Alphaherpesvirus proteins related to herpes simplex virus type 1 ICP0 affect cellular structures and proteins

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP0 intera cts with several cellular proteins and induces the proteasome-dependent deg radation of others during infection. In this study we show that ICP0 is req uired for the proteasome-dependent degradation of the ND10 protein Sp100 an d, as with the other target proteins, the ICP0 RING finger domain is essent ial. Further, comparison of the kinetics and ICP0 domain requirements for t he degradation of PMI and Sp100 suggests that a common mechanism is involve d. Homologues of ICP0 are encoded by other members of the alphaherpesvirus family. These proteins show strong sequence homology to ICP0 within the RIN G finger domain but limited similarity elsewhere. Using transfection assays , we have shown that all the ICP0 homologues that we tested have significan t effects on the immunofluorescence staining character of at least one of t he proteins destabilized by ICP0, and by using a recombinant virus, we foun d that the equine herpesvirus ICP0 homologue induced the proteasome-depende nt degradation of endogenous CENP-C and modified forms of PML and Sp100. Ho wever, in contrast to ICP0, the homologue proteins had no effect on the dis tribution of the ubiquitin-specific protease USP7 within the cell, consiste nt with their lack of a USP7 binding domain. We also found that ICP0 by its elf could induce the abrogation of SUMO-1 conjugation and then the proteaso me-dependent degradation of unmodified exogenous PML in transfected cells, thus demonstrating that other HSV-I proteins are not required. Surprisingly , the ICP0 homologues were unable to cause these effects. Overall, these da ta suggest that the members of the ICP0 family of proteins may act via a si milar mechanism or pathway involving their RING finger domain but that thei r intrinsic activities and effects on endogenous and exogenous proteins dif fer in detail.