Structure and domain organization of the CD19 antigen of human, mouse, and guinea pig B lymphocytes. Conservation of the extensive cytoplasmic domain.

The CD19 molecule is a 95,000 Mr cell-surface protein of human B lymphocytes with two extracellular Ig-like domains and a 240 amino acid cytoplasmic tail. cDNA encoding human CD19 and the cytoplasmic domain of the mouse CD19 Ag were previously isolated. In this report, those cDNA were used to isolate cDNA or genomic DNA encoding the complete mCD19 protein and a portion of CD19 from the guinea pig. Mouse pre-B and B cell lines expressed two CD19 mRNA species of 2.7 and 2.2 kb, whereas myeloma cell lines were negative as were T cell lines. Similarly, among mouse organs, only spleen contained detectable CD19 mRNA. These results suggest that only B cells express CD19 in mouse, as in man. Sequence determination revealed substantial conservation, with hCD19 and mCD19 being 66% and hCD19 and gpCD19 being 73% identical in amino acid sequence. The cytoplasmic region of CD19 was most highly conserved with human/mouse being 73% identical and human/guinea pig being 83% identical in amino acid sequence. Isolation of the hCD19 and mCD19 genes and determination of exon/intron boundaries revealed that both genes were structurally similar and were composed of at least 15 exons, 4 encoded extracellular domains, and 9 encoded cytoplasmic domains. Six of the exons that encoded cytoplasmic domains were essentially identical in sequence in all three species indicating that these regions have undergone considerable selective pressure to conserve sequences. Thus, CD19 appears to be well conserved in structure and expression through recent mammalian evolution and the highly conserved cytoplasmic domains may play a critical role in the transduction of CD19-mediated signals.