Inhibition of SUMO-independent PML oligomerization by the human cytomegalovirus IE1 protein

In human cytomegalovirus-infected cells, the immediate-early IE1 protein disrupts the subnuclear structures known as the PML oncogenic domains or PODs, via the induction of PML desumoylation. This activity correlates with the functions of IE1 in transcriptional regulation and in the stimulation of lytic infection. Here, the effects of IE1 in induction of desumoylation of PML were characterized. IE1 did not interfere with the formation of sumoylated forms of PML in vitro. In in vitro assays using the sumoylated proteins, a SUMO-specific protease SENP1 desumoylated both PML and IE1. However, the IE1 proteins generated from bacteria or insect cells were unable to desumoylate PML in the same conditions. Although both IE1 and SUMO proteases such as SENP1, Axam and SuPr-1 efficiently desumoylated PML in co-transfection assays, they exerted different effects on the localization of PML. In cells transfected with either SENP1 or SuPr-1, the number of PML foci was reduced significantly and these remnant PML foci were devoid of SUMO-1 signals. However, in cells co-transfected with both SUMO proteases and IE1, these SUMO-independent PML foci were also completely disrupted. Furthermore, IE1, but not SENP1, was shown to disrupt the PML foci generated via transfection of a sumoylation-deficient mutant of PML. These data suggest that IE1 exhibits neither an inhibitory effect on sumoylation of PML nor intrinsic SUMO protease activity against PML in vitro. The finding that IE1 is capable of disrupting SUMO-independent PML aggregates suggests that inhibition of PML oligomerization by IE1 may play an important role in inducing PML desumoylation in vivo.