A PLPPV sequence in the p8 region of Gag provides late domain function for mouse mammary tumor virus

Area of Science:

• Virology
• Molecular Biology
• Cellular Biology

Background:

• The late (L) domain sequence in mouse mammary tumor virus (MMTV) Gag proteins, crucial for viral release, was previously undefined.
• Monoubiquitination of MMTV p8 and p14^NC proteins suggests a role in L domain function, similar to other retroviruses.

Purpose of the Study:

• To identify and characterize the specific late (L) domain sequence responsible for MMTV virion release.
• To determine the functional significance of identified MMTV Gag sequences in the viral budding process.

Main Methods:

• Site-directed mutagenesis was employed to alter specific sequences (PLPPV, PLPPL) within MMTV Gag proteins.
• Electron microscopy was used to examine the morphology of Gag mutants with defects in viral budding.
• Reciprocal Gag exchange mutants between MMTV and equine infectious anemia virus (EIAV) were constructed to assess L domain function.

Main Results:

• Mutagenesis revealed that the PLPPV sequence, but not PLPPL, is essential for MMTV virion release in a position-dependent manner.
• Electron microscopy confirmed a budding defect in a Gag mutant lacking functional L domain sequence.
• The MMTV PLPPV sequence conferred L domain function to EIAV Gag, and the EIAV YPDL sequence conferred function to MMTV Gag, demonstrating functional conservation and specificity.

Conclusions:

• The PLPPV sequence functions as the late (L) domain for MMTV, representing a novel fourth class of retroviral L domains.
• This L domain enables MMTV Gag proteins to utilize cellular budding machinery for efficient viral release.
• Understanding MMTV L domain function provides insights into the broader mechanisms of retroviral particle assembly and egress.