By using the mouse Thy-1 gene as a model, we have developed a procedure to distinguish functional vs nonfunctional cDNA of lymphocyte surface antigens by transfecting COS-7 monkey cells and testing for expression of cell surface products encoded by the cDNA inserts. By cross-hybridization with a mouse Thy-1 probe, we isolated cDNA clones from a pcD-expression library prepared from mRNA of C5 cells. Two functional clones were distinguished from the remainder by detection of Thy-1.2 on the surface of 0.5% of COS-7 cells transiently transfected by the DEAE-Dextran method. Inclusion of chloroquine in the transfection procedure greatly facilitated the detection of functional cDNA by raising the percentage of expressing cells to 30%. Nucleotide sequencing of one functional cDNA, about 1700 bp long, confirmed that the gene encodes a protein whose sequence agrees with the published Thy-1.2 protein sequence with the additional 31 amino acids attached at the COOH-terminus. A 75 bp 5' untranslated region preceding the coding region contains 50 bp not found in the genomic clones. Comparison indicates that one or more introns are present in the 5' untranslated region, but are not found in the mature mRNA. The first exon may be separated by at least 1 kb intron from the initiation codon. Because the expressible clones are approximately the size of the mRNA seen on Northern blots, we believe that these clones are nearly full-length cDNA. Dilution experiments indicate that this strategy should also be useful for identifying functional cDNA clones for cell surface proteins solely on the basis of their expression in mammalian cells.

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