Abstract
Studies on dsDNA bacteriophages have revealed that a DNA packaging complex assembles at a special vertex called the ‘portal vertex’ and consists of a portal, a DNA packaging ATPase and other components. AdV protein IVa2 is presumed to function as a DNA packaging ATPase. However, a protein that functions as a portal is not yet identified in AdVs. To identify the AdV portal, we performed secondary structure analysis on a set of AdV proteins and compared them with the clip region of the portal proteins of bacteriophages phi29, SPP1 and T4. Our analysis revealed that the E4 34K protein of HAdV-C5 contains a region of strong similarity with the clip region of the known portal proteins. E4 34K was found to be present in empty as well as mature AdV particles. In addition, E4 34K co-immunoprecipitates and colocalizes with AdV packaging proteins. Immunogold electron microscopy demonstrated that E4 34K is located at a single site on the virus surface. Finally, tertiary structure prediction of E4 34K and its comparison with that of single subunits of Phi29, SPP1 and T4 portal proteins revealed remarkable similarity. In conclusion, our results suggest that E4 34K is the putative AdV portal protein.
Highlights
Adenovirus (AdV) morphogenesis appears to follow a pathway similar to that observed in dsDNA containing bacteriophages and herpesviruses[1,2,3]
Due to the presence of a packaging signal located close to the left end of the viral genome between the left inverted terminal repeat and the E1A transcription start site, the viral genome is recognized by AdV proteins that constitute the viral packaging machinery
Once the capsids are assembled, a complex of viral packaging ATPase associated with the viral genome and other viral proteins assembles as a ring on the portal
Summary
Adenovirus (AdV) morphogenesis appears to follow a pathway similar to that observed in dsDNA containing bacteriophages and herpesviruses[1,2,3]. The extracts of the mock-infected and AdV-infected cells, as well as purified preparations of empty and mature virus particles were separated by SDS–PAGE for immunoblot analysis with anti-E4 34K antibody. In order to test this hypothesis, we performed immunoprecipitation of nuclear extracts of HAdV-C5-infected 293 cells with anti-IVa2, anti-33K, anti-DBP or anti-hexon antibodies using a protocol as described earlier[10].
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