Preterminal protein, the achilles heel of adenoviridae: Implications for adenoviral infections

Abstract

BACKGROUND

Adenoviruses pose a serious health risk particularly in the absence of any clinically approved treatment. As adenoviral infections are quite frequent and recent outbreaks manifest more virulent variant strains, the need to develop an effective treatment remains a priority. The adenoviral protein, preterminal protein (pTP), is one of the key common products of the viral lifecycle as it is necessary to initiate viral replication and hence the infection process. This makes pTP a potential chemotherapeutic target in the search for and development of an effective treatment for adenoviral induced infections. Here we report, for the first time, that glycosylation of pTP in situ prevents binding to ssDNA in vitro.

AIM

To explore whether specific structural tailoring of the adenoviral protein pTP, imparts the potential to scupper the viral replication process.

METHODS

All chemicals used were of reagent grade. Overexpression of pTP was achieved using the ‘BAC to BAC’ expression system. The presence and relative concentration of the protein was determined throughout the incubation period by the Bradford assay. The pTP was identified by MALDI-TOFF and sodium dodecyl sulphate polyacrylamide gel electrophoresis. For the removal of the aminosugar, a deglycosylase enzyme kit from PROZYME was used. Purification of cloned pTP (6xHis) was done with a ssDNA cellulose column followed by a Ni-NTA column. His-tags were excised with the Tobacco etch virus protease. Protein fractionation was performed with a fraction collector coupled to a UV detector (280 nm) from Pharmacia.

RESULTS

The pTP overexpressed in insect cells (Spodoptera frugiperda) (> 96 hours), is unable to bind to ssDNA in vitro. Treatment of this unbound protein with a deglycosidase enzyme that is specific for the removal of truncated unsubstituted O-linked Galβ(1-3)GalNAc-α1 disaccharides bound to Thr or Ser in a glycoprotein, restores binding to ssDNA. Data is presented as a linegraph for both the glycosylated and the deglycosylated proteins. Each point represents the mean of triplicate experiments (from different batches). Means and standard deviation were calculated and plotted on a line graph (with error bars).

CONCLUSION

The finding that glycosylation of cloned pTP in situ prevents binding to ssDNA in vitro could aid in the development of an effective treatment of adenoviral infections and/or as an adjunct to complement other anti-adenoviral chemotherapeutic strategies.

Keywords: Thomsen-Friedenreich antigen; Adenoviral; Preterminal protein; Spodoptera frugiperda cells; Protein priming mechanism; Glycosylation

Core Tip: A key objective in antiviral research is to recognize appropriate targets within the viral life cycle that are independent of normal host cellular functions. Adenoviruses (Ad) utilize a ‘unique’ protein priming mechanism that requires a viral protein, preterminal protein (pTP), for the initiation of replication. Therefore, it appears that pTP could potentially be a target for chemotherapeutic intervention. The finding that glycosylation of cloned pTP (in situ) in insect cells (Spodoptera frugiperda) prevents binding to ssDNA (in vitro) could prove useful in developing an effective treatment that facilitates the generation of an abortive adenoviral replication complex that would adversely affect human Ad virulence and propagation.