The acidic domain of the human cytomegalovirus UL37 immediate early glycoprotein is dispensable for its transactivating activity and localization but is not for its synergism.

The product of the human cytomegalovirus (HCMV) immediate early (IE) UL37 gene, gpUL37, is predicted to be a type I membrane-bound glycoprotein. Typically for HCMV IE proteins, gpUL37 transactivates nuclear gene expression and acts synergistically with other IE proteins. We have initiated mutational analysis of the gpUL37 domains to determine which are required for its transactivating activity. The acidic domain, a feature notably required for the activity of many nuclear transcription factors, was deleted from gpUL37. Similar to wild-type gpUL37, the mutant retained a dose responsive transactivating activity in transiently transfected HeLa cells. Transactivating activity of the mutant was also observed in permissive human diploid fibroblasts when it was cotransfected with IE1. However, the gpUL37 acidic domain mutant is defective for synergism with another HCMV IE protein, pUS3. We found that wild-type gpUL37 and its acidic domain mutant (delta aa53-140) are nonnuclear proteins and are indistinguishable in localization. Confocal microscopy of human cell types coexpressing both HCMV IE regulatory proteins, IE1 and gpUL37, showed gpUL37 does not colocalize with the IE1 nuclear protein. Taken together, our results establish that gpUL37 is a nonnuclear protein that requires its acidic domain for synergism with pUS3 but not for its transactivating activity or its localization.