α3β1 (VLA-3), a member of the integrin family of cell adhesion receptors, may function as a receptor for fibronectin, laminin, and collagen. A partial cDNA clone (2.4 kb) for the human α3 subunit was selected from an endothelial cell λgt11 cDNA library by specific antibody screening. Several overlapping cDNA clones were subsequently obtained, of a total length of 4.6 kb from various cDNA libraries. The reconstructed α3 cDNA was expressed on the surface of chinese hamster ovary cells as detected by an α3-specific mAb after transfection, suggesting that the cDNA is authentic. Within this sequence was an open reading frame, encoding for 1,051 amino acids, including a signal peptide of 32 residues, a long extracellular domain (959 residues), a transmembrane domain (28 residues), and a short cytoplasmic segment (32 residues). Overall, the α3 amino acid sequence was 25-37% similar to the other integrin α subunits that are cleaved, with most similarity to the α6 sequence (37%), and less similarity to those α subunits that have I domains (15-20%, excluding the I domain sequence itself). Features most like those in other α subunits are (a) the positions of 18/19 cysteine residues, (b) three potential metal binding domains of the general structure DX(D/N)X(D/N)GXXD, and (c) the predicted transmembrane domain. The mass of α3 calculated from its amino acid sequence is 113,505. The human α3 sequence was 89% identical to hamster galactoprotein b3, and 70% similar to the chicken CSAT antigen band 2 protein partial sequence, suggesting that these two polypeptides are homologues of human α3.
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