Intracellular localization and expression of the human cytomegalovirus matrix phosphoprotein pp71 (ppUL82): evidence for its translocation into the nucleus.

A polyclonal antiserum, raised against a pp71 fusion protein, was prepared in order to investigate the biosynthesis and localization of the matrix protein pp71 of human cytomegalovirus (HCMV), the UL82 gene product, during the HCMV infectious cycle in human fibroblasts. Transcription of the pp71-specific bicistronic 4.0 kb mRNA and pp71 biosynthesis exhibited a biphasic pattern during one round of the HCMV infectious cycle, with a first peak at 12 h and a second at 72 h post-infection (p.i.). Cycloheximide treatment of infected human fibroblasts revealed that the presence of pp71 in total cell extracts prior to 3 h p.i. was due to the input virus inoculum. Transcription of the two specific pp71 mRNAs commenced 5-7 h p.i. as shown by Northern blot analysis of total cellular RNA. Western blot analysis of isolated nuclei and indirect immunofluorescence experiments indicated that pp71, like the major tegument protein pp65, is present in the nucleus shortly after infection as well as during the late phase of viral morphogenesis. Also, after transient transfection of UL82 into U37 3MG cells, pp71 was found to be present in the nucleus of the transfected cells. By immunogold labelling, pp71 was detected in the nucleoplasm in association with nucleocapsids in electron-dense nuclear skein structures at late stages of the infection cycle. These findings suggest functions of pp71 in viral maturation in addition to that as an early transactivator of viral gene transcription described recently.