The role of PML proteins in adenovirus type 5 infection and the type I interferon response

Abstract

Promyelocytic leukaemia (PML) proteins consist of a collection of related isoforms that are the nucleating components of sub-nuclear structures termed PML nuclear bodies (PML NBs). Numerous functions are attributed to PML and PML NBs, including a role in antiviral responses. Adenovirus type 5 (Ad5) has previously been shown both to disrupt PML NBs and to cause the appearance of a novel PML protein species, termed PML-A, in a manner dependent on the viral E4 Orf3 protein. Here, the interactions between Ad5 and PML proteins were further investigated. The E4 Orf3-dependent species of PML (PML-A) was found to be related to the full-length PML II isoform. In addition, two previously unknown infection-specific species of PML were detected (PML-B and PML-C). Both PML-B and PML-C were produced independently of Ad5 E4 Orf3, though their appearance required viral DNA replication. PML-C was found to be localised within the cytoplasm of infected cells and to be related to PML I. To test the hypothesis that these changes in PML were related to virus effects on antiviral responses, the roles of individual PML proteins in type I interferon (IFN) signalling were investigated. Depletion of PML II by siRNA resulted in a 50% decrease in activity of an IFNβ promoter reporter plasmid during stimulation by poly (I:C). Exogenous expression of a ΔRBCC truncated mutant, but not full-length, PML II resulted in enhanced IFNβ promoter activity, suggesting a role of a PML II-related species in type I IFN signalling. Moreover, expression of the Ad5 E4 Orf3 protein was sufficient to inhibit the activity of the IFNβ promoter, an activity that correlated with Orf3’s ability to bind PML II. Collectively, these data indicate that a PML II-related molecule is involved in the interferon response and that the E4 Orf3- PML II interaction may therefore facilitate Ad5 in mitigating this activity