Characterization of a 52K protein of murine cytomegalovirus and its immunological cross-reactivity with the DNA-binding protein ICP36 of human cytomegalovirus.

We have developed a hybridoma, designated 25G11, which produced a monoclonal antibody (MAb) reactive with a 52K protein of murine cytomegalovirus (MCMV). This MAb, 25G11, was reactive with a protein band of 52K in MCMV-infected cell lysates and with a protein of 49K in human CMV (HCMV)-infected cell lysates as detected by immunoblot analysis. With purified MCMV virions, 25G11 gave a faintly immunoreactive band of 52K. However, no immunoreactive protein band was detected with purified HCMV virions, nor with purified HCMV or MCMV envelope preparations. By immunocytochemistry, 25G11 detected viral antigen primarily in the nucleus of HCMV- or MCMV-infected cells. The antibody 25G11 was used to screen a lambda gt11 library of HCMV DNA fragments. One of the isolated clones (lambda 32323B) was employed for gene mapping on the HCMV genome, which suggested that the immunoreactive HCMV protein was the DNA-binding protein (ICP36). Analysis of the recombinant fusion protein with antibody 25G11 and with an MAb (CH16) specific for an HCMV DNA-binding protein confirmed the identity of the cross-reacting protein as ICP36. Furthermore, we found that whereas the epitope recognized by 25G11 was conserved between HCMV and MCMV proteins, the epitope recognized by CH16 was unique to HCMV and thus represents a variable region in the protein.