HUMAN CYTOMEGALOVIRUS CARRIES SERINE THREONINE PROTEIN PHOSPHATASES PP1 AND A HOST-CELL DERIVED PP2A

Human cytomegalovirus (CMV), a herpesvirus, is an important cause of m orbidity and mortality in immunocompromised patients. When studying hy per-immediate-early events after contact between CMV virions and the c ell membrane, we observed a hypophosphorylation of cellular proteins w ithin 10 min. This can be explained in part by our finding that purifi ed CMV contains serine/threonine protein phosphatase activities. Bioch emical analyses indicate that this protein phosphatase activity has al l characteristics of type 1 and 2A protein phosphatases (PP1 and PP2A) . Specifically, PP1 accounts for approximately 30% and PP2A accounts f or the remaining 70% of the phosphorylase phosphatase activity found. CMV produced in astrocytoma cells stably expressing an amino-terminall y tagged PP2A catalytic subunit contained tagged enzyme, thus demonstr ating the cellular origin of CMV-associated PP2A. PP2A is specifically found inside the virus, associated ,vith the nucleocapsid fraction, W estern blot (immunoblot) analysis of purified virus revealed the prese nce of the catalytic subunits of PP2A and PP1, Furthermore, the cataly tic subunit of PP2A appears to be complexed to the regulatory subunits PR65 and PR55, which is also the most abundant configuration of this enzyme found in the host cells. Incubation of virus with okadaic acid before contact of CMV with cells prevented hypophosphorylation of cell ular proteins, thus demonstrating the role of CMV-associated phosphata ses in this phenomenon, CMV can thus transport an active enzyme from o ne cell to another.