Cloning and Expression of a Divergent Integrin Subunit BS

Rabbit and human cDNA clones have been identified that encode a novel integrin /3 subunit. The sequences that encode this subunit, which has been designated as BE, were isolated initially from rabbit placental cDNA libraries using an oligonucleotide probe derived from a highly conserved region of integrin 6 subunit sequences. The rabbit clone was used to isolate human @E cDNA clones from human placental and MG-63 osteosarcoma cell libraries. The putative BE polypeptides, which comprise 769 and 768 residues in human and rabbit, respectively, show a high degree of inter-species conservation (-90% identity). In contrast, Bs is distinct from the other integrin f i subunits. At the amino acid level human @E ranges from 31 to 37% identity with human B1-,. The domain structure of BS is typical of the integrin f l subunits. Human has a 42-residue N-terminal signal peptide, a large extracellular domain (-639 residues) that contains four cysteine-rich repeats, a transmembrane domain (-30 residues), and a C-terminal cytoplasmic domain (-68 residues). There are several structural features that are unique to the BE polypeptide, as compared with the other integrin @ subunits. Six of the 56 cysteine residues that are conserved within the extracellular domains of B1, &, &, &, BS, and the f l subunit from Drosophila are absent in the BE polypeptide. Also, the cytoplasmic domain of the BS subunit shares no homology with the cytoplasmic regions of any of the other integrin B subunits. Northern analysis demonstrated an -8-kilobase BE mRNA in rabbit placenta, kidney, brain, ovary, and uterus. PCR analysis revealed that f ls mRNA is also present in several transformed human cell lines. The BE polypeptide has been transiently expressed in 293 human embryonic kidney cells. A polyclonal antipeptide antibody specific for BE and a polyclonal antibody that recognizes a, epitopes were used to show that can complex with the endogenous a, subunit in 293 cells and that the resulting integrin is expressed as a cell surface complex.