Molecular defects caused by temperature-sensitive mutations in Semliki Forest virus nsP1

Area of Science:

• Virology
• Molecular Biology
• Protein Function

Background:

• Alphavirus nonstructural protein 1 (nsP1) is essential for viral RNA synthesis, possessing methyltransferase and guanylyltransferase activities for mRNA capping.
• nsP1 also facilitates viral replication complex membrane attachment and is critical for minus-strand RNA synthesis.

Purpose of the Study:

• To investigate the specific functions of Semliki Forest virus (SFV) nsP1 protein affected by temperature-sensitive mutations.
• To elucidate the role of nsP1 domains in viral RNA synthesis, particularly minus-strand synthesis.

Main Methods:

• Construction and characterization of recombinant SFV viruses with specific nsP1 mutations (ts10 and ts14).
• Assay of enzymatic activities (methyltransferase, guanylyltransferase) of mutant nsP1 proteins.
• Analysis of protein-protein interactions, membrane binding, and polyprotein processing in mutant viruses.
• Assessment of viral RNA synthesis, focusing on minus-strand production at restrictive temperatures.

Main Results:

• Mutant nsP1 proteins (ts10, ts14) retained normal capping enzyme activities and membrane binding capabilities.
• Interaction of nsP1 with other nonstructural proteins and polyprotein processing were largely unaffected.
• The mutant viruses exhibited a specific defect in minus-strand RNA synthesis at the restrictive temperature.

Conclusions:

• The temperature-sensitive mutations in nsP1 primarily disrupt minus-strand RNA synthesis, rather than mRNA capping or membrane association.
• Data suggest nsP1 contains a distinct subdomain crucial for minus-strand synthesis, potentially interacting with nsP4 (polymerase) and nsP2 (protease).
• This highlights a specialized role for nsP1 beyond its enzymatic functions in alphavirus replication.