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MAK3 encodes an N-acetyltransferase whose modification of the L-A gag NH2 terminus is necessary for virus particle
1Section on Genetics of Simple Eukaryotes, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Abstract:
The MAK3 gene is necessary for propagation of the L-A double-stranded RNA virus of Saccharomyces cerevisiae. MAK3 encodes a protein with substantial homology to the Escherichia coli rimI N-acetyltransferase that acetylates the NH2 terminus of ribosomal protein S18, and shares consensus sequences with a group of N-acetyltransferases. The NH2 terminus of the viral major coat protein encoded by L-A is normally blocked, but we find that it is unblocked in a mak3-1 mutant. L-A virus-encoded proteins produced from a cDNA clone of L-A can encapsidate the L-A (+)-strands in a wild-type host, but not in a mak3-1 mutant strain. The amount of major coat protein found in the particle fraction is reduced greater than 100-fold, and the amount in the total cell extract is reduced 5-10-fold. A modified beta-galactosidase, having as its NH2-terminal the NH2-terminal 13 residues of the L-A-encoded major coat protein, is blocked in a wild-type host, but not in a mak3-1 host. We propose that MAK3 encodes an N-acetyltransferase whose modification of the L-A major coat protein NH2 terminus is essential for viral assembly, and that unassembled coat protein is unstable.
Insights
The MAK3 gene product is an N-acetyltransferase essential for Saccharomyces cerevisiae L-A virus assembly. This enzyme modifies the viral coat protein, facilitating viral propagation and preventing coat protein degradation.
Area of Science:
- Molecular Biology
- Virology
- Yeast Genetics
Background:
- The L-A double-stranded RNA virus of Saccharomyces cerevisiae relies on host factors for its replication and propagation.
- Viral assembly and propagation often involve post-translational modifications of viral proteins.
Purpose of the Study:
- To investigate the function of the MAK3 gene in the life cycle of the L-A virus.
- To determine the molecular mechanism by which MAK3 facilitates viral propagation.
Main Methods:
- Genetic analysis of a mak3-1 mutant strain.
- Biochemical characterization of viral proteins in wild-type and mutant yeast.
- Analysis of protein modification using modified beta-galactosidase.
Main Results:
- MAK3 encodes a protein homologous to bacterial N-acetyltransferases.
- The L-A viral major coat protein's N-terminus is unblocked in mak3-1 mutants.
- Viral RNA encapsidation and coat protein stability are severely impaired in mak3-1 mutants.
- A reporter protein mimicking the coat protein N-terminus is modified in wild-type but not mak3-1 hosts.
Conclusions:
- MAK3 is an N-acetyltransferase that modifies the L-A viral coat protein's N-terminus.
- This modification is crucial for viral assembly and stability.
- Unmodified coat protein is unstable, leading to reduced viral propagation.