The interaction of the tumor cell with its extracellular matrix may play an important role in determining its metastatic and invasive properties. To better understand the protein components that make up the extracellular matrix and how they are regulated, we have undertaken to construct, isolate, and characterize molecular clones of laminin receptor and of several different collagens. Laminin receptor is a cell surface portein to which laminin (a major component of basement membrane) specifically binds. Using a monoclonal antibody directed against the laminin-binding domain of laminin receptor, we screened a human endothelial cell cDNA Lambdagt11 library. Six plaques showed an intense reaction with the antilaminin receptor monoclonal but showed no reactivity toward a variety of control antibodies. The sizes of the cDNA inserts of the six clones ranged from 450 to 950 base pairs. Restriction enzyme mapping and Southern hybridization identified a 400 base pair fragment which was identical in each cDNA clone, suggesting that this fragment may represent the laminin-binding domain of laminin receptor. Definitive identification of the cDNA clones was based on comparison of cDNA sequence with the amino acid sequence of a cyanogen bromide-generated octapeptide of purified placental laminin receptor. The laminin receptor cDNA clone recognizes a 1700 base mRNA, which would be sufficient in length to code for a protein with the expected size of laminin receptor. The level of laminin receptor mRNA in a variety of human carcinoma-derived cell lines correlated with the number of laminin receptors on the cell surfaces of those cells. This suggests that the amount of laminin receptor mRNA may be a rate-limiting control step in the biosynthesis of laminin receptor, and hence in the regulation of cellular attachment to basement membranes via laminin.
