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Structural role of the matrix protein of type D retroviruses in gag polyprotein stability and capsid assembly
Abstract:
To obtain a better understanding of the role of the gag gene-encoded matrix (MA) protein in the assembly and maturation of type D retroviruses, we have made five mutants with specific in-frame deletions within the p10-coding region by the use of oligonucleotide-directed mutagenesis. The changes in the Gag polyprotein made by these mutations resulted in almost identical phenotypes. In cells expressing mutant genomes, the mutant Gag polyproteins were synthesized and modified with myristic acid in a normal manner. However, they were so unstable that the bulk of the newly synthesized polyproteins was degraded within 1 h without being processed into mature structural polypeptides. In contrast, wild-type polyproteins have a processing half-life of 3.0 to 3.5 h. The mutant Gag polyproteins were assembled with very low efficiency into capsids in the cytoplasm of the mutant-infected cells. Moreover, the few capsids that formed were neither released from nor accumulated in the cells. These results suggest that the matrix protein plays an important role in guiding the correct folding of the Gag polyprotein, which is presumably crucial for both stabilizing the molecule and facilitating the intermolecular interactions that occur during assembly of immature capsids.
Insights
The matrix protein (MA) is crucial for type D retrovirus assembly. Mutations destabilized Gag polyproteins, preventing capsid formation and maturation, highlighting MA's role in folding and stability.
Area of Science:
- Virology
- Molecular Biology
- Structural Biology
Background:
- Type D retroviruses are significant pathogens.
- The gag gene encodes structural proteins essential for viral assembly.
- The matrix (MA) protein's role in retroviral maturation is not fully understood.
Purpose of the Study:
- To investigate the function of the gag gene-encoded matrix (MA) protein in type D retrovirus assembly and maturation.
- To elucidate the MA protein's role in Gag polyprotein stability and capsid formation.
Main Methods:
- Oligonucleotide-directed mutagenesis was used to create five mutants with in-frame deletions in the p10-coding region of the gag gene.
- Analysis of Gag polyprotein synthesis, myristoylation, stability, processing, and capsid assembly in cells expressing mutant genomes.
Main Results:
- Mutant Gag polyproteins were synthesized and myristoylated normally but were highly unstable, degrading within 1 hour.
- Wild-type polyproteins had a processing half-life of 3.0–3.5 hours.
- Mutant Gag polyproteins assembled inefficiently into capsids, which were neither released nor accumulated.
- The matrix protein is essential for stabilizing the Gag polyprotein and facilitating intermolecular interactions during capsid assembly.
Conclusions:
- The matrix protein plays a critical role in guiding the correct folding of the Gag polyprotein.
- Proper folding, influenced by the matrix protein, is crucial for Gag polyprotein stability and intermolecular interactions during immature capsid assembly.
- These findings provide insights into the mechanisms of retroviral assembly and maturation.