Retroviruses produce polyproteins that are later cleaved into functional units. The gag-pro-pol precursor in HTLV-3 encodes three major domains:

• Gag (Group-specific antigen): Responsible for capsid formation, matrix association, and viral core structure.
• Pro (Protease): Processes polyproteins into functional subunits.
• Pol (Polymerase): Encodes enzymes essential for viral replication, including reverse transcriptase (RT), RNase H, and integrase.

In HTLV-3, the gag-pro-pol gene is translated via a ribosomal frameshift mechanism, a hallmark of retroviral gene expression, ensuring precise stoichiometry of structural and enzymatic components.

Recombinant versions of this polyprotein are typically expressed in E. coli or baculovirus-insect systems, enabling researchers to isolate the protein in large quantities for structural and functional assays. Challenges in expression often include:

• Toxicity to the host due to active protease domains.
• Misfolding or aggregation, especially in bacterial systems.
• Need for codon optimization and solubility tags.

Purified recombinant Gag-Pro-Pol is used to study proteolytic processing, enzyme kinetics, and interactions with host cell factors.

Gag Domain: Structural Dynamics

• MA (Matrix): Binds to the plasma membrane and helps incorporate viral RNA.
• CA (Capsid): Forms the shell around the viral genome.
• NC (Nucleocapsid): Contains zinc-finger motifs crucial for RNA binding.

These subdomains are critical for virus assembly, budding, and early stages of infection.

Protease (Pro): Processing Hub

• A dimeric aspartic protease, highly conserved among retroviruses.
• Catalyzes cleavage at specific sites in Gag and Pol to activate the structural and enzymatic proteins.
• Inhibitable by standard retroviral protease inhibitors, making it a target for drug development.

Pol Domain: Replication Engine

• Reverse Transcriptase (RT): Converts viral RNA into proviral DNA.
• RNase H: Degrades RNA strands of RNA-DNA hybrids during reverse transcription.
• Integrase (IN): Inserts proviral DNA into host chromatin.

Each domain is functionally essential, and the correct processing of the Gag-Pro-Pol polyprotein is a precondition for the viral life cycle.