Involvement of the C-terminal disulfide-bonded loop of murine leukemia virus SU protein in a postbinding step critical for viral entry

Area of Science:

• Virology
• Molecular Biology
• Immunology

Background:

• The C-terminal domain (CTD) of the murine leukemia virus (MuLV) Env protein plays a role in viral entry.
• Monoclonal antibodies (MAbs) targeting the CTD can inhibit MuLV-induced fusion.
• Understanding the specific epitopes and their function is key to understanding viral fusion mechanisms.

Purpose of the Study:

• To investigate the role of the CTD of MuLV Env protein in viral fusion.
• To map the epitopes recognized by fusion-inhibiting MAbs.
• To determine the structural requirements of these epitopes for MAb binding and fusion inhibition.

Main Methods:

• Utilized rat monoclonal antibodies (MAbs) 35/56 and 83A25 specific for MuLV Env CTD.
• Mapped MAb epitopes to the C-terminal disulfide-bonded loop (loop 10) of the SU protein.
• Generated and analyzed Env mutants with alterations in loop 10 to assess fusion activity, infectivity, and protein processing.

Main Results:

• Two MAbs inhibited MuLV fusion but not receptor binding, indicating a role for CTD epitopes in fusion.
• Epitope mapping localized determinants to loop 10, with adjacent residues influencing MAb specificity and epitope formation.
• Mutations within loop 10 significantly attenuated viral infectivity and fusion, despite normal expression, processing, and receptor binding.
• Loop 10 mutations also affected Env precursor processing and SU shedding, suggesting roles in Env folding and stability.

Conclusions:

• The C-terminal loop 10 of MuLV SU is critical for viral fusion.
• This loop is essential for proper Env protein folding and stability, influencing both fusion and precursor processing.
• The findings provide insights into the molecular mechanisms of retroviral fusion and Env protein function.