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The HIV life cycle is typically divided into seven distinct stages, from the attachment of the virus to the host cell to the budding of new free-circulating HIV virions pictured. The stages are outlined in sequential orders as follows:. Interrupt any stage of the life cycle and the next cannot occur, making it impossible for the virus to multiply and spread. Once HIV enters the body typically through sexual contact, blood exposure, or mother-to-child transmission , it seeks out a host cell in order to reproduce.
The host in the case is the CD4 T-cell used to signal an immune defense. In order to infect the cell, HIV must attach itself by way of a lock-and-key type system. The keys are proteins on the surface of HIV which attach to a complimentary protein on the CD4 cell much in the way a key fits into a lock. This is what is known as viral attachment.
Viral attachment can be blocked by an entry inhibitor-class drug called Selzentry maraviroc. Once attached to the cell, HIV injects proteins of its own into the cellular fluids cytoplasm of the T-cell. This causes a fusion of the cell membrane to the outer envelope of the HIV virion. This is the stage known as viral fusion. Once fused, the virus is able to enter the cell.
An injectable HIV drug called Fuzeon enfuvirtide is able to interfere with viral fusion. In doing so, it can churn out multiple copies of itself. The process, called viral uncoating , requires that the protective coating surrounding the RNA must be dissolved. It accomplishes this with the help of the enzyme called reverse transcriptase. Once converted DNA, the genetic machine has the coding needed to enable viral replication. Drugs called reverse transcriptase inhibitors can block this process entirely.
In doing so, the double-stranded DNA chain cannot be fully formed, and replication is blocked. Ziagen abacavir , Sustiva efavirenz , Viread tenofovir , and Pifeltro doravirine are just some of the reverse transcriptase inhibitors commonly used to treat HIV. In order for HIV to hijack the host cell's genetic machinery, it must integrate the newly formed DNA into the nucleus of the cell.
Drugs called integrase inhibitors are highly capable of blocking the integration stage by blocking the integrase enzyme used to transfer the genetic material. More importantly, the viruses associated with cells are physically protected from neutralizing antibodies.
In fact, cell—cell transmission was uncovered by an experiment characterizing the neutralizing antibodies-resistant viral transmission. Notably, cell—cell transmission was found only in enveloped viruses, but not in naked viruses. Perhaps, cell—cell transmission is useless for naked viruses, where a bulk of virion is abruptly released upon cell lysis.
As stated above, most viruses enter the cell via receptor-mediated endocytosis. What would be the advantages of receptor-mediated endocytosis, as opposed to direct fusion? Unlike direct fusion, evidently, receptor-mediated endocytosis bypasses the actin cortex or the meshwork of microfilaments in the cortex that presents an obstacle for the penetration see Fig. Moreover, by being taken up by endocytosis, animal viruses can avoid leaving the viral envelope glycoprotein on the plasma membrane, thus likely causing a delay in detection by immune system.
Typically, receptor-mediated endocytosis proceeds via a clathrin-dependent manner Fig. Receptor-mediated endocytosis is the mechanism intrinsic to the cells, which is utilized to take extracellular molecules into the cells. Clathrin -mediated 6 endocytosis, which is also the pathway utilized for uptake of LDL, is employed by many viruses, such as influenza virus and adenovirus.
Upon the binding of the virus particle with the receptor, a clathrin-coated pit is formed, as clathrins are recruited near the plasma membrane. Following the formation of an endocytic vesicle, the vesicles are fused with early endosomes. The virus particles are now located inside the early endosomes. Upon the binding of virus particles to the receptor, clathrins are recruited to form the clathrin-coated pit via its interaction with AP-2 adapter.
Clathrin-coated pits are pinched off by dynamins. After the vesicle coat is shed, the uncoated endocytic vesicle fuses with the early endosome. The capsids are released from the endosome by membrane fusion between viral envelope and endosome that is triggered by low pH inside the endosome. In addition to receptor-mediated endocytosis, a few other endocytic mechanisms are utilized by animal viruses Fig.
For instance, caveolin -mediated 7 endocytosis is used for the entry of polyomaviruses, such as SV40 see Fig. In this case, caveolin, instead of clathrin, serves as a coat protein; otherwise it is similar to clathrin-mediated endocytosis.
Macropinocytosis 8 is utilized for the entry of particles with a larger size, such as vaccinia virus and herpes viruses. The virus particle first activates the signaling pathways that trigger actin-mediated membrane ruffling and blebbing.
The formation of large vacuoles macropinosomes at the plasma membrane is followed by the internalization of virus particles and penetration into the cytosol by the viruses or their capsids. Clathrin-mediated endocytosis. This pathway is the most commonly observed uptake pathway for viruses.
The viruses are transported via the early endosome to the late endosome and eventually to the lysosome. The caveola pathway brings viruses to caveosomes.
By the second vesicle transport step, viruses are transported to Golgi, and then to ER. Macropinocytosis is utilized for the entry of particles with larger size, such as vaccinia viruses and herpes viruses.
