Isolation and structure of a cDNA encoding the B1 (CD20) cell-surface antigen of human B lymphocytes

The B1 (CD20) molecule is a Mr 33,000 phosphoprotein on the surface of human B lymphocytes that may serve a central role in the humoral immune response by regulating B-cell proliferation and differentiation. In this report, a cDNA clone that encodes the B1 molecule was isolated and the amino acid sequence of B1 was determined. B-cell-specific cDNA clones were selected from a human tonsillar cDNA library by differential hybridization with labeled cDNA derived from either size-fractionated B-cell mRNA or size-fractionated T-cell mRNA. Of the 261 cDNA clones isolated, 3 cross-hybridizing cDNA clones were chosen as potential candidates for encoding B1 based on their selective hybridization to RNA from B1-positive cell lines. The longest clone, pB1-21, contained a 2.8-kilobase insert with an 891-base-pair open reading frame that encodes a protein of 33 kDa. mRNA synthesized from the pB1-21 cDNA clone in vitro was translated into a protein of the same apparent molecular weight as B1. Limited proteinase digestion of the pB1-21 translation product and B1 generated peptides of the same sizes, indicating that the pB1-21 cDNA encodes the B1 molecule. Gel blot analysis indicated that pB1-21 hybridized with two mRNA species of 2.8 and 3.4 kilobases only in B1-positive cell lines. The amino acid sequence deduced from the pB1-21 nucleotide sequence apparently lacks a signal sequence and contains three extensive hydrophobic regions. The deduced B1 amino acid sequence shows no significant homology with other known proteins.