Identification and initial characterization of the murine gammaherpesvirus68 gene M3, encoding an abundantly secreted protein

Several viruses, including members of the gammaherpesvirus family, encode p roteins that are secreted into the extracellular environment. We have ident ified an abundant 44-kDa secreted protein that is present in the supernatan t of fibroblasts infected with murine gammaherpesvirus 68 (gamma HV68; also referred to as MHV-68) but not in that of uninfected fibroblasts, Sequence analysis of the amino terminus and of internal peptides revealed that this protein is encoded by the gamma HV68 M3 open reading frame (ORF), The amin o-terminal sequence of the secreted protein starts at residue 25 of the M3 ORF, consistent with the first 24 residues functioning as a signal peptide. Northern blot analysis revealed a single abundant similar to 1.4-kb early- late lytic transcript encoded by the M3 ORF. Analysis of a partial cDNA clo ne and subsequent analyses of products of rapid amplification of cDNA ends coupled with S1 nuclease protection assays demonstrate that the M3 protein is encoded by an unspliced, polyadenylated mRNA initiating at bp 7294 and t erminating at bp 6007 of the gamma HV68 genome. The 3' end of the M3 transc ript maps 9 bp downstream of a consensus polyadenylation signal. Thus, the predicted M3 ORF is a functional gene that encodes an abundant secreted pro tein which is a candidate for interacting with host cellular receptors or c ytokines.