Following successful penetration inside cells, the virus particles need to get to an appropriate site in the cell for genome replication.
This process is termed intracellular trafficking. In fact, the biological importance of the cytoplasmic trafficking was not realized until the invention of live cell imaging technology. For viruses that replicate in the cytoplasm, the viral nucleocapsids need to be routed to the site for replication.
In fact, microtubule-mediated transport coupled with receptor-mediated endocytosis is the mechanism for the transport Fig. In addition, for viruses that replicate in the nucleus, the viral nucleocapsids need to enter the nucleus. For many DNA viruses, the viral nucleocapsids are routed to the perinuclear area via microtubule-mediated transport.
In this process, a dynein motor powers the movement of virus particles. As an analogy, the viral nucleocapsids can be envisioned as a train in a railroad. Two distinct viruses are used to explain how the entry is linked to cytoplasmic trafficking: A adenovirus naked and B herpes virus enveloped. Incoming viruses can enter cells by endocytosis A or direct fusion B.
Following penetration into cytoplasm, either endocytic vesicles or viral capsids exploit dynein motors to traffic toward the minus ends of microtubules. Either the endocytic vesicles A or the capsids B interact directly with the microtubules. The virus can also lyse the endocytic membrane, releasing the capsid into the cytosol A. As the virus particles approach to the site of replication, from the cell periphery to the perinuclear space, the viral genome becomes exposed to cellular machinery for viral gene expression, a process termed uncoating.
Uncoating is often linked with the endocytic route or cytoplasmic trafficking see Fig. For viruses that replicate in the nucleus, the viral genome needs to enter the nucleus via a nuclear pore.
Multiple distinct strategies are utilized, largely depending on their genome size Fig. For the virus with a smaller genome, such as polyomavirus, the viral capsid itself enters the nucleus. For viruses with a larger genome, the docking of nucleocapsids to a nuclear pore complex causes a partial disruption of the capsid eg, adenovirus or induces a minimal change in the viral capsid eg, herpes virus , allowing the transit of DNA genome into the nucleus.
A Polyomavirus capsids are small enough to enter the nucleus directly via the nuclear pore complex without disassembly. Uncoating of the polyomavirus genome takes place in the nucleus. B The adenovirus capsids are partially disrupted upon binding to the nuclear pore complex, allowing the transit of the DNA genome into the nucleus. C For herpesvirus, the nucleocapsids are minimally disassembled to allow transit of the DNA genome into the nucleus.
The viral genome replication strategies are distinct from each other among the virus families. In fact, the genome replication mechanism is the one that defines the identity of each virus family. Furthermore, the extent to which each virus family relies on host machinery is also diverse, ranging from one that entirely depends on host machinery to one that is quite independent.
However, all viruses, without exception, entirely rely on host translation machinery, ribosomes, for their protein synthesis. Exit can be divided into three steps: capsid assembly, release, and maturation. The capsid assembly follows as the viral genome as well as the viral proteins abundantly accumulates.
The capsid assembly can be divided into two processes: capsid assembly and genome packaging. Depending on viruses, these two processes can occur sequentially or simultaneously in a coupled manner. Picornavirus is an example of the former, while adenovirus is an example of the latter Fig. In the case of picornavirus, the capsids ie, immature capsid or procapsid are assembled first without the RNA genome.
Subsequently, the RNA genome is packaged or inserted via a pore formed in the procapsid structure. By contrast, in the case of adenovirus, the capsid assembly is coupled with the DNA genome packaging. Then, a question that arises is how does the virus selectively package the viral genome?
A packaging signal , 9 a cis -acting element present in the viral genome, is specifically recognized by the viral capsid proteins, which selectively package either RNA or DNA. A Sequential mechanism.
For picornavirus, the procapsid, a precursor of the capsids, is preassembled without RNA genome. Subsequently, the RNA genome penetrates into the procapsid via a pore. B Coupled mechanism. For adenovirus, the DNA genome is packaged into the capsid during capsid assembly. For naked viruses, the virus particles are released via cell lysis of the infected cells. Thus, no specific exit mechanism is necessary, because the cell membrane that traps the assembled virus particles are dismantled.
Examples of naked viruses are polyomavirus ie, SV40 and adenovirus. By contrast, in cases of enveloped viruses, envelopment , a process in which the capsids become surrounded by lipid bilayer, takes place prior to the release.
With respect to the relatedness of the capsid assembly to the envelopment, two mechanisms exist. First, the envelopment can proceed after the completion of capsid assembly Fig. In this sequential mechanism, the fully assembled capsids are recruited to the membrane by interaction of the viral capsids with viral envelope glycoprotein.
Examples of this include herpesvirus and hepatitis B virus. Alternatively, the envelopment can occur simultaneously with the capsid assembly Fig. Retrovirus is the representative of this coupled mechanism.
