Nuclear translocation of mutagenized forms of human cytomegalovirus glycoprotein B (gpUL55)

This study investigated how human cytomegalovirus glycoprotein B (gB) localizes to the inner nuclear membrane in the absence of other viral proteins. Researchers created stable transfectants of human astrocytoma cells expressing wild-type gB or mutant versions with deletions in membrane anchor domains or cytoplasmic tail regions. Using immunofluorescence, cell fractionation, pulse-chase labeling with radioactive methionine, and phosphate labeling, they demonstrated that all gB derivatives reached the inner nuclear membrane. However, deletion of the extreme carboxy terminus (amino acids 856-906) significantly reduced nuclear accumulation and retention. Pulse-chase analysis revealed that this region is critical for nuclear retention, as the truncated protein showed impaired accumulation in nuclear fractions over time. The carboxy terminus contains a nucleoplasmin-like nuclear localization signal and is phosphorylated at a casein kinase II consensus motif. The findings suggest that both membrane anchor domains and the cytoplasmic tail contribute to nuclear localization, potentially serving distinct functions in translocation versus retention of the viral protein at the nuclear envelope.

Key findings

• All gB derivatives were transported to the inner nuclear membrane, but deletion of amino acids 856-906 significantly reduced nuclear localization and retention
• Pulse-chase analysis revealed a carboxy-terminal nucleoplasmin-like signal (within the deleted region) is essential for efficient nuclear accumulation and retention
• The gB molecule is phosphorylated at a carboxy-terminal casein kinase II consensus motif (position 900-903), which is absent in the Del3 mutant
• Both membrane anchor domains and cytoplasmic tail regions appear to participate in nuclear localization, possibly serving different functions in translocation and retention