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Truncated versions of the human cytomegalovirus (CMV) strain Towne glycoprotein B (gB) gene were stably expressed in CHO cell lines. The calcium-specific ionophore A23187 inhibited proteolytic cleavage of C-terminal-truncated gB expressed by cell line 67.77. These inhibition studies also showed that the 93-kilodalton cleavage product most likely represents the N-terminal cleavage fragment of gB. The ionophore carboxyl cyanide m-chlorophenyl-hydrazone was used to show that proteolytic cleavage of gB did not occur in the endoplasmic reticulum. Two-dimensional polyacrylamide gel electrophoresis demonstrated that the N- and C-terminal cleavage products of gB remained associated by disulfide linkages after cleavage. Expression studies using constructs in which 80% or all of the N terminus was deleted demonstrated that the N terminus was required for secretion of the gB molecule. The amino acid sequence at the site of cleavage was shown to be critical for cleavage by a cellular protease. Our results indicate that an arginine-to-threonine change at either amino acid 457 or 460, a lysine-to-glutamine change at amino acid 459, or all three substitutions together block gB cleavage. The effect on proteolysis of the arginine-to-threonine amino acid change at residue 457 (position -4 relative to the cleavage site) demonstrated that a basic pair of amino acids at the endoproteolytic processing site is not the only requirement in cis for gB cleavage.

An epitope common to all classes of murine leukemia viruses (MuLVs) was detected by reactivity of MuLVs with a rat monoclonal antibody (MAb) termed 83A25. The antibody is of the immunoglobulin G2a isotype and was derived after fusion of NS-1 myeloma cells with spleen cells from a Fischer rat immunized with a Friend polytropic MuLV. The antibody reacted with nearly all members of the ecotropic, polytropic, xenotropic, and amphotropic classes of MuLVs. Unreactive viruses were limited to the Friend ecotropic MuLV, Rauscher MuLV, and certain recombinant derivatives of Friend ecotropic MuLV. The presence of an epitope common to nearly all MuLVs facilitated a direct quantitative focal immunofluorescence assay for MuLVs, including the amphotropic MuLVs for which no direct assay has been previously available. Previously described MAbs which react with all classes of MuLVs have been limited to those which react with virion core or transmembrane proteins. In contrast, protein immunoblot and immunoprecipitation analyses established that the epitope reactive with MAb 83A25 resides in the envelope glycoproteins of the viruses. Structural comparisons of reactive and nonreactive Friend polytropic viruses localized the epitope near the carboxyl terminus of the glycoprotein. The epitope served as a target for neutralization of all classes of MuLV with MAb 83A25. The efficiency of neutralization varied with different MuLV isolates but did not correlate with MuLV interference groups.

Late in the course of Friend virus (FV)-induced erythroleukemia, leukemic spleen cells express a cell surface retroviral gp70 envelope protein not detected during the early proliferative phase of the disease. Characterization of this gp70 revealed it was unrelated to the input Friend murine leukemia virus (F-MuLV), but antigenically similar to a unique subset of endogenous xenotropic viruses. This gp70 was expressed by murine erythroleukemia cell lines but has not been identified on cell lines of other lineages. A monoclonal antibody (18-6) specifically reactive with this polypeptide was used to examine hematopoietic organs of normal uninoculated mice. This antibody detected a gp70 expressed by a majority of erythroid cells in fetal liver and by a small but significant percentage of normal adult spleen and bone marrow cells. Increased erythropoietic activity induced by treatment of adult mice with phenylhydrazine ( PHZ ) resulted in a seven- to eightfold increase in the frequency of spleen and bone marrow cells expressing this gp70. Peptide map analysis indicated that the 18-6 reactive gp70 expressed by Friend erythroleukemia cells and by cells from normal fetal liver were structurally identical. These results suggested that this unique gp70 was an erythroid-specific differentiation antigen.

Protein kinase activity was detected in immunoprecipitates of human cytomegalovirus virions and infected cells by using a monoclonal antibody directed against an abundant 68,000-dalton virion structural protein. Purification of this protein by electrophoresis confirmed that the kinase activity was associated with this protein. The kinase activity was dependent on divalent cations (Mg2+, Mn2+) and cyclic nucleotide independent and exhibited optimal activity at pH 7 to 8. The kinase phosphorylated threonine and serine but not tyrosine.

The envelope of human cytomegalovirus has been reported to contain between three and eight glycoproteins. Major constituents of the envelope include two abundant glycoproteins with estimated molecular weights of 55,000 (gp55) and 116,000 (gp116). These two glycoproteins have been shown to exist as a disulfide-linked complex (gp55-116) within the envelope of mature virions. Utilizing a panel of monoclonal antibodies reactive with the gp55-116 complex, we characterized the synthesis and processing of these two virion proteins. Infected cells were shown to contain two glycosylated proteins of 160,000 and 150,000 daltons as well as the mature gp55 and gp116. Pulse-chase analysis indicated that gp150 was a precursor protein of gp160. The mature gp55 and gp116 were generated, in turn, by cleavage of gp160. Antigenic and structural analysis revealed that gp55 and gp116 shared little structural homology and no detectable antigenic cross-reactivity. The results of this study are discussed in relation to the synthesis of envelope proteins of other herpesviruses.