Revisiting the role of PML protein targeting and disruption of PML bodies in human cytomegalovirus infection

Promyelocytic leukaemia (PML) bodies are nuclear organelles implicated in post-translational modification by small ubiquitin-like modifier (SUMO) proteins and in the antiviral host cell response to infection. The 72-kDa immediate-early protein 1 (IE1) is considered the principal antagonist of PML bodies encoded by the human cytomegalovirus, one of eight human herpesviruses. Previous work has suggested that the interaction between IE1 and PML proteins, the central organisers of PML bodies, and the subsequent disruption of these organelles serve a critical role in viral replication by counteracting intrinsic antiviral immunity and the induction of interferon (IFN)-stimulated genes. However, this picture has emerged largely from studying mutant IE1 proteins known or predicted to be globally misfolded und metabolically unstable. We systematically screened for stable IE1 mutants by clustered charge-to-alanine scanning. We identified a mutant protein (IE1cc172-176) selectively defective for PML interaction. Functional comparisons between the mutant and wild-type protein revealed that IE1 can undergo modification by mixed polymeric SUMO chains and that it targets PML and Sp100, the two main constituents of PML bodies, via distinct mechanisms. Unexpectedly, IE1cc172-176 supported viral replication almost as efficiently as wild-type IE1. Moreover, lower instead of higher (as expected) levels of tumor necrosis factor alpha, IFN-beta, IFN-lambda and IFN-stimulated gene expression were observed with the mutant compared to the wild-type protein and virus. These results suggest that the disruption of PML bodies is linked to induction rather than inhibition of antiviral gene expression. Our findings challenge current views regarding the role of PML bodies in viral infection.