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Avian polyomavirus agnoprotein 1a is incorporated into the virus particle as a fourth structural protein, VP4
Reimar Johne1, Hermann Müller1
1Institute of Virology, Faculty of Veterinary Medicine, University of Leipzig, An den Tierkliniken 29, D-04103 Leipzig, Germany1.
Abstract:
Agnoproteins, encoded by the 5'-region of the late bicistronic mRNA of some polyomaviruses, are small proteins with largely unknown functions. In avian polyomavirus (APV)-infected cells, mRNAs of seven putative agnoproteins have been observed. Recently, it has been shown that agnoprotein 1a and its truncated variant agnoprotein 1b, encoded by the predominant mRNA species, are essential for APV replication. Here, the presence of agnoprotein 1a is demonstrated in the nucleus of APV-infected cells and in purified APV particles. Interaction between agnoprotein 1a and the major structural protein, VP1, was demonstrated by co-immunoprecipitation experiments using lysates of recombinant baculovirus-infected insect cells. With proteins expressed in E. coli, binding to double-stranded DNA in a sequence-unspecific manner was shown for agnoprotein 1a, whereas agnoprotein 1b failed to bind. A leucine zipper-like motif present in agnoprotein 1a is considered to be involved in DNA binding. Due to the absence of any structural or functional homologies between APV agnoprotein 1a and the agnoproteins of mammalian polyomaviruses, it is suggested that this protein should be renamed VP4, indicating its function as a fourth structural protein of APV.
Insights
Avian polyomavirus (APV) agnoprotein 1a is found in infected cells and virus particles, interacting with VP1 and binding DNA. Due to its unique nature, APV agnoprotein 1a is proposed to be renamed VP4, a fourth structural protein.
Area of Science:
- Virology
- Molecular Biology
- Structural Biology
Background:
- Agnoproteins are small proteins with poorly understood functions encoded by polyomaviruses.
- Seven agnoprotein mRNAs have been identified in avian polyomavirus (APV) infections.
- Agnoproteins 1a and 1b are crucial for APV replication.
Purpose of the Study:
- To investigate the localization and function of APV agnoprotein 1a.
- To determine the interaction of agnoprotein 1a with viral proteins.
- To characterize the DNA-binding properties of agnoprotein 1a and 1b.
Main Methods:
- Immunofluorescence microscopy to detect agnoprotein 1a in infected cells and purified virions.
- Co-immunoprecipitation assays to study protein-protein interactions.
- Electrophoretic mobility shift assays (EMSAs) to assess DNA binding of expressed proteins.
Main Results:
- Agnoprotein 1a was localized to the nucleus of APV-infected cells and detected in purified APV particles.
- Agnoprotein 1a directly interacts with the major structural protein VP1.
- Agnoprotein 1a binds double-stranded DNA non-specifically, potentially via a leucine zipper-like motif, while agnoprotein 1b does not.
Conclusions:
- APV agnoprotein 1a exhibits distinct characteristics compared to mammalian polyomavirus agnoproteins.
- The findings suggest agnoprotein 1a functions as a structural component of the APV virion.
- A renaming of APV agnoprotein 1a to VP4 is proposed, reflecting its role as a fourth structural protein.