Human beta 2 chain of laminin (formerly S chain): cDNA cloning, chromosomal localization, and expression in carcinomas

Overlapping cDNA clones that encode the full-length human laminin beta 2 chain, formerly called the S chain, were isolated. The cDNA of 5680 nt contains a 5391-nt open reading frame encoding 1797 amino acids. At the amino terminus is a 32-amino-acid signal peptide that is followed by the mature beta 2 chain polypeptide of 1765 amino acids with a calculated molecular mass of 192,389 Da. The human beta 2 chain is predicted to have all of the seven structural domains typical of the beta chains of laminin, including the short cysteine-rich alpha region. The amino acid sequence of human beta 2 chain showed 86.1% sequence identity to the rat beta 2 chain, 50.0% to the human beta 1 chain, and 36.3% to the human beta 3 chain. The greatest sequence identity was in domains VI, V, and III. The sequence of a 24-amino-acid peptide fragment isolated from the beta 2 chain of laminin purified from human amniotic basement membrane matched the sequence predicted from the cDNA, confirming that the cDNA encodes human beta 2 laminin. The cDNA was used to assign the gene (LAMB2) to human chromosome 3p21 by in situ hybridization. It is not linked to genes for human laminin chains alpha 1, beta 1, and gamma 1 or other known laminin genes. Immunostaining showed that the beta 2 chain is localized to the smooth muscle basement membranes of the arteries, while the homologous beta 1 chain is confined to the subendothelial basement membranes. The beta 2 chain was found in the basement membranes of ovarian carcinomas but not colon carcinomas. These results indicate that the expression of the beta 2 chain gene is tightly regulated in normal human tissues and in disease.