The capsid assembly occurs prior to the envelopment. The assembled capsid is then targeted to the membrane for envelopment. Togavirus constitutes a family of positive-strand RNA viruses see Table Capsid proteins and the viral genome are recruited together to the budding site on the membrane.
Author information Copyright and License information Disclaimer. Correspondence to: Alan Engelman 1 ude. Correspondence to: Peter Cherepanov 2 ku. Copyright notice. The publisher's final edited version of this article is available at Nat Rev Microbiol. See other articles in PMC that cite the published article. Abstract Three-dimensional molecular structures can provide detailed information on biological mechanisms and, in cases where molecular function impacts on human health, significantly aid in the development of therapeutic interventions.
Box 1 Highly active anti-retroviral therapy. Open in a separate window. Figure 1. Virus entry The HIV-1 envelope spikes, which comprise trimers of non-covalently linked heterodimers of the surface gp and transmembrane gp41 glycoproteins 7 — 9 , initiate a cascade of conformational changes that culminates in fusion between the viral and host cell membranes and the release of the viral core into the cytoplasm. Figure 2.
Uncoating Partial CA shell dissolution, which is required for reverse transcription 33 , 34 , is a recently verified therapeutic target 35 Fig. Figure 3. Viral DNA synthesis Reverse transcription and integration of the resultant linear viral DNA molecule into a host cell chromosome occurs within the context of the nucleoprotein complex structures that are derived from the viral core Fig.
Figure 4. Figure 5. Viral mRNA biogenesis and transport Integration marks the transition from the early to late phase of HIV-1 replication, in which the focus shifts to viral gene expression followed by the assembly and egress of nascent viral particles. Transcriptional elongation Tat recruits the cellular positive transcription elongation factor P-TEFb, comprising the Cdk9 kinase and cyclin T1 CycT1 subunits, to the viral trans-activation response TAR element present in stalled transcripts , Figure 6.
Viral egress and maturation The retroviral structural proteins CA, matrix MA and NC are synthesized as parts of the Gag precursor polypeptide, and HIV-1 Gag is sufficient to assemble virus-like particles at the plasma membrane and bud from cells Fig.
Viral late domains and the cellular ESCRT machinery Retroviral budding is orchestrated by interactions between Pro-rich motifs in Gag that are known as late L domains and cellular class E vacuolar protein sorting Vps proteins, the actions of which are required to form the nascent particle and sever it from the plasma membrane.
Figure 7. Protease and virus maturation The final step of the viral lifecycle, which is mediated by PR and occurs concomitant with or soon after budding, converts immature particles to infectious virions via the proteolysis of Gag and Gag-Pol precursor polypeptides to yield the structural components MA, CA and NC, and the PR, RT and IN enzymes Fig.
Conclusions and perspectives HIV-1 has been analyzed by structural biology techniques more so than any other virus, with partial or complete structures known for all 16 of its protein components and additional structures determined for substrate- and host factor-bound complexes. References 1. Gao F, et al. Origin of HIV-1 in the chimpanzee Pan troglodytes troglodytes. Korber B, et al. Timing the ancestor of the HIV-1 pandemic strains. Lemey P, et al. Tracing the origin and history of the HIV-2 epidemic.
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Atomic-level modelling of the HIV capsid. The crystal structure reveals the basic building block of the HIV-1 capsid pentamer that affords critical shape declinations to the conical capsid shell and accordingly leads to an atomicscale model of overallshell structure.
Fitzon T, et al. Proline residues in the HIV-1 NH2-terminal capsid domain: structure determinants for proper core assembly and subsequent steps of early replication. Formation of a human immunodeficiency virus type 1 core of optimal stability is crucial for viral replication.
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Hexagonal assembly of a restricting TRIM5 alpha protein. Small-molecule inhibition of human immunodeficiency virus type 1 infection by virus capsid destabilization. Crystal structure at 3. The X-ray crystal structure of the HIV-1 reverse transcriptase heterodimer reveals the asymmetric nature of the protein complex and the binding site for the non-nucleoside reverse transcriptase inhibitor nevirapine.
Jacobo-Molina A, et al. Crystal structure of human immunodeficiency virus type 1 reverse transcriptase complexed with double-stranded DNA at 3. The crystal structure of HIV-1 reverse transcriptase reveals the positioning of template nucleic acid. Rodgers DW, et al. The structure of unliganded reverse transcriptase from the human immunodeficiency virus type 1. Structure of a covalently trapped catalytic complex of HIV-1 reverse transcriptase: implications for drug resistance.
The X-ray crystal structure of the reverse transcriptase heterodimer with covalently trapped nucleic acid template, primer and deoxynucleoside triphosphate reveals the mechanism of DNA polymerization. The protein—nucleic acid covalent linkage is adopted as a field standard technique moving forward. Sarafianos SG, et al. Lamivudine 3 TC resistance in HIV-1 reverse transcriptase involves steric hindrance with beta-branched amino acids.
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