Biochemical characterization of 39-kDa class I histocompatibility antigen in plasma. A secretable membrane protein derived from transmembrane domain deletion

Three human class I major histocompatibility antigens (HLA) with molecular masses of 44, 39, and 36 kDa were identified in plasma by immunoprecipitation and immunoblotting. Further biochemical characterization showed that these antigens in plasma could be fractionated by Sephacryl S-300 column chromatography into two different pools. The 44-kDa intact HLA heavy chains are detected only in pool I and have an apparent molecular weight of 200,000 as determined by calibrated gel filtration column chromatography. The 39- and 36-kDa HLA heavy chains are present only in pool II and have an apparent molecular weight of 50,000. HLA in pool I can be extracted by Triton X-114 detergent, but 39- and 36-kDa plasma HLA in pool II are water soluble and not extractable by Triton X-114. Amino acid sequences of NH2 termini for 44- and 39-kDa plasma HLA are identical to that of cellular HLA. In contrast, the NH2-terminal amino acid sequence for 36-kDa plasma HLA has not been reported previously for any other proteins. Since the loss of both transmembrane domain and cytoplasmic tail at the carboxyl terminus of HLA will generate a 36-kDa protein, the findings suggest that the 39-kDa HLA might be the product of alternatively spliced mRNA with deletion of the exon coding for transmembrane domain. By using polymerase chain reaction and DNA sequencing, the presence of alternatively spliced mRNA with deletion of the transmembrane domain exon was identified in mononuclear leukocytes of peripheral blood. This alternatively spliced HLA mRNA was not detectable in mononuclear leukocytes of an individual who had no 39-kDa plasma HLA. This finding indicates that the alternatively spliced mRNA in mononuclear leukocytes is responsible for the synthesis of a secretable class I HLA.