THE HUMAN CYTOMEGALOVIRUS UL74 GENE ENCODES THE 3RD COMPONENT OF THE GLYCOPROTEIN H-GLYCOPROTEIN L-CONTAINING ENVELOPE COMPLEX

The human cytomegalovirus (HCMV) gCIII envelope complex is composed of glycoprotein H (gH; gpUL75), glycoprotein L (gL; gpUL115), and a thir d, 125-kDa protein not related to gH dr gL (M. T. Huber and T. Compton , J. Virol. 71:5391-5398, 1997; L. Li, J. A. Nelson, and W. J. Britt, J. Virol. 71:3090-3097, 1997). Glycosidase digestion analysis demonstr ated that the 125-kDa protein was a glycoprotein containing ca. 60 kDa of N-linked oligosaccharides on a peptide backbone of 65 kDa or less. Based on these biochemical characteristics, two HCMV open reading fra mes, UL74 and TRL/IRL12, were identified as candidate genes for the 12 5-kDa glycoprotein. To identify the gene encoding the 125-kDa glycopro tein, we purified the gCIII complex, separated the components by sodiu m dodecyl sulfate-polyacrylamide gel electrophoresis, and subjected gH and the 125-kDa glycoprotein to amino acid microsequence analysis. Mi crosequencing of an internal peptide derived from purified 125-kDa gly coprotein yielded the amino acid sequence LYVGPTK. A FASTA search reve aled an exact match of this sequence to amino acids 188 to 195 of the predicted product of the candidate gene UL74, which we have designated glycoprotein O (gO). Anti-gO antibodies reacted in immunoblots with a protein species migrating at ca. 100 to 125 kDa in lysates of HCMV-in fected cells and,vith 100- and 125-kDa protein species in purified vir ions. Anti-gO antibodies also immunoprecipitated the gCIII complex and recognized the 125-kDa glycoprotein component of the gCIII complex. P ositional homologs of the UL74 gene were found in other betaherpesviru ses, and comparisons of the predicted products of the UL74 homolog gen es demonstrated a number of conserved biochemical features.