Role of preterminal protein processing in adenovirus replication

Preterminal protein (pTP), the protein primer for adenovirus DNA replication, is processed at two sites by the virus-encoded protease to yield mature terminal protein (TP). Here we demonstrate that processing to TP, via an intermediate (iTP), is conserved in all serotypes sequenced to date; and in determining the sites cleaved in Ad4 pTP, we extend the previously published substrate specificity of human adenovirus proteases to include a glutamine residue at P4. Furthermore, using monoclonal antibodies raised against pTP, we show that processing to iTP and TP are temporally separated in the infectious cycle, with processing to iTP taking place outside the virus particles. In vitro and in vivo studies of viral DNA replication reveal that iTP can act as a template for initiation and elongation and argue against a role for virus-encoded protease in switching off DNA replication. Virus DNA with TP attached to its 5' end (TP-DNA) has been studied extensively in in vitro DNA replication assays. Given that in vivo pTP-DNA, not TP-DNA, is the template for all but the first round of replication, the two templates were compared in vitro and shown to have different properties. Immunofluorescence studies suggest that a region spanning the TP cleavage site is involved in defining the subnuclear localization of pTP. Therefore, a likely role for the processing of pTP-DNA is to create a distinct template for early transcription (TP-DNA), while the terminal protein moiety, be it TP or pTP, serves to guide the template to the appropriate subcellular location through the course of infection.