Question

Describe the role of the Gag polyprotein during the assembly of HIV components at the host-cell plasma membrane.

Short answer

The Gag polyprotein is crucial in the assembly of HIV components at the host-cell plasma membrane. It targets the assembly site, helps assemble the immature HIV particle, and is involved in the maturation of the virus into an infectious form.

Step-by-step solution

Understanding the Gag Polyprotein

Firstly, it's important to understand the Gag polyprotein. Gag is a HIV polyprotein that plays critical roles in the assembly of an HIV particle. It is encoded by the gag gene in the HIV genome and is initially synthesized in the infected cell as a single, long polypeptide chain. The Gag polyprotein serves as the building block for the immature HIV particle.

Role in Virus Assembly

The primary function of the Gag polyprotein is to mediate the assembly of the virus. HIV-1 Gag directs the assembly of virus-like particles at the plasma membrane of the host cell. It has multiple domains, each with a specific function: the Matrix (MA) domain targets Gag to the plasma membrane, the Capsid (CA) domain forms the core of the virus, and the Nucleocapsid (NC) domain binds to the viral RNA.

Interaction with Plasma Membrane

Gag interacts with the inner leaflet of the plasma membrane with the help of its N-terminally located myristoylated MA domain. This interaction targets the assembly of the virus to the plasma membrane of the host cell.

Formation of Immature HIV Particles

Once at the plasma membrane, Gag molecules bind to one another and to the viral RNA to form immature HIV particles. These budding virions will then begin to pinch off from the plasma membrane, becoming independent HIV particles.

Maturation into Infectious HIV

Upon budding from the host cell, the HIV protease (which is part of the Gag-Pol polyprotein) cleaves the Gag polyproteins into their functional subunits, resulting in structural rearrangement. This process, known as maturation, transforms the virus from an immature non-infectious particle to an infectious HIV virion.