Crystal Structure of a Retroviral Polyprotein: Prototype Foamy Virus Protease-Reverse Transcriptase (PR-RT)

Area of Science:

• Structural Biology
• Virology
• Biochemistry

Background:

• Retroviral Pol polyproteins are typically processed into protease (PR), reverse transcriptase (RT), and integrase (IN).
• Foamy viruses (FVs) differ by expressing Pol separately and releasing only IN upon processing.
• Understanding FV Pol processing is key to comprehending retroviral evolution and enzyme function.

Purpose of the Study:

• To determine the 3D structure of the mature prototype foamy virus (PFV) protease-reverse transcriptase (PR-RT) complex.
• To elucidate the structural basis for the distinct processing and activity of FV PR-RT compared to other retroviruses.
• To gain insights into the conformational changes involved in retroviral Pol polyprotein maturation.

Main Methods:

• X-ray crystallography at 2.9 Å resolution.
• Analysis of the quaternary structure and domain arrangements of PFV PR-RT.
• Comparison of PFV PR-RT architecture with known retroviral enzymes like HIV-1.

Main Results:

• The crystal structure of PFV PR-RT was determined, revealing a monomeric complex.
• PFV PR-RT is in a conformation incompetent for proteolytic or polymerase activity.
• The structure shows a unique C-terminal helical extension in PFV PR anchoring it to the RT palm subdomain.
• PFV RT domain organization resembles the p51 subunit of HIV-1 RT, with flexible linkers connecting RNase H and polymerase domains.

Conclusions:

• The PFV PR-RT structure provides a unique snapshot of a retroviral Pol enzyme in an inactive state.
• This structure highlights significant conformational rearrangements necessary for nucleic acid binding and enzymatic activity.
• Findings offer critical insights into the conformational maturation of retroviral Pol polyproteins and FV-specific mechanisms.