ROLES OF HIV-1 REGULATORY AND ACCESSORY PROTEINS
A unique feature of HIV-1 and other members of the lentivirus subfamily is the ability to produce a complex array of regulatory and accessory proteins that appear to be responsible for staging the infection, increasing the efficiency and yield of the infection, and in some cases contributing to viral latency. These proteins also appear to in-teract with cellular factors to modulate the infection differently in different host cells. The roles of the two HIV-1 regulatory genes, tat and rev, and the four accessory pro-teins, nef, vpu,vpr, and vif, are discussed below and summarized in Table 42 – 2. Although the four accessory proteins are dispensable in many cell culture systems, they appear to be important for the maximum pathogenic potential of the virus in infected individuals.
The products of the tat and rev regulatory genes are the Tat and Rev proteins, respec-tively. Both of these proteins have the effect of staging the infection, so that in the ab-sence of abundant transcription of the proviral genome, only limited gene expression is possible. When the infected T-lymphocyte is stimulated, for example by antigen presenta-tion, Tat and Rev play a positive role in promoting viral gene expression. In the absence of high levels of Tat, the host RNA polymerase initiates properly at the LTR promoter, but transcription is usually prematurely terminated leading to the production of short, dead-end transcripts. Tat is a transcriptional activator that acts at a sequence near the beginning of the viral mRNA, called TAR, to recruit cellular proteins to the transcribing RNA poly-merase, resulting in a modification to the polymerase that prevents premature termination and allows complete transcription of the proviral genome.
The Rev protein acts at the level of mRNA splicing. Normally, unspliced cellular tran-scripts are retained in the nucleus and only fully spliced mRNAs are transported to the cytoplasm for translation. The only viral proteins that are made from fully spliced mRNAs are Tat, Rev, and Nef, and consequently only these proteins are found early after infection, when there is no mechanism to prevent complete splicing of pre-mRNAs. To express the Vif, Vpr, and Vpu proteins, and the Env polyprotein, which are all made from singly spliced transcripts, as well as the Gag and Pol polyproteins, which are translated from the unspliced genomic RNA, it is necessary to transport incompletely spliced RNAs to the cytoplasm. Transport of partially spliced transcripts is accomplished by Rev bind-ing to a site on the viral RNA within the env gene called the Rev-responsive element (RRE). The RNA-bound Rev then interacts with normal cellular machinery responsible for protein export from the nucleus to mediate the movement of the RNA through the nu-clear pore. By promoting translation of the virion structural proteins and some of the accessory proteins, Rev turns up late gene expression that leads directly to a high rate of virus production.
The Nef accessory protein enhances virus production and virion infectivity and also ap-pears to interfere with immune recognition of infected cells. Nef causes the internalization and degradation of the CD4 protein, which likely contributes to virus release by preventing the formation of complexes between the cellular receptor and newly synthesized virions. Nef also causes the downregulation of cell surface major histocompatibility com-plex (MHC) I molecules, which may prevent recognition of infected cells by cytotoxic T lymphocytes. In addition, virions produced in the absence of the Nef protein are at least partially blocked at some step prior to integration. The combination of these and perhaps other effects allows the Nef protein to play an essential pathogenic role in an infected in-dividual.
The Vpu protein appears to play two separate roles during the late stages of infection. In the absence of Vpu, the Env protein forms complexes with CD4 in the endoplasmic reticulum and fails to reach the plasma membrane of the cell. One of the roles of Vpu is to target the destruction of CD4 in the endoplasmic reticulum to allow for incorporation of Env into newly synthesized virions. The second role of Vpu is to promote the release of virions from the infected cell by an unknown mechanism.
The Vpr protein is involved in promoting import of subviral particles into the nucleus after reverse transcription. Thus, the protein has little or no effect in proliferating T cells where nuclear access is ensured with each mitosis. However, successful infection of non-dividing cells such as macrophages requires Vpr to allow the newly synthesized viral DNA to reach the nucleus and be integrated into the cellular DNA.
Vif (virion infectivity factor) increases the infectivity of HIV-1 in primary T cells and certain “nonpermissive” cells in culture. In the absence of Vif, the virus fails to complete reverse transcription in these cell types. “Permissive” cell lines infected by mutants defec-tive in the vif gene produce normal yields of infectious virus. One possible explanation for this observation is that “permissive” cells contain a factor that can substitute for the missing Vif protein. Thus, one role of Vif may be to extend the host range of HIV-1 to cell types that would otherwise not be infected.
Superimposed on this complex regulatory network is the fact that the viral pro-moter contains elements that are sensitive to specific cellular transcription factors. This observation may help explain why virus production in CD4 T lymphocytes is greatly increased when the cells are activated. Clearly the outcome of an HIV-1 infec-tion is determined by a complex interplay between a very large number of different factors.