif and could be
restored when this motif alone was reinserted into a Gag
polyprotein lacking the entire pp16 domain. Single-amino-acid substitutions for any of the residues within this motif confer a
similar virion release-defective phenotype. It is unlikely that the
function of the proline-rich motif is simply to inhibit premature activation of protease, since the PPPY deletion blocked virion release
in the context of a protease-defective provirus. These results
demonstrate that in type D retroviruses a PPPY motif plays a key role
in a late stage of virus budding that is independent of and occurs
prior to virion maturation.
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
A Proline-Rich Motif (PPPY) in the Gag Polyprotein of
Mason-Pfizer Monkey Virus Plays a Maturation-Independent Role in
Virion Release
and
*
Corresponding author. Mailing address: Department of
Microbiology, University of Alabama at Birmingham, 256 Bevill
Biomedical Research Building, 845 19th St. South, Birmingham, AL
35294-2170. Phone: (205) 934-2437. Fax: (205) 934-1640. E-mail:
ehunter{at}uab.edu.
Present address: Laboratory Animal Research Center, Institute of
Medical Science, University of Tokyo, Tokyo 108, Japan.
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