We have identified an abundant 50K phosphoprotein (pp50) in MCMV-infected 3T3-L1 cells and shown by immunofluorescence microscopy and surface-iodination experiments that pp50 is localized to the plasma membrane of the infected cell. Furthermore, the kinetics of its synthesis suggests that it belongs to the early-late class of herpesvirus proteins. Using monoclonal antibodies specific for pp50 to screen a λ ZAP II expression library constructed from poly(A)+ mRNA of MCMV-infected cells, we have isolated a cDNA clone that synthesizes a truncated form of pp50 as a β-galactosidase fusion protein. This allowed us to localize the partial pp50 transcript to a region between map coordinates 0.228 and 0.260 of the MCMV genome (Smith strain, Vancouver). Finally, we demonstrated that the MAb 5H10.21 A recognizes an antigenic determinant that is conserved between pp50 and a 50K human cytomegalovirus (HCMV) nonstructural protein. © 1991.
