Molecular cloning of cDNA encoding MS2 antigen, a novel cell surface antigen strongly expressed in murine monocytic lineage

cDNA clones which strongly hybridized with a 3.1 kb mRNA from mouse macrophages and macrophage cell lines and weakly with mRNA from P815 but not from a variety of other cell lines and tissues were isolated from cDNA libraries constructed using mRNA from murine macrophage cell lines and peritoneal macrophages. Treatment of a macrophage cell line with macrophage stimulators significantly enhanced transcription of the mRNA. Sequencing analysis of these clones demonstrated that the cDNA consisted of 3036 bp insert containing a 2478 bp open reading frame followed by a 538 bp 3' untranslated region. The amino acid sequence, deduced from the nucleotide sequence of the cDNA, predicted a protein containing a signal peptide, an extracellular region, a transmembrane domain, and a cytoplasmic tail. The extracellular region had five putative N-glycosylation sites and a cysteine-rich domain, whereas the cytoplasmic region consisted of a proline-rich amino acid sequence significantly similar to CD2. SDS-PAGE and NEPHGE SDS-PAGE analysis of the immunoprecipitated membrane of the macrophage cell lines prepared by using rabbit anti-MS2 peptide antibody raised against a synthetic peptide preparation relative to a hydrophilic region of the MS2 amino acid sequence confirmed that MS2 protein is a cross-linked protein having approximate molecular sizes of 89 kd and pl 6.5-7.0. These results show that MS2 protein is a novel cell surface antigen expressed mainly in monocytic lineages.