Assembly of HIV in step 5 (viral assembly) which occurs in 2 stages
The correct answer and explanation is:
Correct Answer:
The assembly of HIV during step 5 (viral assembly) occurs in two main stages:
- Gag polyprotein multimerization and viral particle formation at the plasma membrane.
- Incorporation of viral RNA and other viral proteins into the assembling virion.
Detailed Explanation (300 words):
HIV assembly is a critical step in the viral life cycle that occurs at the host cell’s plasma membrane. This process can be divided into two major stages:
1. Gag Polyprotein Multimerization and Particle Formation
The Gag polyprotein (Pr55^Gag) plays the central role in HIV assembly. It is synthesized in the cytoplasm and contains several domains responsible for targeting, multimerization, and budding. The first stage involves the transport of Gag molecules to the plasma membrane. Gag contains a matrix (MA) domain that binds to the inner leaflet of the plasma membrane, primarily via interactions with phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P2).
At the membrane, Gag molecules multimerize through interactions involving their capsid (CA) and nucleocapsid (NC) domains, creating a lattice-like structure that forms the immature viral particle’s shell. This multimerization drives the curvature of the membrane, leading to budding of the virus particle.
2. Packaging of Viral RNA and Other Proteins
Simultaneously, the viral genomic RNA (gRNA) is selectively incorporated into the assembling virion. This packaging is mediated by the nucleocapsid (NC) domain of Gag, which specifically binds to the psi (Ψ) packaging signal on the viral RNA.
In addition to the RNA genome, other viral proteins—such as the envelope glycoproteins (Env), reverse transcriptase, integrase, and protease—are incorporated during this stage. Env glycoproteins are transported through the secretory pathway to the plasma membrane, where they become incorporated into the budding virion.
Budding and Maturation
After assembly, the immature virion buds from the host cell. HIV protease then cleaves the Gag polyprotein during maturation, transforming the virion into an infectious particle.
In summary, HIV assembly involves Gag polyprotein multimerization at the plasma membrane and selective packaging of viral RNA and proteins, setting the stage for viral budding and maturation.