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Alternate splicing in a parvoviral nonstructural gene links a common amino-terminal sequence to downstream domains
1Department of Laboratory Medicine, Yale University School of Medicine, New Haven, Connecticut 06510.
Abstract:
Minute virus of mice (MVM) encodes two groups of nonstructural proteins, the 83-kDa NS-1 polypeptides encoded from a contiguous sequence in the left half of the genome and the 25-kDa NS-2 polypeptides, which share a common amino-terminal domain with NS-1 but are multiply spliced. Peptide-specific antibodies were used to demonstrate that three alternatively spliced forms of NS-2 are synthesized when synchronized A9 cells are infected with the prototype strains of MVM, MVM(p), and that each of these species migrates as two bands on sodium dodecyl sulfate-gel electrophoresis, due to the presence of both phosphorylated and unphosphorylated forms. While most NS-1 molecules are located in the nucleus, all three species of NS-2 are predominantly cytoplasmic, and their phosphorylated forms are exclusively cytoplasmic. Although both NS-1 and NS-2 molecules are synthesized early in infection, all forms of NS-2 are synthesized and accumulate three to four times as NS-1 molecules, making them the predominant virally coded proteins in the cell at this time. Despite their common amino-terminal domain, NS-2 molecules turn over rapidly while NS-1 polypeptides persist for many hours. Apart from the fact that the three NS-2 gene products are synthesized in different molar amounts, we were unable to detect any differences in the expression, stability, distribution, or phosphorylation of the various molecular forms, suggesting that these latter characteristics are mediated by their common internal exon.
Insights
Minute virus of mice (MVM) produces distinct NS-1 and NS-2 proteins. While NS-1 is nuclear, NS-2 proteins are cytoplasmic and accumulate more, despite rapid turnover, suggesting shared exon mediation.
Area of Science:
- Virology
- Molecular Biology
- Cell Biology
Background:
- Minute virus of mice (MVM) is a parvovirus encoding nonstructural (NS) proteins.
- NS-1 and NS-2 proteins share a common amino-terminal domain but differ in splicing and function.
- Understanding MVM protein dynamics is crucial for deciphering viral replication and pathogenesis.
Purpose of the Study:
- To characterize the synthesis, distribution, and stability of MVM NS-1 and NS-2 proteins.
- To investigate the role of alternative splicing and post-translational modifications in NS-2 protein expression.
- To compare the cellular localization and turnover rates of NS-1 and NS-2 variants.
Main Methods:
- Infection of synchronized A9 cells with MVM prototype strains.
- Use of peptide-specific antibodies for protein detection and characterization.
- Sodium dodecyl sulfate-gel electrophoresis to analyze protein size and phosphorylation status.
- Cellular fractionation to determine protein localization (nucleus vs. cytoplasm).
Main Results:
- Three alternatively spliced forms of NS-2 were identified, each existing in phosphorylated and unphosphorylated states.
- NS-1 proteins were predominantly nuclear, while all NS-2 forms were primarily cytoplasmic.
- NS-2 proteins were synthesized and accumulated at higher levels than NS-1 but exhibited rapid turnover.
- Despite differences in abundance and localization, NS-2 variants showed similar stability and phosphorylation patterns, suggesting a common regulatory mechanism.
Conclusions:
- MVM NS-2 proteins are the predominant viral proteins synthesized early in infection.
- The distinct cellular localization and rapid turnover of NS-2 suggest specific roles in the viral life cycle.
- Shared characteristics among NS-2 forms point to the involvement of their common internal exon in mediating these properties.