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J. Virol., 07 1996, 4283-4290, Vol 70, No. 7
MW Pandori, NJ Fitch, HM Craig, DD Richman, CA Spina and JC Guatelli
Type 1 human immunodeficiency viruses encoding mutated nef reading frames
are 10- to 30-fold less infectious than are isogenic viruses in which the
nef gene is intact. This defect in infectivity causes nef- negative viruses
to grow at an attenuated rate in vitro. To investigate the mechanism of
Nef-mediated enhancement of viral growth rate and infectivity, a
complementation analysis of nef mutant viruses was performed. To provide
Nef in trans upon viral infection, a CEM derivative cell line (designated
CLN) that expresses Nef under the control of the viral long terminal repeat
was constructed. When nef- negative virus was grown in CLN cells, its
growth rate was restored to wild-type levels. However, the output of
nef-negative virus during the first 72 h after infection of CLN cells was
not restored, suggesting that provision of Nef within the newly infected
cell does not enhance the productivity of a nef-negative provirus. The
genetically nef- negative virions produced by the CLN cells, however, were
restored to wild-type levels of infectivity as measured in a syncytium
formation assay in which CD4-expressing HeLa cells were targets. These
trans- complemented, genetically nef-negative virions yielded wild-type
levels of viral output following a single cycle of replication in primary
CD4 T cells as well as in parental CEM cells. To define the determinants
for producer cell modification of virions by Nef, the role of
myristoylation was investigated. Virus that encodes a myristoylation-
negative nef was as impaired in infectivity as was virus encoding a deleted
nef gene. Because myristoylation is required for both membrane association
of Nef and optimal viral infectivity, the possibility that Nef protein is
included in the virion was investigated. Wild-type virions were purified by
filtration and exclusion chromatography. A Western blot (immunoblot) of the
eluate fractions revealed a correlation between peak Nef signal and peak
levels of p24 antigen. Although virion-associated Nef was detected in part
as the 27-kDa full- length protein, the majority of immunoreactive protein
was detected as a 20-kDa isoform. nef-negative virus lacked both 27- and
20-kDa immunoreactive species. Production of wild-type virions in the
presence of a specific inhibitor of the human immunodeficiency virus type 1
protease resulted in virions which contained only 27-kDa full-length Nef
protein. These data indicate that Nef is a virion protein which is
processed by the viral protease into a 20-kDa isoform within the virion
particle.
Copyright © 1996, American Society for Microbiology
Producer-cell modification of human immunodeficiency virus type 1: Nef is a virion protein
Department of Medicine, University of California San Diego School of Medicine, California 92093-0619, USA.
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