AIDS/HIV, Molecular and Cell Biology
107
ribosomes. The earliest transcripts code
for the regulatory proteins, Tat, Rev, and
Nef (Fig. 2). Tat is a transcriptional activa-
tor that shuttles back into the nucleus and
interacts with an RNA stem loop struc-
ture, the Tat Response Element (TAR) at
the 5
0
end of all viral transcripts (Fig. 2).
Tat bound to TAR, recruits cyclin T and
CDK9, which cause hyperphosphorylation
of the C-terminal domain of the RNA
polymerase II, massively enhancing its
transcriptional ef±ciency. The sequence
of transcription, export, and translation
is replicated and ampli±ed and more Tat
protein is produced in what amounts to
a positive feedback loop. The spliced RNA
species encoding Rev is also translated and
like the Tat protein, it shuttles back into the
nucleus. It interacts with an RNA structure
called the
Rev Responsive Element
(
RRE
)in
the envelope intron (Fig. 2), enhancing ex-
port of both the full-length genomic RNA
and the singly spliced RNA that encodes
the envelope glycoprotein. Thus, the Tat
protein causes an early ampli±cation of
transcription, and Rev subsequently leads
to an early to late shift in the species of
RNA, which are exported from the nucleus
from those encoding regulatory factors
to mRNAs for structural and enzymatic
proteins. Unspliced full-length RNAs are
translated on free cytoplasmic ribosomes
to produce the Gag and Pol proteins. The
singly spliced envelope encoding RNA
binds to a ribosome and with the trans-
lation of its signal peptide, the complex
is transported to the rough endoplasmic
reticulum where envelope glycoproteins
are translated and glycosylated using the
conventional cellular glycoprotein secre-
tory route.
The Gag and Pol proteins are encoded on
the same transcript but in different read-
ing frames. Translation of the polyprotein
is interrupted by a frameshift sequence at
the Gag/Pol overlap with an ef±ciency of
about 5%. The ribosome shifts into the
1
frame, so that for approximately every 19
Gag proteins that are produced, 1 Gag/Pol
fusion protein is synthesized. Gag and
Gag/Pol proteins assemble at the plasma
membrane in approximately the same
stoichiometric ratio. At some stage, fol-
lowing transcription, the Gag protein has
interacted with full-length genomic RNA
transcripts and captured these through
binding to a complex folded region of
the RNA termed the
packaging signal
or
9
region. This RNA is then transported
into the assembling virion where it binds
other Gag proteins through the zinc ±n-
gers of their nucleocapsid subfragments,
the RNA acting to some extent as a scaf-
fold for viral assembly. RNA–protein and
protein–protein interactions contribute to
the generation of a convex curved raft of
protein underneath the cell membrane.
Assembly continues with the viral bud
bulging out of the cell and the Gag and
Gag/Pol proteins accumulating to form a
spherical viral core. This mode of assem-
bly at the membrane de±nes lentiviruses
as having a C-type morphology.
The viral core that forms is composed
of Gag and Gag/Pol polyproteins, all
aligned with the matrix region outermost
and the NC region toward the center of
the particle. At the extreme C-terminal
of the Gag protein is a small region
termed p6, which is intimately involved
in the budding process. Mutations in
p6 have been shown to lead to arrested
budding in which stalked but otherwise
intact particles remain attached to the
plasma membrane of the cell. p6 has
recently been shown to interact with a
cellular protein, TSG101. In the normal
cell, TSG101 is involved in the budding
process of endosomes into multivesicular
endosomes. It appears that the virus has
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