Retrovirus Replication Process: From Entry to Maturation

Retroviruses use RNA as their genetic material and require a DNA intermediate to replicate. This allows them to integrate into the host genome, making infections persistent and difficult to treat. Understanding their replication is crucial for developing antiviral therapies and vaccines, particularly for viruses like HIV.

Their replication cycle involves multiple essential steps for producing new infectious particles.

Cell Binding And Entry

A retrovirus begins its replication cycle by binding to a susceptible host cell. This interaction is mediated by viral envelope glycoproteins that recognize and attach to specific receptors on the host membrane. For example, HIV-1 uses its envelope glycoprotein gp120 to bind to the CD4 receptor on T-helper cells, a process stabilized by co-receptors such as CCR5 or CXCR4. These receptor interactions determine the virus’s tropism, restricting infection to cells that express the necessary receptors.

Once attached, conformational changes in the viral glycoproteins facilitate membrane fusion or endocytosis. In HIV-1, binding to CD4 triggers structural rearrangements in gp120, exposing regions that interact with the co-receptor. This activates gp41, a transmembrane protein that drives the fusion of the viral envelope with the host membrane. Other retroviruses, like murine leukemia virus (MLV), enter through receptor-mediated endocytosis, where the acidic endosomal environment induces membrane fusion. Regardless of the mechanism, this step delivers the viral core into the cytoplasm.

Following entry, the viral capsid is released into the cytoplasm and undergoes uncoating. Proper timing of this step is critical—premature disassembly exposes viral components to host defenses, while delayed uncoating hinders replication. In HIV-1, host proteins such as cyclophilin A and the nuclear pore complex influence this process, guiding the viral genome toward the nucleus.

Role Of Reverse Transcriptase

Once inside the cytoplasm, the viral enzyme reverse transcriptase (RT) converts single-stranded RNA into double-stranded DNA, enabling integration into the host genome. RT has two functions: RNA-dependent DNA polymerase synthesizes a complementary DNA (cDNA) strand from the viral RNA, and ribonuclease H (RNase H) degrades the original RNA template. Together, these activities create a stable DNA intermediate necessary for infection.

The process begins when a host-derived tRNA anneals to a complementary sequence on the viral RNA, providing a primer for DNA synthesis. RT extends this primer, synthesizing the first DNA strand while overcoming structural challenges such as RNA secondary structures. Lacking proofreading capabilities, RT introduces frequent mutations, particularly in HIV-1, contributing to drug resistance and immune evasion.

As the first DNA strand elongates, RNase H degrades the RNA template, leaving fragments that serve as primers for the complementary DNA strand. A critical strand-transfer event, the “strong-stop” transfer, allows the new DNA to relocate and continue elongation. The final product is a double-stranded DNA molecule with long terminal repeats (LTRs) at both ends, which regulate viral gene expression after integration.

Integration In The Host Genome

Once synthesized, the retroviral DNA must enter the nucleus. Unlike many viruses that replicate in the cytoplasm, retroviruses integrate into the host genome for long-term survival. The pre-integration complex (PIC), a multiprotein structure, shields the viral genome from degradation and guides it to the nuclear pore. Some retroviruses, like HIV-1, can infect non-dividing cells due to nuclear localization signals in their integrase and capsid proteins. Others, like MLV, require mitosis for nuclear entry.

Inside the nucleus, the viral enzyme integrase facilitates genome integration. It removes two terminal nucleotides from each LTR, creating sticky ends for insertion. The enzyme then cleaves host DNA and integrates the viral genome, resulting in short duplications of host sequences flanking the provirus. While integrase does not target specific sequences, it favors transcriptionally active regions, enhancing viral gene expression.

Integration location affects infection outcomes. In transcriptionally active regions, viral gene expression proceeds efficiently, while insertion into heterochromatin may induce latency. In HIV-1, latent reservoirs in resting CD4+ T cells complicate eradication efforts. Additionally, integration near proto-oncogenes can disrupt host genes, potentially leading to oncogenesis. Studies on retroviral vectors in gene therapy have highlighted cases where insertion near oncogenes triggered abnormal cell proliferation.

Gene Expression And Protein Synthesis

After integration, the viral genome becomes a permanent part of the host DNA, driving the production of viral components. Transcription is carried out by the host RNA polymerase II, which recognizes promoter elements in the viral LTRs. These regulatory regions interact with host transcription factors, influencing RNA synthesis. HIV-1 enhances transcription through its Tat protein, which stimulates elongation for efficient replication.

Viral RNA serves as both genomic material for new virions and messenger RNA (mRNA) for protein synthesis. Alternative splicing generates multiple viral proteins from a limited number of genes. Spliced mRNAs encode regulatory and accessory proteins, while unspliced or partially spliced transcripts produce structural and enzymatic components. This splicing strategy ensures efficient protein expression while maintaining proper virion assembly.

Virion Assembly

With viral gene expression underway, new virions begin to form within the host cell. Assembly involves packaging genomic RNA, structural proteins, and enzymatic components to produce infectious particles. This process is highly regulated to ensure proper virion formation.

Assembly starts in the cytoplasm, where Gag, Gag-Pol, and Env proteins accumulate. The Gag polyprotein plays a central role, containing domains for membrane targeting, RNA binding, and particle formation. Its matrix (MA) domain associates with the plasma membrane, while the nucleocapsid (NC) domain selectively binds full-length viral RNA via packaging signals.

As Gag multimerizes at the membrane, spherical particles form, encapsulating the viral genome and enzymatic components. Meanwhile, Env glycoproteins are processed in the endoplasmic reticulum and Golgi apparatus before being transported to the plasma membrane, where they incorporate into budding virions. At this stage, virions remain immature and non-infectious, requiring further processing after release.

Release And Maturation

Newly assembled virions exit the host cell through budding, acquiring a lipid envelope in the process. Efficient budding depends on interactions between the Gag protein and the host ESCRT system, which facilitates membrane remodeling and virus release. Mutations in Gag’s late domain can impair release, leaving virions tethered to the cell surface.

After release, virions undergo maturation, a crucial step for infectivity. The viral protease enzyme cleaves Gag and Gag-Pol polyproteins into functional subunits. Cleavage of Gag results in the formation of mature matrix, capsid, and nucleocapsid proteins, condensing the viral core into a stable, infectious structure. In HIV-1, protease inhibitors target this step, preventing proper maturation and rendering the virus non-infectious.